EP4701725A1 - Isolation breach detection - Google Patents

Abstract

A medical device is configured to deliver a test pulse to an electrode terminal of the medical device. The medical device may measure a response signal to the test pulse for detecting a leakage current pathway based on the response signal. The medical device may determine that a leakage current pathway detection threshold is met based on the response signal. The medical device may generate and transmit an alert in response to the leakage current pathway detection threshold being met.

Description

ISOLATION BREACH DETECTION

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63/499,114, filed April 28, 2023, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The disclosure relates generally to a medical device and method for detecting an isolation breach in the medical device.

BACKGROUND

[0003] Medical devices may sense electrophysiological signals from the heart, brain, nerve, muscle or other tissue. Such devices may be implantable, wearable or external devices using implantable and/or surface (skin) electrodes for sensing the electrophysiological signals. In some cases, such devices may be configured to deliver a therapy based on the sensed electrophysiological signals. For example, implantable or external cardiac pacemakers, cardioverter defibrillators, cardiac monitors and the like, sense cardiac electrical signals from a patient’s heart. The medical device may sense cardiac electrical signals from the heart and deliver electrical stimulation therapies, such as cardiac pacing pulses and/or cardioversion or defibrillation (CV/DF) shocks, to the heart using electrodes, which may be carried by medical electrical leads extending from the medical device to position electrodes within or near the patient’s heart.

[0004] A cardiac pacemaker or cardioverter defibrillator may deliver therapeutic electrical stimulation to the heart via electrodes carried by one or more medical electrical leads coupled to the medical device. Cardiac signals sensed from the heart may be analyzed for detecting an abnormal rhythm. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation pulse or pulses may be delivered to restore or maintain a more normal rhythm of the heart. For example, an implantable cardioverter defibrillator (ICD) may deliver bradycardia pacing pulses to the heart of the patient in the absence of sensed intrinsic myocardial depolarization signals, e.g., R-waves, deliver anti-tachycardia pacing pulses in response to detecting tachycardia, or deliver CV/DF shocks to the heart upon detecting tachycardia or fibrillation. SUMMARY

[0005] In general, the disclosure is directed to a medical device and techniques for detecting an isolation breach. The medical device may be a pacemaker or ICD configured to deliver cardiac pacing pulses and/or high voltage CV/DF shocks. An electrical current leakage pathway may exist when an isolation breach occurs. The electrical leakage pathway may reduce the energy delivered to a therapeutic load. For example, the delivered energy to a patient’s heart during a CV/DF shock or a cardiac pacing pulse may be lower than a programmed energy or voltage amplitude of the respective CV/DF shock or cardiac pacing pulse. The medical device, operating according to methods disclosed herein, may apply a test pulse to an internal electrode terminal. In some examples, the test pulse may be applied without enabling an electrical return path for the current through the patient’ s body. A voltage or current measurement within the medical device during the test pulse can indicate that a leakage current pathway exists, which may reduce the delivered energy and/or cause damage to internal device components when a therapeutic electrical stimulation pulse is delivered by the medical device.

[0006] In one example, the disclosure provides a medical device including multiple electrode terminals and a therapy delivery circuit configured to deliver electrical stimulation pulses via the electrode terminals. The therapy delivery circuit may include one or more low side switches that can each be selectively enabled for providing a return current path from at least one of the electrode terminals to an electrical ground of the medical device during delivery of an electrical stimulation pulse by the therapy delivery circuit. The medical device further includes a control circuit that may be configured to simultaneously hold each of the one or more low side switches in a non-conducting state. The control circuit may control the therapy delivery circuit to deliver a test pulse to a first electrode terminal of the electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open. The control circuit may measure a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state. The control circuit may determine that a leakage current pathway detection threshold is met based on the response signal. The control circuit may generate an alert in response to the leakage current pathway detection threshold being met. The medical device may include a communication circuit configured to transmit the alert.

[0007] In another example, the disclosure provides a method including delivering a therapeutic electrical stimulation pulse via electrode terminals when at least one of one or more low side switches of a medical device is selectively enabled for providing a return current path from at least one of the electrode terminals to an electrical ground of the medical device. The method further includes simultaneously holding each of the one or more low side switches in a non-conducting state. The method further includes delivering a test pulse to one of the electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open. The method may include measuring a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state. The method may include determining that a leakage current pathway detection threshold is met based on the response signal. The method may include generating an alert in response to the leakage current pathway detection threshold being met and transmitting the alert.

[0008] In yet another example, the disclosure provides a non-transitory computer readable medium storing a set of instructions that, when executed by a control circuit of a medical device, cause the medical device to deliver a therapeutic electrical stimulation pulse via electrode terminals of the medical device when at least one of one or more low side switches of the medical device is selectively enabled for providing a return current path from at least one of the electrode terminals to an electrical ground of the medical device. The instructions further cause the medical device to simultaneously hold each of the one or more low side switches in a non-conducting state and deliver a test pulse to one of the electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the electrode terminals is open. The instructions may further cause the device to measure a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state. The instructions may cause the medical device to determine that a leakage current pathway detection threshold is met based on the response signal, generate an alert in response to the leakage current pathway detection threshold being met and transmit the alert. [0009] In another example the disclosure provides a medical device comprising electrode terminals that include one or more low impedance electrode terminals each associated with a corresponding low impedance electrode and one or more high impedance electrode terminals each associated with a corresponding high impedance electrode. The medical device further includes a therapy delivery circuit configured to deliver electrical stimulation pulses via the electrode terminals. The medical device includes a control circuit configured to control the therapy delivery circuit to deliver a first test pulse to a first low impedance electrode terminal of the one or more low impedance electrode terminals and enable a first return current path via a first high impedance electrode terminal of the one or more high impedance electrode terminals during the first test pulse. The control circuit is further configured to measure a first response signal while the first return current path is enabled. The control circuit is further configured to control the therapy delivery circuit to deliver a second test pulse to the first high impedance electrode terminal and enable a second return current path via the first low impedance electrode terminal during the second test pulse. The control circuit is further configured to measure a second response signal while the second return current path is enabled and detect a leakage current pathway based on the first response signal and the second response signal. The control circuit may be further configured to generate an alert in response to detecting the leakage current pathway. The medical device may include a communication circuit configured to transmit the alert.

[0010] In yet another example the disclosure provides a method including delivering a first test pulse to a low impedance electrode terminal associated with a corresponding low impedance electrode, enabling a first return current path via a high impedance electrode terminal associated with a corresponding high impedance electrode during the first test pulse and measuring a first response signal while the first return current path is enabled. The method may further include delivering a second test pulse to the high impedance electrode terminal, enabling a second return current path via the low impedance electrode terminal during the second test pulse and measuring a second response signal while the second return current path is enabled. The method may include detecting a leakage current pathway based on the first response signal and the second response signal, generating an alert in response to detecting the leakage current pathway and transmitting the alert. [0011] In another example the disclosure provides a non-transitory computer readable medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to deliver a first test pulse to a low impedance electrode terminal associated with a corresponding low impedance electrode and enable a first return current path via a high impedance electrode terminal associated with a corresponding high impedance electrode during the first test pulse. The instructions may cause the medical device to measure a first response signal while the first return current path is enabled. The instructions may further cause the medical device to deliver a second test pulse to the high impedance electrode terminal, enable a second return current path via the low impedance electrode terminal during the second test pulse and measure a second response signal while the second return current path is enabled. The instructions may further cause the medical device to detect a leakage current pathway based on the first response signal and the second response signal, generate an alert in response to detecting the leakage current pathway and transmit the alert.

[0012] This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the accompanying drawings and description below. Further details of one or more examples are set forth in the accompanying drawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIGs. 1A and IB are conceptual diagrams of one example of an ICD system that may be configured to sense cardiac event signals, deliver electrical stimulation therapy and detect a leakage current pathway according to the techniques disclosed herein.

[0014] FIGs. 2A-2C are conceptual diagrams of a patient implanted with an ICD system in a different implant configuration than the arrangement shown in FIGs. 1A-1B.

[0015] FIG. 3 is a conceptual diagram of a medical device system including an ICD coupled to a patient’s heart via transvenous electrical leads.

[0016] FIG. 4 is a conceptual diagram of the ICD shown in FIGs. lA-2c or FIG. 3 according to some examples.

[0017] FIG. 5 is a conceptual diagram of circuitry that can be included in the therapy delivery circuit of the ICD of FIG. 4 according to some examples. [0018] FIG. 6 is a diagram of an electrical stimulation pulse that may be delivered by the therapy delivery circuit of FIG. 5 according to one example.

[0019] FIG. 7A is a conceptual diagram of a current leakage pathway to internal ground of the ICD of FIG. 4 that could occur during electrical stimulation pulse delivery by the ICD therapy delivery circuit.

[0020] FIG. 7B is a conceptual diagram of circuitry and a method for performing a leakage current test according to some examples.

[0021] FIG. 8 is a flow chart of a method for performing a leakage current test by an ICD according to some examples.

[0022] FIG. 9 is a conceptual diagram of a connector assembly coupled to an ICD housing according to some examples.

[0023] FIG. 10 is a conceptual diagram of another technique for performing a leakage current test by an ICD according to some examples.

[0024] FIG. 11 is a flow chart of the method for performing the leakage current test according to the techniques depicted by the diagram of FIG. 10.

DETAILED DESCRIPTION

[0025] In general, this disclosure describes medical devices and techniques for detecting an electrical leakage path that may provide a current pathway to the medical device ground during electrical stimulation therapy delivery. A leakage pathway may exist that shunts electrical stimulation current away from a therapy delivery pathway, e.g., a therapy delivery pathway to a patient’s heart via electrodes coupled to the medical device. In various examples, the leakage pathway may exist due to a mechanical breach of insulation around an electrically conductive component of an electrical feedthrough that passes through the medical device housing, fluid accumulation in the medical device header, or foreign material inside the medical device housing, e.g., on a hybrid circuit board or feedthrough assembly.

[0026] The techniques disclosed herein may be implemented in any implantable, partially implantable, or external or wearable medical device system configured to deliver electrical stimulation therapies. In the illustrative examples, the techniques are described in conjunction with an ICD that is configured to deliver CV/DF shock pulses and/or cardiac pacing pulses via extra-cardiac electrodes that are not in direct contact with the heart or pericardium. The electrodes may be carried by an implantable medical electrical lead extending from the pacemaker or ICD and/or carried by the housing of the pacemaker or ICD. The CV/DF shock pulses or cardiac pacing pulses may be delivered with a relatively high pulse energy using extra-cardiac electrodes compared to CV/DF shock pulses or cardiac pacing pulses, respectively, that are delivered using electrodes that are implanted in or on the heart. The disclosed techniques are not necessarily limited to a medical device system that delivers electrical stimulation therapy to the patient’s heart via extra-cardiac electrodes. The techniques disclosed herein can be implemented in conjunction with a pacemaker or ICD that delivers electrical stimulation therapy to a patient’s heart via transvenous, endocardial, epicardial or pericardial electrodes. Furthermore, the techniques disclosed herein are not necessarily limited to implantable systems. The disclosed techniques may be implemented in an external pacemaker or ICD using cutaneous surface electrodes or transcutaneous electrodes.

[0027] FIGs. 1A and IB are conceptual diagrams of one example of an ICD system 10 that may be configured to detect a leakage current pathway according to the techniques disclosed herein. ICD system 10 is configured to sense cardiac electrical signals and deliver electrical stimulation therapy. FIG. 1 A is a front view of ICD system 10 implanted within patient 12. FIG. IB is a side view of ICD system 10 implanted within patient 12. ICD system 10 includes an ICD 14 connected to an electrical stimulation and sensing lead 16, positioned in an extra-cardiovascular location in this example. FIGs. 1A and IB are described in the context of an ICD system 10 capable of providing high voltage CV/DF shocks and relatively lower voltage cardiac pacing pulses in response to detecting a cardiac arrhythmia based on processing of sensed cardiac electrical signals.

[0028] ICD 14 includes a housing 15 that forms a hermetic seal that protects internal components of ICD 14. The housing 15 of ICD 14 may be formed of a conductive material, such as titanium or titanium alloy. The housing 15 may function as an electrode (sometimes referred to as a “can” electrode). Housing 15 may be used as an active can electrode for use in delivering CV/DF shocks or other high voltage pulses delivered using a high voltage therapy circuit. In other examples, housing 15 may be available for use in delivering unipolar, relatively lower voltage cardiac pacing pulses and/or for sensing cardiac electrical signals in combination with electrodes carried by lead 16. In other instances, the housing 15 of ICD 14 may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing 15 functioning as an electrode(s) may be coated with a material, such as titanium nitride, e.g., for reducing post- stimulation polarization artifact.

[0029] ICD 14 includes a connector assembly 17 (also referred to as a connector block or header) that includes one or more electrical feedthroughs crossing housing 15 to provide electrical connections between conductors extending within the lead body 18 of lead 16 and electronic components included within the housing 15 of ICD 14. The electrical feedthroughs (not shown in FIG. 1A) may include feedthrough components such as insulative materials, electrical conductors, ferrules, capacitors, sealing members, etc. As will be described in further detail herein, housing 15 may house one or more processing circuits, memories, transceivers or other communication circuits, cardiac electrical signal sensing circuitry, therapy delivery circuitry, impedance measurement circuitry, power sources and other components for sensing cardiac electrical signals, detecting a heart rhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm.

[0030] Elongated lead body 18 has a proximal end 27 that includes a lead connector (not shown) configured to be connected to ICD connector assembly 17 and a distal portion 25 that includes one or more electrodes. In the example illustrated in FIGs. 1A and IB, the distal portion 25 of lead body 18 includes high surface area, low impedance electrodes 24 and 26 and relatively low surface area, higher impedance electrodes 28 and 30. Electrodes 24 and 26 are elongated electrodes that may extend along a portion of the length of lead body 18 to form relatively high surface area, low impedance electrodes that can be used for delivering high voltage CV/DF pulses. A CV shock pulse may be synchronized to an intrinsic R-wave sensed by ICD 14 for terminating non-sinus, tachycardia. A DF shock pulse may be delivered without synchronization to a sensed R-wave for terminating fibrillation. In either case, the high voltage, high energy CV/DF shock pulse can be delivered to the heart using high surface area electrodes, e.g., elongated coil electrodes, to cause depolarization of a large mass of the myocardial tissue simultaneously. The simultaneous depolarization of the large mass of myocardial tissue is followed by repolarization and an associated state of physiological refractoriness of the large mass, which disrupts the conduction of aberrant depolarizations through the heart that are causing the tachyarrhythmia. In this way, the tachyarrhythmia may be successfully terminated because the heart’s normal, intrinsic electrical conduction system (or a cardiac pacing pulse) may initiate the next heartbeat to restore a more normal, organized propagation and conduction of the myocardial depolarizations through the heart.

[0031] High surface area electrodes, such as electrodes 24 and 26 and/or housing 15, are used to deliver CV/DF shocks in order to encompass a large mass of the heart within the electrical field between the electrodes selected in the CV/DF electrode vector and to avoid tissue injury at the electrode sites that could occur when delivering high voltage shocks via a lower electrode surface area, resulting in a high current density at a more localized tissue site. Electrodes 24 and 26 may be configured to be activated concurrently to form one, large surface area, low impedance anode or cathode. Alternatively, electrodes 24 and 26 may form separate high surface area, low impedance electrodes in which case each of the electrodes 24 and 26 may be activated independently, e.g., as an anode or cathode, for delivering CV/DF shock pulses.

[0032] In some examples, electrodes 24 and 26 may be selected in a low impedance pacing electrode vector for delivering cardiac pacing pulses, having a much lower voltage amplitude than a CV/DF shock but a voltage that is typically higher than the voltage amplitude required of cardiac pacing pulses delivered using endocardial or epicardial pacing electrodes that are in intimate contact with the heart 8. One electrode 24 or 26 may serve as a pacing cathode with the other electrode 26 or 24 serving as the return anode. In other examples, one electrode 24 or 26, or concurrently selected electrodes 24 and 26, may serve as the pacing cathode with the housing 15 or another available electrode serving as the return anode electrode.

[0033] For the sake of convenience, electrodes 24 and 26 are referred to herein as “coil electrodes” because they may take the form of an elongated, coiled electrode (which may include a single wire or filar or multiple wires or filars, e.g., a braided multi-filar wire, a stranded multi-filar wire, etc.) winding around a longitudinal portion of lead body 18 to provide a relatively high surface area for delivering high voltage CV/DF shocks. However, it is to be understood that electrodes 24 and 26 may be configured as other types of high surface area electrodes that can be used for delivering CV/DF shocks, which may include ribbon electrodes, plate electrodes, serpentine electrodes, zig-zagging electrodes, or other types of physical electrode configurations that provide a relatively large surface area and low impedance and do not necessarily include a coiled wire.

[0034] Coil electrodes 24 and 26 (and in some examples housing 15) are sometimes referred to as “defibrillation electrodes” or “CV/DF electrodes” because they are utilized, individually or collectively, for delivering high voltage CV/DF shocks. However, coil electrodes 24 and 26 (and in some examples housing 15) may be utilized in a cardiac pacing electrode vector to provide cardiac pacing pulse delivery in some instances. Furthermore, in some examples, coil electrodes 24 and 26 may be utilized in a sensing electrode vector for providing sensing functionality in addition to being utilized for delivering high voltage CV/DF shocks and/or cardiac pacing pulses. In this sense, the use of the term “defibrillation electrode” or “CV/DF electrode” herein should not be considered as limiting the electrodes 24 and 26 for use in only high voltage CV/DF shock therapy applications. For example, either of coil electrodes 24 and 26 may be used as a sensing electrode in a sensing electrode vector for sensing cardiac electrical signals and determining a need for an electrical stimulation therapy. Furthermore, either or both coil electrodes 24 and 26 may be used in a cardiac pacing electrode vector for delivering cardiac pacing pulses. While two coil electrodes 24 and 26 are shown along lead body 18, in other examples only one coil electrode (which may be used in combination with housing 15 for delivering high voltage pulses) or three or more coil electrodes may be carried by lead body 18. In still other examples, two or more coil electrodes may be carried by two or more different lead bodies extending from ICD 14.

[0035] Electrodes 28 and 30 are relatively smaller surface area electrodes which are available for use in sensing electrode vectors for sensing cardiac electrical signals and may be used for delivering relatively low voltage cardiac pacing pulses in some examples. Electrodes 28 and 30 are sometimes referred to as “pace/sense electrodes” because they are generally configured for use in relatively low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses and/or sensing of cardiac electrical signals, as opposed to delivering high voltage CV/DF shocks. In some instances, electrodes 28 and 30 may provide only pacing functionality, only sensing functionality or both.

[0036] Electrodes 28 and 30 may be ring electrodes extending around the circumference of lead body 18 and having a relatively short longitudinal dimension along the length of lead body 18 compared to coil electrodes 24 and 26. For the sake of convenience, electrodes 28 and 30 are referred to herein as “ring electrodes” or “high impedance electrodes” to distinguish them from the relatively larger surface area, low impedance electrodes 24 and 26, also referred to herein as “coil electrodes.” However, electrodes 28 and 30 may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, button electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, helical electrodes, fishhook electrodes, tip electrodes, or the like (all of which may have a relatively lower surface area and higher impedance than the coil electrodes 24 and 26) and are not limited to being exclusively ring electrodes. [0037] In the example illustrated in FIGs. 1A and IB, ring electrode 28 is located proximal to coil electrode 24, and ring electrode 30 is located between coil electrodes 24 and 26. Ring electrodes 28 and 30 may be positioned at other locations along lead body 18 and are not limited to the positions shown. One, two or more ring or other low surface area electrodes used for sensing and/or low voltage cardiac pacing pulse delivery may be carried by lead body 18. For instance, a third ring electrode may be located distal to coil electrode 26 in some examples. In other examples, lead 16 may include fewer or more ring electrodes and/or coil electrodes than the example shown here.

[0038] In some cases, post-shock cardiac pacing pulses are needed to prevent asystole following a CV/DF shock until the intrinsic conduction system initiates an intrinsic heart rhythm. In other cases, cardiac pacing may be needed to treat bradycardia, asystole or deliver anti-tachycardia pacing (ATP), as examples. Cardiac pacing pulses are generally much lower in voltage than CV/DF shock pulses because a much smaller, relatively local volume of cardiac tissue can be captured by a pacing pulse to cause a heartbeat than the relatively large mass of cardiac tissue that is simultaneously depolarized during a CV/DF shock. Cardiac pacing pulses are delivered to cause depolarization of myocardial tissue at one or more local pacing sites. The pacing evoked depolarization of local cardiac cells captured in the vicinity of the current field of the pacing cathode electrode is conducted through the heart via the myocardium in a coordinated manner to cause a paced heartbeat. [0039] As used herein, the term “extra-cardiac” refers to a position outside the heart and may refer to a position outside of the pericardium surrounding the heart of a patient. Extracardiac electrodes, e.g., electrodes 24, 26, 28 and 30, can be carried by a non-transvenous lead, e.g., lead 16, or a transvenous lead. A transvenous extra-cardiac lead may carry implantable electrodes that can be positioned intravenously but outside the heart in an extra-cardiac location, e.g., within the internal thoracic vein, jugular vein, or another vein. As used herein, the term “extra-cardiovascular” refers to a position outside the blood vessels and heart, which may also be outside the pericardium surrounding the heart of a patient. Implantable electrodes carried by non-transvenous, extra-cardiovascular leads, such as lead 16, may be positioned extra-thoracically (outside the ribcage and sternum) or intra-thoracically (beneath the ribcage or sternum) but may not be in intimate contact with myocardial tissue. In general, the techniques disclosed herein for detecting a leakage current pathway may be utilized in conjunction with a medical device system including therapy delivery electrodes that may or may not be in contact with the myocardial tissue of the patient’ s heart.

[0040] Cardiac pacing pulses that are delivered using extra-cardiac electrodes that are not in contact with cardiac tissue generally require higher energy (e.g., higher pulse amplitude and/or pulse width) than cardiac pacing pulses that are delivered using endocardial or epicardial electrodes. However, these cardiac pacing pulses delivered using extra-cardiac electrodes are still much lower in voltage amplitude and overall pulse energy than that required for CV/DF shocks. Relatively higher voltage cardiac pacing pulses are required when pacing using extracardiac electrodes than endocardial or epicardial electrodes in order to deliver enough energy within the pacing pulse width to capture the heart. A limitation of the maximum pacing pulse width may exist due in part to the decay rate of the pacing pulse amplitude delivered by the ICD therapy delivery circuitry. The decay rate can be dependent on the capacitance of a holding capacitor being discharged to deliver the pacing pulse and the impedance of the pacing electrode vector. In order to achieve capture of cardiac tissue within a limited pulse width, e.g., 8 ms or less, 4 ms or less or 2 ms or less, a high pacing voltage amplitude may be required to deliver sufficient pacing pulse energy. Cardiac pacing pulses delivered using extra-cardiac electrodes may be in the range of 8 V to 40 V with a pacing pulse width of 2 ms to 8 ms, as examples. By comparison CV/DF shocks may be greater than 100 V or on the order of several hundred volts.

[0041] The high surface area coil electrodes 24 and 26 may be employed for delivering cardiac pacing pulses. Relatively higher pacing pulse voltage amplitudes may be used with lower current density at the electrode tissue interface of the high surface area coil electrodes 24 and 26 compared to the low surface area electrodes 28 and 30. The surface area of a coil electrode 24 or 26 may be 50 to 100 times larger than the surface area of the ring electrodes 28 and 30. High current density at the ring electrode-tissue interface during relatively high voltage cardiac pacing could cause local tissue injury. The electrical field of current traveling through conductive tissues toward the heart between a cardiac pacing electrode vector that includes at least one or both high surface area coil electrodes 24 and 26 may be more effective in capturing the heart for cardiac pacing than the electrical field between a cardiac pacing electrode vector that includes lower surface area ring electrodes 28 and 30 or one of ring electrodes 28 or 30 and housing 15. A higher voltage cardiac pacing pulse that can be delivered via the coil electrodes 24 and/or 26 and/or housing 15 can have a relatively short pulse width so that the pacing pulse decay rate does not become a limiting factor of pacing pulse energy delivered for capturing the heart.

[0042] Accordingly, ICD 14 may be configured to deliver cardiac pacing pulses using coil electrodes 24 and/or 26, e.g., as a cathode and anode pair. High voltage output circuitry of ICD 14 can be enabled by therapy delivery control circuitry of ICD 14 when a CV/DF shock is needed for delivery via coil electrodes 24 and/or 26. However, when a cardiac pacing pulse is needed, that is a much lower voltage than the CV/DF shock pulse, ICD 14 may be configured to enable the high voltage output circuitry for delivering a cardiac pacing pulse using a low impedance pacing electrode vector that includes one or both of coil electrodes 24 and 26. In some examples, current required to operate the high voltage output circuitry can be controlled using an internal adjustable load that is electrically connected in parallel with electrode terminals that can be coupled to the external pacing load. The internal adjustable load can be configured to pull a controlled, adjustable current that maintains charge coupled components of the high voltage output circuitry in a conducting state for delivery of cardiac pacing pulses.

[0043] In the example shown in FIGs. 1A and IB, lead 16 extends subcutaneously or submuscularly over the ribcage 32 medially from the connector assembly 27 of ICD 14 toward a center of the torso of patient 12, e.g., toward xiphoid process 20 of patient 12. At a location near xiphoid process 20, lead 16 bends or turns and extends superiorly, subcutaneously or submuscularly, over the ribcage and/or sternum, substantially parallel to sternum 22. Although illustrated in FIG. 1A as being offset laterally from and extending substantially parallel to sternum 22, the distal portion 25 of lead 16 may be implanted at other locations, such as over sternum 22, offset to the right or left of sternum 22, angled laterally from sternum 22 toward the left or the right, or the like. Alternatively, lead 16 may be placed along other subcutaneous or submuscular paths. The path of extra- cardiovascular lead 16 may depend on the location of ICD 14, the arrangement and position of electrodes carried by the lead body 18, and/or other factors. The techniques disclosed herein are not limited to a particular path of lead 16 or final locations of electrodes 24, 26, 28 and 30.

[0044] Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body 18 of lead 16 from the lead connector at the proximal lead end 27 to electrodes 24, 26, 28, and 30 located along the distal portion 25 of the lead body 18. The elongated electrical conductors contained within the lead body 18, which may be separate respective insulated conductors within the lead body 18, are each electrically coupled with respective coil electrodes 24 and 26 and ring electrodes 28 and 30. The respective conductors electrically couple the electrodes 24, 26, 28, and 30 to circuitry, such as a therapy delivery circuit and/or a sensing circuit, of ICD 14 via connections in the connector assembly 17, including associated electrical feedthroughs (e.g., insulated feedthrough conductors) crossing housing 15. The electrical conductors transmit electrical stimulation pulses from therapy delivery circuitry within ICD 14 to one or more of coil electrodes 24 and 26 and/or ring electrodes 28 and 30 and transmit electrical signals produced by the patient’s heart 8 from one or more of coil electrodes 24 and 26 and/or ring electrodes 28 and 30 to the sensing circuitry within ICD 14.

[0045] The lead body 18 of lead 16 may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and/or other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. Lead body 18 may be tubular or cylindrical in shape. In other examples, the distal portion 25 (or all of) the elongated lead body 18 may have a flat, ribbon or paddle shape. Lead body 18 may be formed having a preformed distal portion 25 that is generally straight, curving, bending, serpentine, undulating or zig-zagging.

[0046] In the example shown, lead body 18 includes a curving distal portion 25 having two “C” shaped curves, which together may resemble the Greek letter epsilon, “e.” Defibrillation electrodes 24 and 26 are each carried by one of the two respective C-shaped portions of the lead body distal portion 25. The two C-shaped curves are seen to extend or curve in the same direction away from a central axis of lead body 18, along which ring electrodes 28 and 30 are positioned. Ring electrodes 28 and 30 may, in some instances, be approximately aligned with the central axis of the straight, proximal portion of lead body 18 such that mid-points of coil electrodes 24 and 26 are laterally offset from ring electrodes 28 and 30.

[0047] Other extra-cardiovascular leads including one or more coil or other high surface area electrodes and optionally one or more ring or other relatively low surface area may be implemented with the techniques described herein. The techniques disclosed herein are not limited to any particular lead body design. In other examples, lead body 18 can be a flexible elongated lead body without any pre-formed shape, bends or curves.

[0048] ICD 14 may obtain cardiac electrical signals corresponding to electrical activity of heart 8 via a combination of sensing electrode vectors that include combinations of electrodes 24, 26, 28 and/or 30. In some examples, housing 15 of ICD 14 is used in combination with one or more of electrodes 24, 26, 28 and/or 30 in at least one sensing electrode vector. Each cardiac electrical signal received via a selected sensing electrode vector may be used by ICD 14 for sensing cardiac event signals attendant to intrinsic depolarizations of the myocardium, e.g., R-waves attendant to ventricular depolarizations and in some cases P-waves attendant to atrial depolarizations. Sensed cardiac event signals may be used for determining the heart rate and determining a need for cardiac pacing, e.g., for treating bradycardia or asystole for preventing a long ventricular pause, or for determining a need for tachyarrhythmia therapy, e.g., ATP and/or CV/DF shocks.

[0049] ICD 14 analyzes the cardiac electrical signal(s) received from one or more sensing electrode vectors to monitor for abnormal rhythms, such as asystole, bradycardia, ventricular tachycardia (VT) and/or ventricular fibrillation (VF). ICD 14 may analyze the heart rate and/or morphology of the cardiac electrical signals to monitor for tachyarrhythmia in accordance with any tachyarrhythmia detection techniques. ICD 14 generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia, e.g., VT or VF (VT/VF) using a therapy delivery electrode vector which may be selected from any of the available electrodes 24, 26, 28 30 and/or housing 15. ICD 14 may deliver ATP in response to VT detection and in some cases may deliver ATP prior to a CV/DF shock or during high voltage holding capacitor charging in an attempt to avert the need for delivering a CV/DF shock. If ATP does not successfully terminate VT or when VF is detected, ICD 14 may deliver one or more CV/DF shocks via one or both of coil electrodes 24 and 26 and/or housing 15.

[0050] In the absence of a ventricular event signal, e.g., a sensed R-wave, ICD 14 may generate and deliver a cardiac pacing pulse, such as a post-shock pacing pulse or bradycardia pacing pulse. When asystole is detected or when a pacing escape interval expires prior to sensing a ventricular event signal (e.g., and R-wave), one or more cardiac pacing pulses may be delivered by ICD 14. The cardiac pacing pulses may be delivered using a low impedance pacing electrode vector that includes at least one or both coil electrodes 24 and 26 according to the techniques disclosed herein. In some examples, housing 15 of ICD 14 is used in combination with one or both coil electrodes 24 and 26 to deliver cardiac pacing pulses.

[0051] ICD 14 is shown implanted subcutaneously on the left side of patient 12 along the ribcage 32. ICD 14 may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient 12. ICD 14 may, however, be implanted at other subcutaneous or submuscular locations in patient 12. For example, ICD 14 may be implanted in a subcutaneous pocket in the pectoral region. In this case, lead 16 may extend subcutaneously or submuscularly from ICD 14 toward the manubrium of sternum 22 and bend or turn and extend inferiorly from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD 14 may be placed abdominally. Lead 16 may be implanted in other extra-cardiovascular locations as well. For instance, as described with respect to FIGs. 2A-2C, the distal portion 25 of lead 16 may be implanted underneath the sternum/ribcage in the substernal space. FIGs. 1A and IB are illustrative in nature and should not be considered limiting in the practice of the techniques disclosed herein.

[0052] A medical device operating according to techniques disclosed herein may be coupled to one or more transvenous or non-transvenous leads in various examples for carrying electrodes for sensing cardiac electrical signals and delivering electrical stimulation therapy. For example, the medical device, such as ICD 14, may be coupled to an extra-cardiovascular lead as illustrated in the accompanying drawings, referring to a lead that positions electrodes outside the blood vessels, heart, and pericardium surrounding the heart of a patient. Implantable electrodes carried by extra-cardiovascular leads may be positioned extra- thoracic ally (outside the ribcage and sternum), subcutaneously or submuscularly, or intra-thoracically (beneath the ribcage or sternum, sometimes referred to as a sub-sternal position) and may not necessarily be in intimate contact with myocardial tissue. An extra-cardiovascular lead may also be referred to as a “non-transvenous” lead. [0053] In other examples, the medical device may be coupled to one or more transvenous leads that position electrodes within a blood vessel, which may remain outside the heart in an extra-cardiac location. For instance, a transvenous medical lead may be advanced along a venous pathway to position electrodes in an extra-cardiac location within the internal thoracic vein (ITV), an intercostal vein, the superior epigastric vein, or the azygos, hemiazygos, or accessory hemiazygos veins, as examples. In still other examples, an ICD configured to operate according to techniques disclosed herein may be coupled to one or more transvenous leads that can be advanced to position electrodes within the heart, e.g., within an atrial and/or ventricular heart chamber. An example of a transvenous ICD system is described below in conjunction with FIG. 3.

[0054] An external device 50 is shown in telemetric communication with ICD 14 by a wireless communication link 51 in FIG. 1A. External device 50 may be embodied as a programmer used in a hospital, clinic or physician’s office to retrieve data from ICD 14 and to program operating parameters and algorithms in ICD 14 for controlling ICD functions. External device 50 may alternatively be embodied as a home monitor or handheld device for retrieving data from ICD 14. External device 50 may be used to program cardiac signal sensing parameters, cardiac rhythm detection parameters, pacing and CV/DF therapy control parameters and other operating and control parameters used by ICD 14.

[0055] External device 50 may include a processor 52, memory 53, display unit 54, user interface 56 and telemetry unit 58. Processor 52 executes instructions stored in memory 53. Processor 52 may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field- programmable gate array (FPGA), or equivalent discrete or analog logic circuitry. In some examples, processor 52 may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor 52 herein may be embodied as software, firmware, hardware or any combination thereof. [0056] Memory 53 may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random-access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital or analog media. Memory 53 may be configured to store instructions executed by processor 52 for obtaining data received from ICD 14 and for generating a GUI on display unit 54 according to the techniques disclosed herein. Memory 53 may store various operating parameter settings of ICD 14 that may be used in generating various GUI windows, menus, reports, etc. by processor 52.

[0057] Display unit 54 may generate a display of cardiac electrical signals, lead impedance measurements, leakage current pathway test results, programmed operating settings of ICD 14 and other device and patient related data received from processor 52. Display unit 54 may be configured to generate a GUI including various windows, icons, user selectable menus, etc. to facilitate interaction by a user with the external device 50. Display unit 54 may function as an input and/or output device using technologies including liquid crystal displays (LCD), quantum dot display, dot matrix displays, light emitting diode (LED) displays, organic light-emitting diode (OLED) displays, cathode ray tube displays, e-ink, or monochrome, color, or any other type of display capable of generating tactile, audio, and/or visual output. In some examples, display unit 54 is a presence- sensitive display that may serve as a user interface device that operates both as one or more input devices and one or more output devices.

[0058] User interface unit 56 may include a mouse, touch screen, keypad or the like to enable a user to interact with external device 50, e.g., to initiate and terminate an interrogation session for retrieving data from ICD 14, adjust settings of display unit 54, enter programming commands or selections or make other user requests. Telemetry unit 58 includes a transceiver and antenna configured for bidirectional communication with a telemetry circuit included in an ICD 14, e.g., in response to user requests.

[0059] Telemetry unit 58 is configured to operate in conjunction with processor 52 for sending and receiving data relating to ICD functions via a wireless communication link 51 with ICD 14. Communication link 51 may be established using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, Medical Implant Communication Service (MICS) or other communication bandwidth. In some examples, external device 50 may include a programming head that is placed proximate ICD 14 to establish and maintain a communication link 51, and in other examples external device 50 and ICD 14 may be configured to communicate using a distance telemetry algorithm and circuitry that does not require the use of a programming head and does not require user intervention to maintain a communication link.

[0060] It is contemplated that external device 50 may be in wired or wireless connection to a communications network via telemetry circuit 58 that includes a transceiver and antenna or via a hardwired communication line for transferring data to a centralized database or computer to allow remote management of the patient. One example of a remote patient management system is the CARELINK® Network (Medtronic, Inc. Minneapolis, MN). Review of operating parameter settings and other data collected from ICD 14 may be performed remotely by a clinician who may authorize programming of operating parameters in ICD 14, e.g., after viewing reports and cardiac electrical signals and other device related data, such as marker channel data and therapy delivery history. [0061] External device 50 may be configured to generate a GUI on display unit 54, which may include an alert received from ICD 14 indicating that a leakage current pathway has been detected and corrective action may be needed, such as ICD reprogramming or replacement. External device 50 may transmit the alert and associated data and information received from ICD 14 to a centralized database or computer or other device in a clinic or hospital to notify a clinician.

[0062] FIGs. 2A-2C are conceptual diagrams of patient 12 implanted with extra- cardiovascular ICD system 10 in a different implant configuration than the arrangement shown in FIGs. 1A-1B. FIG. 2A is a front view of patient 12 implanted with ICD system 10. FIG. 2B is a side view of patient 12 implanted with ICD system 10. FIG. 2C is a transverse view of patient 12 implanted with ICD system 10. In this arrangement, lead 16 of system 10 is implanted at least partially underneath sternum 22 of patient 12. Lead 16 extends subcutaneously or submuscularly from ICD 14 toward xiphoid process 20 and at a location near xiphoid process 20 bends or turns and extends superiorly within anterior mediastinum 36 (see FIG. 2C) in a substemal position.

[0063] Anterior mediastinum 36 may be viewed as being bounded laterally by pleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22 (see FIG. 2C). The distal portion 25 of lead 16 may extend along the posterior side of sternum 22 substantially within the loose connective tissue and/or substernal musculature of anterior mediastinum 36. A lead implanted such that the distal portion 25 is substantially within anterior mediastinum 36, may be referred to as a “substemal lead.”

[0064] In the example illustrated in FIGS. 2A-2C, lead 16 is located substantially centered under sternum 22. In other instances, however, lead 16 may be implanted such that it is offset laterally from the center of sternum 22. In some instances, lead 16 may extend laterally such that distal portion 25 of lead 16 is undemeath/below the ribcage 32 in addition to or instead of sternum 22. In other examples, the distal portion 25 of lead 16 may be implanted in other extra-cardiac, intra-thoracic locations, including in the pleural cavity or around the perimeter of and adjacent to the pericardium 38 of heart 8.

[0065] FIG. 3 is a conceptual diagram of a medical device system 100 including an ICD

114 coupled to transvenous electrical leads 116, 118, and 121. ICD 114 is shown as a multi-chamber device capable of delivering electrical stimulation pulses and sensing cardiac electrical signals in the right atrium (RA), the right ventricle (RV) and the left ventricle (LV). ICD housing 115 encloses internal circuitry corresponding to the various circuits and components described in conjunction with FIG. 4 below, for performing the functionality of ICD 114 as disclosed herein, including performing leakage current tests. [0066] ICD housing 115 forms a hermetic seal that protects internal components of ICD 114. As described above, housing 115 may be formed of a conductive material, such as titanium or titanium alloy. Housing 115 may function as an electrode (sometimes referred to as a “can” electrode). Housing 115 may be used as an active can electrode for use in delivering high voltage CV/DF shock pulses to heart 8 for terminating a tachyarrhythmia, e.g., ventricular tachycardia or fibrillation. In other examples, housing 115 may be available for use in delivering unipolar, relatively lower voltage cardiac pacing pulses and/or for sensing cardiac electrical signals in combination with electrodes carried by a lead coupled to ICD 114. In other instances, the housing 115 of ICD 114 may include multiple electrodes on an outer portion of the housing. The outer portion(s) of the housing

115 functioning as an electrode(s) may be coated with a material, such as titanium nitride, e.g., for reducing post-stimulation polarization artifact.

[0067] ICD 114 includes a connector assembly (or “connector block”) 117 that includes insulated electrical feedthroughs crossing housing 115 to provide electrical connections between conductors extending within the leads 116, 118 and 121 coupled to ICD 114 and the electronic components enclosed by housing 115. An antenna (not shown in FIG. 3) may be carried in connector assembly 117 for coupling RF signals transmitted to/from external device 50 to a telemetry circuit enclosed by housing 115. As described below housing 115 may enclose one or more processing circuits, memories, transceivers, cardiac electrical signal sensing circuitry, therapy delivery circuitry, power sources and other components for sensing cardiac electrical signals, processing and analyzing sensed cardiac electrical signals, and delivering electrical stimulation pulses as needed.

[0068] In the example shown, connector assembly 117 is configured to receive a proximal lead connector 140, 142 and 144 of each of RA lead 116, RV lead 118 and LV lead 121, respectively. Each lead 116, 118, and 121 can be advanced transvenously for positioning electrodes for sensing and stimulation in the atria or ventricles of heart 8. The proximal portion of each of lead 116, 118, and 121 may be configured as an industry standard or custom lead connector 140, 142 and 144, respectively. Connector assembly 117 includes connector bores that are appropriately sized for receiving the proximal portion of each lead 116, 118 and 121, e.g., lead connector assemblies 140, 142 or 144. Each connector bore includes electrical contacts that become aligned with and physically mate with a corresponding electrical contact of the respective lead connector 140, 142 or 144 providing physical and electrical connection of each lead 116, 118 and 121 to ICD 114. [0069] RA lead 116 includes an elongated lead body 141, a proximal lead connector 140 and distal electrodes 120 and 122 in the example shown. RA lead 116 may be advanced transvenously for positioning its distal end, carrying electrodes 120 and 122, in the vicinity of the RA. RA lead 116 is equipped with pacing and sensing electrodes 120 and 122, shown as a tip electrode 120 and a ring electrode 122 spaced proximally from tip electrode 120 along RA lead body 141. Tip electrode 120 may be used as a cathode electrode with ring electrode 122 serving as an anode electrode for bipolar pacing and bipolar sensing in the RA. The electrodes 120 and 122 are each connected to a respective insulated conductor extending within the elongated body 141. Each insulated conductor is coupled at its proximal end to an electrical connector (not shown in FIG. 3), e.g., a pin or ring connector, of the proximal lead connector 140, which becomes electrically connected to internal ICD circuitry via various electrical feedthroughs in ICD connector assembly 117.

[0070] RV lead 118 includes an elongated lead body 143 having a proximal connector 142 at its proximal end for coupling lead 118 to ICD connector assembly 117 and electrodes 124, 126, 128 and 130 carried along a distal portion of lead body 143. RV lead 118 may be advanced transvenously through the RA and into the RV to position electrodes 124, 128 and 130 in the RV. RV lead 118 is shown carrying a distal tip electrode 128 and ring electrode 130 spaced proximally from tip electrode 128 for bipolar sensing of cardiac electrical signals in the RV and delivering bipolar cardiac pacing pulses. Tip electrode 128 may be used as a cathode electrode for pacing and sensing with ring electrode 130 serving as an anode electrode.

[0071] RV lead 118 is further shown to be carrying an RV coil electrode 124 spaced proximally from ring electrode 130 and a superior vena cava (SVC) coil electrode 126 spaced proximally from RV coil electrode 124. SVC coil electrode 126 may be carried along RV lead body 143 such that it is positioned at least partially within the RA and/or SVC when the distal end of RV lead 118 is advanced within the right ventricle. Coil electrodes 124 and 126 are elongated electrodes having a relatively high surface area compared to electrodes 120, 122, 128 and 130. Coil electrodes 124 and 126 may have a surface area ranging from 50 to 100 times greater than the surface area of electrodes 120, 122, 128 and 130, for example. For the sake of convenience, electrodes 124 and 126 are referred to herein as “coil electrodes” because they may take the form of a coiled electrode, which may include a single wire or filar or multiple wires or filars (e.g., a braided multi-filar wire, a stranded multi-filar wire, etc.) that winds helically around a longitudinal portion of lead body 143 to provide a relatively high surface area electrode for delivering high voltage CV/DF shocks. However, it is to be understood that electrodes 124 and 126 may be configured as other types of high surface area electrodes that can be used for delivering CV/DF shocks, which may include ribbon electrodes, plate electrodes, serpentine electrodes, zig-zagging electrodes, segmented electrodes or other types of physical electrode configurations that provide a relatively large surface area and low impedance that do not necessarily include a coiled wire.

[0072] As described above, coil electrodes 124 and 126 (and in some examples housing 115) are sometimes referred to as “defibrillation electrodes” or “CV/DF electrodes” because they can be utilized, individually or collectively, for delivering high voltage CV/DF shocks. However, in some examples, a coil electrode available for delivering CV/DF shocks may be utilized in a cardiac sensing electrode vector to sense cardiac electrical signals. In this sense, the use of the term “defibrillation electrode” or “CV/DF electrode” herein should not be considered as limiting the coil electrodes 124 and 126 for use in only high voltage CV/DF shock therapy applications. For example, either of coil electrodes 124 and 126 may be used as a sensing electrode in a sensing electrode vector for sensing cardiac electrical signals and determining a need for an electrical stimulation therapy. While two coil electrodes 124 and 126 are shown along lead body 143 of RV lead 118, in other examples only one coil electrode, e.g., RV coil electrode 124 (which may be used in combination with housing 115 for delivering high voltage shock pulses), or more than two coil electrodes may be carried by lead body 143. In still other examples, two or more coil electrodes may be carried by two or more different lead bodies extending from ICD 114. For example, in some lead and electrode configurations, SVC coil electrode 126 may be carried by lead body 141 of RA lead 116. Housing 115 may function as an active electrode during CV/DF shock delivery in conjunction with RV coil electrode 124 and/or SVC coil electrode 126 in some examples.

[0073] Each of electrodes 124, 126, 128 and 130 carried by RV lead body 143 are connected to a respective insulated conductor extending within lead body 143 of RV lead 118. Lead body 143 may be a multi-lumen lead body in some examples to accommodate multiple, insulated conductors. The proximal ends of the insulated conductors are coupled to corresponding electrical connectors (not illustrated in FIG. 1) of proximal lead connector 142 for providing electrical connection to ICD 114 via electrical feedthroughs in connector assembly 117.

[0074] The RV lead tip electrode 128 and the RA lead tip electrode 120 can be active fixation electrodes providing fixation of the distal ends of leads 118 and 116, respectively, at an implant site in addition to providing cardiac electrical signal sensing and cardiac pacing functionality. In FIG. 3, RA tip electrode 120 and RV tip electrode 128 are each shown as a helical, screw-in electrode that can be rotatably advanced into cardiac tissue to provide lead fixation. In other examples, tip electrodes 120 and 128 may be configured as fishhook electrodes, hemispherical electrodes, button electrodes or other types of electrodes. When the tip electrode of the medical lead does not provide fixation of the distal end of the elongated lead body, the RA lead 116 or RV lead 118 may be equipped with other fixation mechanisms, such as tines or hooks, that may engage with cardiac tissue at an implant site.

[0075] The proximal ring electrode 122 of RA lead 116 and the proximal ring electrode 130 of RV lead 118 may each be ring electrodes that fully or partially circumscribe the respective lead body 141 or 143. In various examples, the relatively low surface area pace/sense electrodes 120, 122, 128 and 130 may be implemented as ring electrodes, short coil electrodes, button electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, helical electrodes, fishhook electrodes, or other shaped electrode and are not limited to being exclusively ring electrodes and helical screw-in electrodes as shown here.

[0076] RA lead electrodes 120 and 122 and RV lead electrodes 128 and 130 are relatively small surface area electrodes which are available for use in sensing cardiac electrical signals and may be used in for delivering relatively low voltage cardiac pacing pulses, e.g., for delivering bradycardia pacing, post-shock pacing, cardiac resynchronization therapy (CRT), ATP therapy or other therapeutic cardiac pacing pulses. In some cases, RV lead electrodes 128 and 130 may be used to deliver high frequency induction pulses delivered to induce a tachyarrhythmia, e.g., during CV/DF threshold testing. Electrodes 120, 122, 128 and 130 are sometimes referred to as “pace/sense electrodes” because they are generally configured for use in low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses and/or sensing of cardiac electrical signals, as opposed to delivering high voltage CV/DF shocks. In some instances, electrodes 120, 122, 128 and 130 may provide only pacing functionality, only sensing functionality or both. [0077] LV lead 121 includes elongated lead body 145 having a proximal connector 144 and distal electrodes 134 and 136, which may be provided as any of the example pacing and sensing electrodes listed above. LV lead 121 may be advanced transvenously into the RA, and further into a cardiac vein 132 via the ostium of the coronary sinus 9 to position electrodes 134 and 136 along the lateral free wall of the left ventricle. Electrodes 134 and 136 may be used as a bipolar sensing and pacing electrode pair for sensing cardiac electrical signals from the LV and for delivering LV pacing pulses. LV lead 121 may be coupled to ICD 114 for providing multi-chamber sensing and therapy delivery to heart 8, such as CRT.

[0078] CRT may be delivered when poor ventricular synchrony between the RV and the LV exists, e.g., due to conduction disease, heart failure or other cardiac conditions. LV lead 121 may be optional in some examples and may not be included in some medical device systems that employ the techniques disclosed herein. While LV lead 121 is shown as a bipolar lead having two electrodes 134 and 136 for the sake of convenience, LV lead 121 may be a unipolar lead having one electrode or a multi-polar lead, e.g., having three or four electrodes. In some examples, LV lead 121 is a quadripolar lead having four electrodes, e.g., four ring electrodes or one tip electrode and three ring electrodes, with proximal connector 144 configured as an industry standard IS -4 connector.

[0079] Electrodes 120, 122, 124, 126, 128, 130, 134 and 136 may be formed from titanium, platinum, iridium or alloys thereof, as examples with no limitation intended, and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black, among others. Lead bodies 141, 143 and 145 may each be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and/or other appropriate materials. Each lead body may be shaped to form one or more lumens within which one or more insulated electrical conductors extend between the electrical connectors of the proximal lead connectors 140, 142 and 144 and the respective electrodes carried by the lead body. The lead bodies 141, 143, and 145 may be generally tubular or cylindrical in shape but may have a flattened or ribbon shape in some examples. Any of the lead bodies 141, 143 and 145 may have a pre-formed shape such as a curve or bend, which may be along a distal portion of the lead body, to facilitate guidance and implantation of the lead body distal end at a targeted implant site. In other examples, the lead bodies 141, 143 and 145 may be elongated flexible bodies without any preformed shapes or curves.

[0080] In the example shown in FIG. 3, RA lead 116 and RV lead 118 are configured as “true bipolar” leads in that a cardiac electrical signal can be sensed between tip electrode 120 and ring electrode 122 in the RA, and a cardiac electrical signal can be sensed between tip electrode 128 and ring electrode 130 in the RV. In other examples, a lead coupled to ICD 114 may be an “integrated bipolar” lead, referring to a lead that is configured to sense cardiac electrical signals using a tip electrode and a coil electrode, e.g., RV tip electrode 128 and RV coil electrode 124, omitting the need for a ring electrode 130. In an “integrated bipolar” lead, for example, the RV coil electrode 124 may serve the dual purposes of bipolar sensing of cardiac electrical signals when paired with tip electrode 128 and delivering high voltage CV/DF shocks in combination with SVC coil electrode 126 and/or housing 115.

[0081] It is to be understood that although ICD 114 is described as a multi-chamber device capable of sensing and pacing in the RA, RV and LV, in other examples, ICD 114 may be a dual chamber device, e.g., coupled to RA lead 116 and RV lead 118. In still other examples, ICD 114 may be a single chamber device, e.g., coupled only to RA lead 116 or only to RV lead 118. While ICD 14 described above and ICD 114 are described as being capable of delivering both low voltage cardiac pacing therapies and high voltage CV/DF shocks, an ICD operating according to techniques disclosed herein may be configured as a single, dual or multi-chamber device capable of delivering cardiac pacing or CV/DF shocks but not necessarily both. External device 50 is shown in telemetric communication with ICD 114 by a communication link 151, as generally described above in conjunction with FIG. 1A.

[0082] FIG. 4 is a conceptual diagram of an ICD according to one example. For the sake of convenience, the diagram in FIG. 4 depicts ICD 14 coupled to electrodes 24, 26, 28 and 30 as shown in FIG. 1A. However, it is to be understood that the circuitry, components and functionality described in conjunction with FIG. 4 may generally correspond to circuitry, components and functionality of ICD 114, adapted to receive one or more medical electrical leads as needed and provide single, dual or multi-chamber sensing and electrical stimulation therapy delivery. As such the electrodes 120, 122, 124, 126, 128, 130, 132, 134, 136 and/or housing 115 shown in the system of FIG. 3 may be connected to therapy delivery circuit 84 and/or sensing circuit 86 of FIG. 4 in other examples.

[0083] The electronic circuitry enclosed within housing 15 (shown schematically in FIG. 4 as an electrode, sometimes referred to as a “can electrode”) includes software, firmware and hardware that cooperatively monitor cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver therapy as needed according to programmed therapy delivery algorithms and control parameters. ICD 14 may include a control circuit 80, memory 82, therapy delivery circuit 84, cardiac electrical signal sensing circuit 86, and telemetry circuit 88. A power source 98 provides power to the circuitry of ICD 14, including each of the components 80, 82, 84, 86, and 88 as needed. Power source 98 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source 98 and each of the other components 80, 82, 84, 86 and 88 are to be understood from the general block diagram of FIG. 4 but are not shown for the sake of clarity. For example, power source 98 may be coupled to one or more charging circuits included in therapy delivery circuit 84 for charging holding capacitors included in therapy delivery circuit 84 and operating output circuitry for discharging the holding capacitor(s) at appropriate times under the control of control circuit 80 for producing electrical pulses according to a therapy protocol. Power source 98 is also coupled to components of cardiac electrical signal sensing circuit 86 (such as sense amplifiers, analog-to-digital converters, switching circuitry, etc.), memory 82, and telemetry circuit 88 as needed.

[0084] The various operating circuits shown in FIG. 4 represent functionality included in ICD 14 (or ICD 114) and may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to the ICD herein. Functionality associated with one or more circuits may be performed by separate hardware, firmware and/or software components, or integrated within common hardware, firmware and/or software components. For example, cardiac electrical signal sensing and analysis for detecting arrhythmia may be performed cooperatively by sensing circuit 86 and control circuit 80 and may include operations implemented in a processor or other signal processing circuitry included in control circuit 80 executing instructions stored in memory 82 and control signals such as blanking and timing intervals and sensing threshold amplitude signals sent from control circuit 80 to sensing circuit 86. Therapy delivery may be performed cooperatively by therapy delivery circuit 84 under the control of signals received from control circuit 80 for controlling the timing, pulse amplitude, pulse width, polarity, rate, electrode vector and other therapy delivery parameters used by therapy delivery circuit 84 to generate and deliver electrical stimulation pulses, which may include CV/DF shock pulses, cardiac pacing pulses, tachyarrhythmia induction pulses, leakage current test pulses, impedance measurement pulses or any other electrical pulses delivered via electrodes 24, 26, 28, 30 and/or housing 15 shown in the system of FIG. 1 A (or electrodes 120, 122, 124, 126, 128, 130 132, 134, 136 and/or housing 115 shown in the system of FIG. 3).

[0085] The various circuits of ICD 14/114 may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine, hardware subroutine, or other suitable components or combinations of components that provide the described functionality. The particular form of software, hardware and/or firmware employed to implement the functionality disclosed herein will be determined primarily by the particular system architecture employed in the ICD and by the particular sensing, detection and therapy delivery methodologies employed by the ICD. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modem medical device system, given the disclosure herein, is within the abilities of one of skill in the art.

[0086] Memory 82 may include any volatile, non-volatile, magnetic, or electrical non- transitory computer readable storage media, such as random access memory (RAM), readonly memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other memory device. Furthermore, memory 82 may include non-transitory computer readable media storing instructions that, when executed by one or more processing circuits, cause control circuit 80 and/or other ICD components to perform various functions attributed to ICD 14/114 or those ICD components. The non-transitory computer-readable media storing the instructions may include any of the media listed above.

[0087] Therapy delivery circuit 84 and sensing circuit 86 are electrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 and the housing 15, which may function as a common or ground electrode for sensing or cardiac pacing pulses or as an active can electrode for delivering CV/DF shock pulses or cardiac pacing pulses. As indicated above, in the example of the transvenous system 100 of FIG. 3, therapy delivery circuit 84 and sensing circuit 86 may be electrically coupled to electrodes 120, 122, 124, 126, 128, 130 132, 134, 136 and/or housing 115. Control circuit 80 communicates, e.g., via a data bus, with therapy delivery circuit 84 and sensing circuit 86 for sensing cardiac electrical signals, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals (or the absence thereof).

Control circuit 80 may include an arrhythmia detection circuit 92, timing circuit 90, and therapy control circuit 94. Arrhythmia detection circuit 92 may be configured to process and analyze signals received from sensing circuit 86, which may be in conjunction with time intervals and/or timing related signals received from timing circuit 90. Timing circuit 90 may generate clock signals and include various timers and/or counters for use in determining time intervals between cardiac events, sensed and/or paced, and control the timing of delivered pacing pulses and/or CV shocks. Control circuit 80 may further include a therapy control circuit 94 configured to pass signals to and receive signals from therapy delivery circuit 84 for controlling and monitoring electrical stimulation therapies delivered by therapy delivery circuit 84.

[0088] Cardiac electrical signal sensing circuit 86 (also referred to herein as “sensing circuit” 86) may be selectively coupled to electrodes 28, 30 and/or housing 15 in order to monitor electrical activity of the patient’s heart. Sensing circuit 86 may additionally be selectively coupled to coil electrodes 24 and/or 26 for use in a sensing electrode vector together or in combination with one or more of electrodes 28, 30 and/or housing 15. Sensing circuit 86 may be enabled to receive cardiac electrical signals from at least one sensing electrode vector selected from the available electrodes 24, 26, 28, 30, and housing 15 in some examples. In some examples, two, three or more cardiac electrical signals from two, three or more different sensing electrode vectors may be received simultaneously by sensing circuit 86. Sensing circuit 86 may monitor one or more cardiac electrical signals for sensing cardiac event signals, e.g., R-waves attendant to intrinsic ventricular myocardial depolarizations. In some examples, sensing circuit 86 may be configured to monitor two cardiac electrical signals simultaneously for sensing cardiac event signals. At least one cardiac electrical signal may be received by sensing circuit 86 and passed to control circuit 80 for processing and analysis, e.g., by arrhythmia detection circuit 92, for determining when morphology-based criteria for detecting arrhythmia are met in some examples. Sensing circuit 86 may include switching circuitry for selecting which of electrodes 24, 26, 28, 30, and housing 15 are coupled to one or more sensing channels included in sensing circuit 86. For example, ICD 14 may include multiple sensing channels for sensing R-waves and analyzing sensed cardiac electrical signals for detecting arrhythmias. ICD 114 may include an atrial sensing channel for receiving signals from electrodes carried by RA lead 116, an RV sensing channel for receiving signals from electrodes carried by RV lead 118, and an LV sensing channel for receiving signals from electrodes carried by LV lead 121.

[0089] Each sensing channel may be configured to amplify, filter and digitize the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for sensing cardiac event signals, such as R-waves or P-waves. The cardiac event sensing circuitry within sensing circuit 86 may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), timers or other analog and/or digital components. A cardiac event sensing threshold may be automatically adjusted by sensing circuit 86 under the control of control circuit 80, based on sensing threshold control parameters, such as various timing intervals and sensing threshold amplitude values that may be determined by control circuit 80, stored in memory 82, and/or controlled by hardware, firmware and/or software of control circuit 80 and/or sensing circuit 86. In response to sensing a cardiac event signal, e.g., an R-wave, sensing circuit 86 may generate a sensed event signal, e.g., a ventricular sensed event signal, that is passed to control circuit 80.

[0090] Ventricular sensed event signals received from sensing circuit 86 by control circuit 80 can be used by control circuit 80 for determining sensed event intervals, which can be referred to as RR intervals (RRIs). An RRI is the time interval between two ventricular sensed event signals received by control circuit 80. Control circuit 80 may include a timing circuit 90 for determining RRIs. Based on RRIs, control circuit 80 may detect VT/VF in some examples. RRIs may include time intervals between consecutive ventricular sensed event signals and intervals between a delivered pacing pulse and a ventricular sensed event signal. In some examples, sensing circuit 86 passes a digitized cardiac electrogram (EGM) or electrocardiogram (ECG) signal to control circuit 80 for morphology analysis for use in detecting cardiac arrhythmias in combination with RRIs according to an implemented arrhythmia detection algorithm. Aspects of the techniques disclosed herein may be implemented in conjunction with a variety of cardiac event signal sensing and arrhythmia detection methods and are not limited to any particular method for determining the need or timing of a cardiac electrical stimulation pulse delivered by therapy delivery circuit 84.

[0091] Timing circuit 90 may be configured to control various timers and/or counters used in setting various intervals and windows used in sensing ventricular event signals, determining time intervals between received ventricular sensed event signals, performing morphology analysis and controlling the timing of cardiac pacing pulses and other electrical pulses generated by therapy delivery circuit 84. Timing circuit 90 may start a timer in response to receiving ventricular sensed event signals from sensing circuit 86 and for timing RRIs. Timing circuit 90 may pass the RRIs to arrhythmia detection circuit 92 for determining and counting tachyarrhythmia intervals.

[0092] Control circuit 80 may include an arrhythmia detection circuit 92 configured to analyze RRIs received from timing circuit 90 and cardiac electrical signals received from sensing circuit 86 for detecting arrhythmia. Arrhythmia detection circuit 92 may be configured to detect asystole and/or tachyarrhythmia based on sensed cardiac electrical signals meeting respective asystole or tachyarrhythmia detection criteria. Arrhythmia detection circuit 92 may be implemented in control circuit 80 as hardware, software and/or firmware that processes and analyzes signals received from sensing circuit 86 for detecting VT/VF. In some examples, arrhythmia detection circuit 92 may include comparators and counters for counting RRIs determined by timing circuit 90 that are tachyarrhythmia intervals. An RRI that is less than the tachyarrhythmia detection interval is referred to as a “tachyarrhythmia interval.” Arrhythmia detection circuit 92 may compare the RRIs determined by timing circuit 90 to one or more tachyarrhythmia detection interval zones, such as a VT detection interval zone and a VF detection interval zone. RRIs falling into a detection interval zone are counted by a respective VT interval counter or VF interval counter and in some cases in a combined VT/VF interval counter. When a threshold number of tachyarrhythmia intervals is reached, control circuit 80 may detect VT or VF. In some examples, a tachyarrhythmia detection based on the threshold number of tachyarrhythmia intervals being reached may be confirmed or rejected based on morphology analysis of a cardiac electrical signal. It is to be understood that the ICD, e.g., ICD 114 coupled to RA lead 116, may sense P-waves, determine PP intervals, and use the timing of sensed P-waves and determined PP intervals in detecting arrhythmias, determining a need for cardiac pacing or CV/DF shock, and/or controlling the timing of atrial and/or ventricular pacing pulses.

[0093] Therapy delivery circuit 84 may include at least one charging circuit and one or more charge storage devices such as one or more high voltage capacitors for generating high voltage shock pulses for treating VT/VF. Therapy delivery circuit 84 may include a high voltage (HV) therapy circuit 83, which may include a HV charging circuit, HV holding capacitor(s), and HV output circuit that are operatively controlled by signals from control circuit 80 for charging and subsequently discharging the high voltage capacitor(s) for CV/DF shock delivery when control circuit 80 detects VT/VF. Examples of circuitry that may be included in therapy delivery circuit 84 are described below in conjunction with FIG. 5.

[0094] In some examples, therapy delivery circuit 84 may include a low voltage therapy delivery circuit 85, which may include a low voltage charging circuit, one or more low voltage holding capacitors and a low voltage output circuit for generating and delivering low voltage cardiac pacing pulses, e.g., cardiac pacing pulses having a pacing pulse amplitude that is 8 V or less, up to 10 V, up to 12 V, up to 16 V, or other maximum voltage amplitude of the low voltage therapy delivery circuit 85. Low voltage cardiac pacing pulses may be delivered by ICD 14 via ring electrodes 28 and/or 30 (together or in combination with housing 15) in some instances for successfully capturing and pacing the heart. Composite cardiac pacing pulses may be delivered by low voltage therapy delivery circuit 85 in some examples for delivering successive low voltage cardiac pacing pulses having a relatively long cumulative pulse width, e.g., up to 4 to 8 ms as examples, for delivering sufficient pulse energy to capture and pace the heart via extra-cardiac electrodes carried by lead 16 shown in FIG. 1A.

[0095] In some patients, the cardiac pacing capture threshold may require a pacing pulse amplitude and/or pulse width that is greater than a maximum pacing pulse amplitude and/or pulse width that can be generated and delivered by the low voltage therapy delivery circuit 85 via ring electrodes 28 and 30 for successfully capturing the heart in the extracardiac ICD system 10 of FIGs. 1A-2C. The pacing capture threshold and/or other factors, such as the electrical field of the pacing electrode vector relative to the patient’s heart, current density at the electrode tissue interface, or extraneous capture of non-cardiac tissue may make cardiac pacing via a low impedance pacing electrode vector including coil electrode 24 and/or coil electrode 26 desirable or preferred.

[0096] In some examples, in addition to being configured to deliver therapeutic electrical stimulation pulses to the patient’s heart under the control of control circuit 80, therapy delivery circuit 84 may be controlled to deliver electrical stimulation pulses for inducing tachyarrhythmia, e.g., T-wave shocks or trains of induction pulses, upon receipt of a programming command from external device 50 (FIG. 1A) by telemetry circuit 88, e.g., during ICD implant or follow-up testing procedures. As further described below, therapy delivery circuit 84 may be controlled to generate a leakage current test pulse for detecting a leakage current pathway according to techniques disclosed herein.

[0097] Telemetry circuit 88 includes a transceiver and antenna for communicating with external device 50 (shown in FIG. 1A) using RF communication or other communication protocols as described above. Control parameters utilized by control circuit 80 for sensing cardiac event signals, detecting arrhythmias, and controlling therapy delivery may be programmed into memory 82 via telemetry circuit 88. Under the control of control circuit 80, telemetry circuit 88 may receive downlink telemetry from and send uplink telemetry to external device 50.

[0098] FIG. 5 is a conceptual diagram of circuitry that can be included in therapy delivery circuit 84 of an ICD, e.g., ICD 14 or ICD 114, according to some examples. Therapy delivery circuit 84 may include a HV therapy delivery circuit 83 that can include a HV charging circuit 152, one or more HV holding capacitors 162, and a HV output circuit 160. The HV charging circuit 152 is configured to charge the HV holding capacitors 162 to deliver CV/DF shocks using coil electrodes coupled to electrode terminals 224, 226 and/or the ICD housing coupled to terminal 215 via HV output circuit 160.

[0099] In response to control circuit 80 detecting a need for CV/DF shock therapy based on an analysis of cardiac electrical signals sensed by sensing circuit 86, HV holding capacitor 162 may be charged to a shock voltage amplitude by HV charging circuit 152 for delivering a CV/DF shock under the control of control circuit 80 (shown in FIG. 4). HV charging circuit 152 may include a transformer to step up the battery voltage of power source 98 (shown in FIG. 4) in order to achieve charging of HV holding capacitor 162 to a voltage greater than the battery voltage. HV charging circuit 152 may include one or more transformers, switches, diodes, and/or other devices for operating to charge HV holding capacitor 162 to a desired voltage.

[0100] Control circuit 80 may pass a charge signal to HV charging circuit 152 to initiate charging and receive feedback signals from the HV charging circuit 152 to determine when HV holding capacitor 162 is charged to a shock voltage amplitude, e.g., corresponding to a programmed CV/DF shock energy, which may be selected based on defibrillation threshold testing or set to a nominal defibrillation energy, e.g., 20 Joules or more. A charge completion signal may be passed from control circuit 80 to HV charging circuit 152 to terminate charging of HV holding capacitor 162 in response to determining that the HV holding capacitor 162 is charged to a desired voltage.

[0101] While HV holding capacitor 162 is illustrated as a single capacitor in FIG. 5, it is to be understood that a combination of capacitors may be configured to function as a HV holding capacitor chargeable to a CV/DF shock voltage amplitude. For example, two or more HV capacitors may be provided in HV therapy circuit 83 having an effective capacitance of 100 to 200 microfarads, or about 140 to 160 microfarads as examples. The HV capacitors may be charged to hold up to 750 to 800 V, for example, in order to deliver CV/DF shocks having a pulse energy of 20 Joules or more, 30 Joules or more or 40 Joules or more, as examples, though lower energy CV/DF shocks could be delivered when the patient’s defibrillation threshold is lower, e.g., when the coil electrodes coupled to electrode terminals 224 and 226 are carried by transvenous leads as shown in FIG. 3. [0102] A CV/DF shock can be delivered to the heart by discharging HV holding capacitor 162 under the control of control circuit 80 according to signals passed to HV output circuit 160, e.g., via a control bus from therapy control circuit 94 (shown in FIG. 4). HV output circuit 160 includes switching circuitry, which may be in the form of an H-bridge including high side switches 180a- 180c and low side switches 182a- 182c, that are selectively biased into a conducting state (e.g., switched ON or enabled) from a nonconducting state (e.g., switched OFF or disabled) by signals from therapy control circuit 94 of control circuit 80.

[0103] As used herein, “low side" generally refers to the current path from the therapy delivery load 192 (e.g., the CV/DF electrodes and the patient’s heart and tissues through which the stimulation pulse energy is delivered) to ground (common) 190. For example, a selected low side switch out of low side switches 182a-c conducts current to ground from an electrode terminal 224, 226 or 215 coupled to a respective coil electrode, e.g., coil electrode 24 or 124 or coil electrode 26 or 126, or to housing 15 or 115 (as shown in FIGs. 1A and 3). As used herein, “high-side" generally refers to the current path from the cardiac electrical stimulation voltage source, e.g., HV capacitor 162, to the therapy delivery load 192. For example, a selected high side switch 180a-c conducts current from the HV capacitor 162 to an electrode terminal 224, 226 or 215 coupled to a respective coil electrode 24/124 or 26/126 or housing 15. Connections between the electrode terminals 224 and 226 and a respective coil electrode 24, 124, 26 or 126 can be made in the connector assembly 17 or 117 via insulated electrical feedthroughs crossing the ICD housing 15 or 115.

[0104] High side switches 180a- 180c may each include one or more electronic switching devices. In some examples, high side switches 180a- 180c may each include an anode gated thyristor (AGT), metal oxide semiconductor field effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), MOS -controlled thyristor (MCT), silicon- controlled rectifier (SCR) or other switching device or combination of switching devices having a high voltage rating. The high side switches 180a-c are generally high voltage rated switches that require a high operating current to bias the switch into a conducting or “on” state from a non-conducting or “off’ state, sometimes referred to as a “trigger” current, such that current leakage from HV holding capacitor 162 can be minimized when high side switches 180a-c are not enabled. High side switches 180a- 180c may be charge coupled devices, such as AGTs, that can be controlled without requiring bootstrapping. [0105] One or a combination of high side switches 180a- 180c is/are switched on by a trigger current signal, e.g., from control circuit 80, for conducting current from the HV capacitor 162 to an electrode terminal 224, 226, or 215 coupled to a respective coil electrode 24/124 or 26/126 or the ICD housing 15/115 selected as the CV/DF cathode electrode. A relatively high current trigger signal may be passed from control circuit 80 to switch a selected high side switch 180a, 180b or 180c to an ON state, to start discharging HV capacitor 162 for shock delivery. A different one of coil electrode 26/126, coil electrode 24/124 or housing 15/115 may be selected as the return anode electrode by switching on a selected one of low side switches 182a, 182b or 182c, which is coupled to the respective electrode terminal 224, 226, or 215 of the selected anode electrode. By providing a return path through a selected low side switch 182a, 182b or 182c, the high side switch 180a-c is held in a conducting state during discharge of HV capacitor 162. For example, as shown by one possible shock delivery path 250, when the high side switch 180a and the low side switch 182b are enabled by control circuit 80, a CV/DF shock pulse can be delivered from electrode terminal 224, electrode 24/124, the external therapy delivery load 192 and return through electrode 26/126 via electrode terminal 226, low side switch 182b to ground 190.

[0106] During discharging of HV capacitor 162 through the selected shock delivery pathway, e.g., shock delivery path 250, the high current flowing through the enabled high side (charge coupled) switch 180a in this example holds the switch in the conducting state until the low side switch 182b in this example, is switched OFF, to a non-conducting state, by control circuit 80. When the low side switches 182a- 182c are switched to a nonconducting state, current flowing through the high side switches 180a- 180c is stopped or falls below a required holding current for high side switches 180a- 180c to remain on. High side switches 180a- 180c are switched OFF in this way, terminating shock delivery (or a phase of the shock pulse as described below in conjunction with FIG. 6). High side switches 180a-c may require a relatively high trigger current from control circuit 80 of 100 to 200 milliamps, for example, to initially bias the selected high side switch 180a-c into a conducting state. A relatively lower current flowing through the enabled high side switch 180a-c can maintain the switch in a conducting state until the low side switch of the therapy delivery path is disabled or switched off.

[0107] Low side switches 182a- 182c may each include one or more switching devices, which may be implemented as SCRs, IGBTs, MOSFETs, MCTs, and/or other components or combinations of components. A selected low side switch 182a, 182b or 182c is biased in a conducting state by a control signal from therapy control circuit 94 of control circuit 80 to select the return path through an anode electrode selected from coil electrodes 24/124 and/or 26/126 or housing 15/115. Low side switches 182a-182c can be relatively low impedance switches, to minimize losses during CV/DF shock delivery, and can be switched to an ON state by a relatively low current control signal, e.g., less than 10 milliamps, from control circuit 80.

[0108] High side switches 180a- 180c and low side switches 182a- 182c are selectively controlled to be ON or OFF by control circuit 80 (e.g., by signals received by therapy delivery circuit 84 from therapy control circuit 94 shown in FIG. 3) at the appropriate times for delivering the CV/DF shock. For instance, one of high side switches 180a, 180b or 180c may be switched to an ON state simultaneously with one of low side switches 182a, 182b, or 182c, without switching on both of the “a,” “b” or “c” switches across a given electrode terminal 224, 226 or 215, respectively, at the same time. To deliver a biphasic electrical stimulation pulse using coil electrode 24/124 and electrode 26/126, for instance, switch 180a and 182b may be switched to ON states to deliver a first phase of the biphasic shock pulse via therapy delivery path 250. Before HV capacitor 162 is fully discharged, low side switch 182b is switched to an OFF state after the first phase, thereby causing high side switch 180a to be starved of the required holding current and also switch off. Switches 180b and 182a can be switched to an ON state by control circuit 80 to reverse the polarity of the biphasic shock pulse and deliver the second phase of the biphasic pulse along the delivery path 252. Switches 180c and 182c remain in an off (nonconducting) state in this example when housing 15/115 is not selected for use in the CV/DF shock delivery vector. In other examples, ICD housing 15/115 may be included instead of a coil electrode 24/124 or coil electrode 26/126 or simultaneously selected with a coil electrode to function as a cathode electrode or an anode electrode.

[0109] In some examples, HV therapy circuit 83 may be controlled by control circuit 80 to deliver cardiac pacing pulses. For instance, when a cardiac pacing pulse is needed and the pacing capture threshold is relatively high, e.g., greater than 8 V, 10 V, 16 V, 20 V, or 30 V, control circuit 80 may control HV charging circuit 152 to charge HV capacitor 162 to a programmed pacing voltage amplitude, less than the voltage required for CV/DF shock delivery. A relatively high voltage cardiac pacing pulse may be delivered via HV output circuit 160 by applying control signals to enable one or more selected high side switches 180a-c and enable one or more selected low side switches 182a-c during each phase of a cardiac pacing pulse as needed for discharging HV capacitor 162 via a selected pacing electrode vector including coil electrode 24/124 and/or coil electrode 26/126 and/or housing 15/115. When the HV output circuit 160 is used for delivering relatively lower voltage cardiac pacing pulses, therapy delivery circuit 84 may include an internal adjustable load in parallel to the therapy delivery load 192 for pulling additional current through the selected high side switch(es) 180a-c to hold the high side switch(es) 180a-c in a conducting state. The additional current pulled by an internal adjustable load can be controlled as needed during pacing pulse delivery when the current through the selected return path low side switch 182a-c is less than the holding current required to maintain the high side switches 180a-c in a conducting state.

[0110] As described above, therapy delivery circuit 84 may include a low voltage therapy circuit 85. The low voltage therapy circuit 85 may include a low voltage charging circuit 232 and a low voltage output circuit 240. The low voltage charging circuit 232 may include one or more charge pumps 234 for charging low voltage holding capacitors 242 and/or 246 to a pacing pulse amplitude. Charge pump 234 may charge low voltage holding capacitors 242 and/or 246 up to a multiple of the battery voltage of power source 98 (shown in FIG. 4). The charge pump 234 may be referred to as an “Nx” charge pump because it may be capable of charging low voltage holding capacitors 242 and 246 up to N times (Nx) the battery voltage of power supply 98, where N may be equal to any selected multiple of the battery voltage, e.g., up to two, three, four, five or six times the battery voltage, as examples. A state machine of control circuit 80 may control charging of low voltage holding capacitors 242 and/or 246 to a programmed pacing pulse amplitude using a multiple of the battery voltage of power source 98. Low voltage holding capacitors 242 and 246 may each have a capacitance of 50 microfarads or less or as low as 10 microfarads or less, as examples.

[0111] A capacitor selection switch 243 or 247 may be biased to a conducting state by a control signal from control circuit 80 for charging a selected low voltage holding capacitor 242 or 246 by a charge pump 234 to achieve a desired pacing pulse amplitude in a lower range of pacing pulse amplitudes. The charged holding capacitor 242 or 246 may be discharged via a tip capacitor 245 or 249, respectively, by switching on an electrode selection switch 255 or 248 after charge completion to deliver a pacing pulse to a selected cathode electrode, e.g., ring electrode 28 (FIG. 1A) or tip electrode 128 (FIG. 3) in electrical contact with electrode terminal 228. Another electrode, e.g., ring electrode 30/130 in electrical contact with electrode terminal 230, may serve as the return anode electrode when low side switch 238 is turned on to provide a return path to ground 190. Electrode terminal 228 may be coupled to ground 190 when low side switch 236 is turned on to provide a return path to ground when an electrode, e.g., 28 or 128 shown in FIGs. 1 and 3 respectively, is selected as the anode electrode during delivery of an electrical stimulation pulse, e.g., a cardiac pacing pulse.

[0112] While not shown in FIG. 5 for the sake of clarity, it is to be understood that low voltage therapy circuit 85 may include multiple channels for delivering atrial pacing pulses, RV pacing pulses and/or LV pacing pulses. As such, therapy delivery circuit 84 may be selectively coupled to multiple electrode terminals including RA electrode terminals, RV electrode terminals, and LV electrode terminals, for example. Each electrode terminal can be coupled to a respective electrode via electrical feedthroughs extending through connector assembly 17/117 that receives the corresponding leads connected to ICD 14/114. Accordingly, while only one pacing channel is shown in low voltage output circuit 240, it is to be understood that multiple low voltage holding capacitors and output capacitors may be provided in low voltage therapy circuit 85 as needed to provide delivery of electrical stimulation pulses via multiple, selectable electrode terminals and corresponding pacing electrode vectors.

[0113] Low voltage therapy circuit 85 may be controlled by control circuit 80 to deliver electrical pulses to various electrodes coupled to ICD 14/114 for measuring lead impedance, perform pacing capture threshold tests and other purposes. According to the techniques disclosed herein, low voltage charging circuit 232 may charge one or more low voltage holding capacitors, e.g., capacitor 242 and/or 246, for delivering a test pulse to a selected electrode terminal for performing a leakage current test. As further described below, a leakage current pathway to ground 190 may exist when the mechanical isolation of one or more connectors, feedthroughs and/or the RF antenna included in connector assembly 17/117 is breached, providing a possible return path to ground 190 when an electrical stimulation pulse is delivered by therapy delivery circuit 84. In some examples as described below, a test pulse may be delivered to electrode terminal 224, 226 or 215 when a corresponding high side switch 180a-c is enabled and all low side switches 182a-c are disabled. In this instance, a leakage current pathway, e.g., due to an isolation breach in the connector assembly 17/117, may provide the only return path for the delivered test pulse. In this way, control circuit 80 may control therapy delivery circuit 84 to perform a leakage current test. While electrical connections between electrode terminals 224, 226 and 215 and the low voltage therapy circuit 85 are not shown in FIG. 5, it is to be understood that terminals 224, 226 and 215 may be electrically connected to low voltage therapy circuit 85 or another test pulse voltage source or test pulse current source, e.g., by switching circuitry of therapy delivery circuit 84. For example, the low voltage holding capacitors 242 and 246 (that may be charged by low voltage charging circuit 232 for generating a test pulse) or another test pulse voltage source or test pulse current source may be electrically connected to electrode terminals 224, 226 and 215, each by respective switches that can be turned on by control circuit 80 for delivering the test pulse to an electrode terminal 224, 226 or 215. The necessary electrical connections, e.g., via switching circuitry of therapy delivery circuit 84 or isolating diodes between a voltage or current source generating the test pulse and an electrode terminal that the test pulse is being applied to are to be understood from the general block diagrams and description provided herein.

[0114] FIG. 6 is a diagram 300 of an electrical stimulation pulse 302 that may be delivered by therapy delivery circuit 84. For sake of illustration, a biphasic CV/DF shock pulse 302 is shown in FIG. 6. Shock pulse 302 has a starting pulse voltage amplitude 304. With continued reference to the therapy delivery circuit 84 shown in FIG. 5, HV charging circuit 152 may charge the HV holding capacitor 162 to the voltage of pulse amplitude 304. CV/DF shock pulse 302 may have a total pulse width 312 defined by the duration 312A of the first phase 303 and the duration 312B of the second phase 305. A negligible time delay between the first phase 303 and the second phase 305 may occur when the high side and low side switches of the H-bridge of HV output circuit 160 are switched to reverse the polarity of the second phase 305 of CV/DF shock pulse 302. In the example shown, the first phase duration 312a and the second phase duration 312b are shown to be equal, but each phase duration 312a and 312b could be different from the other in some examples. CV/DF shock pulse 302 decays exponentially from the starting pulse amplitude 304 to an ending amplitude 306 of the first phase 303 due to HV capacitor 162 being discharged through the therapy delivery load 192, e.g., via the delivery pathway 250 shown in FIG. 5, over the first phase duration 312a.

[0115] The second phase 305 has a starting amplitude 308 corresponding to the ending amplitude 306 of the first phase 303. The HV capacitor 162 providing the voltage signal for generating the CV/DF shock pulse 302 continues to discharge, e.g., via delivery pathway 252 in FIG. 5, during the second phase 305. The starting amplitude 308 of the second phase 305 exponentially decays to the ending, amplitude 310 at the expiration of the pacing pulse width 312.

[0116] To initiate delivery of CV/DF shock pulse 302, control circuit 80 may apply a trigger current 334 to a selected first one of the high side switches 180a, 180b or 180c to turn the switch ON from an OFF state and enable a low side switch 182a, 182b or 182c, as described above, for providing a return path to ground 190. When the trigger current is removed from the high side switch, the current flow through the enabled high side switch maintains the switch in a conducting state.

[0117] The current flowing through the enabled high side switch must be equal to or greater than a specified latching current 324 during a latch period 325 after removing the trigger current and equal to or greater than a specified holding current 326 after the latching period 325 expires. When the low side switch 182a-c is turned off by control circuit 80, the enabled high side switch is disabled because the current flow through the high side switch falls below the required holding current 326. The first phase 303 of CV/DF shock pulse 302 is truncated upon disabling the low side switch 182a, 182b, or 182c.

[0118] Control circuit 80 enables a second, high side switch (different than the first one) to initiate the second phase 305 of CV/DF shock pulse 302 by applying a trigger current 336. Control circuit 80 enables a second, low side switch to provide a return current path to ground as described above. As long as the second, low side switch is enabled to allow current to flow to ground, the second high side switch can remain in a conducting state if the current flow is equal to or greater than the specified latching current 328 during latch period 325 and equal to or greater than the holding current 330 after the latch period 325 expires. When the second low side switch is turned off by control circuit 80 at the expiration of the second phase pulse duration 312B, the second high side switch current falls below holding current 330. The second high side switch turns off. The CV/DF shock pulse 302 is terminated.

[0119] In this way, control circuit 80 can control when each high side switch 180a-c and each low side switch 182a-c are selectively enabled and disabled to control delivery of an electrical stimulation pulse through the HV output circuit 160, such as CV/DF shock pulse 302. When an isolation breach occurs, e.g., in the connector assembly 17/117, however, a return current path from an enabled high side switch to ground 190 may exist. A leakage current pathway, e.g., due to an isolation breach, could result in current flowing through an enabled high side switch that is equal to or greater than the holding current 326 at the expiration of the first phase duration 312A. When the first high side switch remains enabled (or “stuck on”) after the first phase duration 312A due to the leakage current, a shoot through condition may exist after control circuit 80 closes the low side switch that is in the same leg of HV output circuit 160 as the high side switch that is stuck on. For example switch 180a may remain in a conducting state due to leakage current when control circuit 80 disables (opens) switch 182b and enables (closes) switch 182a. The term “shoot through condition” can refer to the condition of current flowing through the high side switch (e.g., 180a) and the low side switch in the same leg (e.g., 182a) of the high voltage output circuit 160 directly to ground 190 without flowing through the therapy delivery load 192. Control circuit 80 may perform a short circuit protection shutdown of the therapy delivery circuit 84 when a shoot through condition occurs. The CV/DF shock pulse 302 is terminated early, before the second phase 305 is delivered when control circuit 80 performs the short circuit protection shutdown. Sufficient energy for terminating a detected tachyarrhythmia may not be delivered under these circumstances. In other instances, the second phase 305 may be started and a leakage current pathway could result in current flowing through an enabled high side switch during the second phase duration 312B that is equal to or greater than the holding current 330 at the expiration of the second phase duration 312B. In this case, there is a risk of inadvertently applying a DC voltage to the heart which can potentially be proarrhythmic. The techniques disclosed herein provide for detecting a current leakage pathway that could lead to a shoot through current flowing through a high side switch 180a, 180b or 180c. By detecting a current leakage pathway, mitigating actions may be taken.

[0120] FIG. 7A is a conceptual diagram 350 of a current leakage pathway to internal ground 190 of ICD 14/114 that could possibly occur during electrical stimulation pulse delivery. The HV output circuit 160 and the telemetry circuit 88 of ICD 14/114 are shown. For the sake of the example described in conjunction with FIG. 7A, HV output circuit 160 is shown including high side switches 180a and 180c and low side switches 182a and 182c as described above in conjunction with FIG. 5. Switches 180b and 182b, selectively enabled when coil electrode 26/126 is used in the CV/DF electrode vector, are not shown for the sake of clarity in the example shown in FIG. 7A. However, it is to be understood that switches 180b and 182b may be selectively enabled for including the coil electrode 26/126 in the therapy delivery vector during delivery of an electrical stimulation pulse via HV output circuit 160 in other examples as generally described above.

[0121] The telemetry circuit 88 includes a transceiver 204, an antenna 216, and an impedance matching circuit 218. Antenna 216 may be located in the ICD connector assembly 17/117 and receives and transmits RF signals to and from transceiver 204 via impedance matching circuit 218. Impedance matching circuit 218 may include one or more capacitors, inductors, diodes and other circuitry for matching the impedance of the antenna 216 to the impedance of the antenna input/output of transceiver 204 and to protect transceiver 204 from RF energy levels higher than the transceiver 204 can handle. For example, a capacitor 213 and inductor 211 may provide impedance matching between antenna 216 and antenna input/output 205 of transceiver 204. When RF energy levels exceed safe limits for transceiver 204, a diode array 214 and inductor 217 can conduct RF energy to ground 190 to protect the transceiver 204.

[0122] Antenna 216 can be mechanically isolated, e.g., surrounded by elastane or other electrically insulating material. An insulated antenna feedthrough (not shown in FIG. 7A) extending from antenna 216 in connector assembly 17/117 across housing 15/115 provides electrical connection between antenna 216 and telemetry circuit 88. In some instances, however, a mechanical isolation breach may exist that exposes antenna 216 and/or the antenna feedthrough (see feedthrough 508 in FIG. 9) that conducts signals between antenna 216 and transceiver 204. When a mechanical isolation breach exists, the exposed antenna and/or antenna feedthrough may conduct electrical signals that are being delivered via HV output circuit 160 directly to a common ground 190 of the circuitry enclosed by housing 15/115. As shown in FIG. 7A, a pathway from the antenna 216 to ground 190 (through inductor 217 if capacitor 222 is not present in the example shown) may pull current through a high side switch 180a or 180c that is enabled for delivering an electrical stimulation pulse to electrode terminal 224 or 215. Current pulled through the leakage pathway 210 (shown by dashed line) via antenna 216 (and/or an associated feedthrough) to ground 190 may hold the high side switch 180c in a conducting state even after the opposite low side switch 182a is disabled, e.g., to terminate the first phase of a biphasic CV/DF shock as described above in conjunction with FIG. 6.

[0123] The leakage current may hold the high side switch 180c on, in a conducting state, when the second high side switch 180a is enabled at the beginning of the second phase of a CV/DF shock and the opposite low side switch 182c is enabled. The result can be a shoot through current flowing through the high side switch 180c directly through the corresponding low side switch, e.g., 182c. Current delivered through the external therapy delivery load 192 can be reduced due to the shoot through current and may fail to deliver sufficient CV/DF shock energy (or other therapeutic stimulation pulse energy such as a cardiac pacing pulse energy) through the external therapy delivery load 192. In some examples, the therapy delivery circuit 84 may include a short circuit monitor 356 that could detect the shoot through current. Short circuit monitor 356 enables control circuit 80 to measure current flowing to ground 190 through a sensing resistor 358. When the current measured by short circuit monitor 356 exceeds a threshold current, control circuit 80 may to perform a short circuit shutdown of HV therapy circuit 83 in response to the detected shoot through current at the start of the second phase such that the second phase of a biphasic electrical stimulation pulse is not delivered at all.

[0124] In some examples, a short circuit shutdown may occur prior to the end of the first phase of the biphasic electrical stimulation pulse when leakage current or a high voltage arc due to the isolation breach causes faster than expected discharge of the HV holding capacitor 162, for example. As such, a leakage current or high voltage arc, e.g., due to a mechanical isolation breach in the connector assembly 17/117 may prevent successful delivery of a CV/DF shock pulse, cardiac pacing pulse or other electrical stimulation pulse. The leakage current pathway may result in early termination of a monophasic pulse, early termination of the first phase of a biphasic pulse, or pulse delivery shutdown prior to delivery of the second phase of a biphasic pulse in various examples. When the leakage pathway impedance 212 is less than 25, 20, 15, or 10 kiloohms, as examples, the leakage current may be high enough to cause a short circuit shutdown of HV therapy circuit 83 or shoot through current holding a high side switch 180a-c open longer than intended.

[0125] In some examples, capacitor 221 and/or capacitor 222 can be provided to protect the telemetry circuit 88 from a high voltage that can be present on ICD housing 15/115 and/or during CV/DF shock delivery by therapy delivery circuit 84 (or by an external defibrillator or other high voltage source). Capacitors 221 and/or 222 can be high voltage rated capacitors. Capacitor 222 can be coupled between antenna 216 and the impedance matching circuit 218 to block high voltage signals that can be on the ICD housing 15/115 and block the leakage current pathway 210 to ground 190. Capacitor 222 can be selected to have a capacitance that, in combination with impedance matching components of impedance matching circuit 218, can provide impedance matching with the antenna input and output 205 of transceiver 204. Capacitor 222 and/or capacitor 221, due to the high voltage ratings required, can be configured as a physically large capacitor relative to other antenna impedance matching capacitors that may be present in impedance matching circuit 218, e.g., relative to capacitor 213 which can be a lower voltage rated capacitor. For example, capacitor 222 may be a 1206 package (0.126 inches long) whereas other capacitors in impedance matching circuit 214 such as capacitor 213 may be an 0402 or 0201 package, e.g., having a length less than 0.04 inches.

[0126] The physically large capacitor 222 and/or capacitor 221 provides proper high voltage spacing between the capacitor electrodes. In order to avoid relatively high parasitic inductance associated with physically large, high voltage rated capacitors that is undesirable in the RF frequency ranges used for telemetry communication with external device 50, capacitor 221 and/or capacitor 222 can be implemented in the printed circuit board of the telemetry circuit 88 using adjacent copper layers as the capacitor plates and the printed circuit board insulative material as the capacitor dielectric. In this way, a leakage current pathway 210 due to an isolation breach of antenna 216 or its associated feedthrough can be blocked by capacitor 222 without introducing excessive parasitic inductance. Other leakage current pathways may occur, however, due to isolation breaches in the connector assembly 17/117 that could provide a return current path to ground 190 or result in a high voltage arc during therapeutic electrical stimulation pulse delivery. Other leakage current pathways that may be detected using the techniques disclosed herein are described below in conjunction with FIG. 9.

[0127] FIG. 7B is a conceptual diagram of circuitry and a method for performing a leakage current test according to some examples. In some examples, the test pulse 352 may be applied to a low impedance electrode terminal 224, 226 or 215 (terminal 224 in the example shown) without requiring turning on any of the high side switches 180a-c. In this example, the test pulse 352 may be applied to a low impedance electrode terminal 224, 226 or 215, e.g., via a leakage current test resistor 355, which enables control circuit 80 to measure the response signal 354, e.g., as the current flow through resistor 355.

[0128] The low side switches 182a-c remain disabled (open) during the leakage current test. In this way, the only return path to ground 190 is via a leakage current pathway 360 if it exists, e.g., due to a mechanical isolation breach, which may be an isolation breach of the telemetry circuit antenna or an electrical feedthrough in the connector assembly 17/117. When the leakage current pathway impedance 362 falls below a threshold impedance, below 15 kiloohms for example, the leakage current may be high enough to cause a short circuit shutdown of HV therapy circuit 83 and/or hold one of high side switches 180a-c in a conducting state resulting in shoot through current through the corresponding low side switch 182a-c. In other instances, a leakage current pathway impedance that is 100 kiloohms or less, for example, may result in a high voltage arc that reduces the current delivered to the therapy delivery load. The response signal 354 may be measured by control circuit 80 as the current delivered to the electrode terminal 224 for detecting the leakage current pathway 360 when the test pulse 352 is applied and return paths via the low side switches 182a-c are held open.

[0129] In other examples, the test pulse 352 may be applied to any of electrode terminals 224, 226 or 215. While examples shown in FIGs. 7A and 7B show the low impedance electrode terminals 224, 226 and 215 used for applying the test pulse for detecting a leakage current pathway, it is contemplated that the test pulse may be applied to an electrode terminal that is electrically coupled to a relatively higher impedance electrode (e.g., any ring electrode or tip electrode coupled to the ICD 14/114 used for pacing and sensing). The examples described herein may detect a leakage current pathway that could occur when a therapeutic electrical stimulation pulse (such as CV/DF shocks and/or cardiac pacing pulses) are delivered via any of the electrodes coupled to ICD 14/114. A return path for the test pulse 352 via any electrode terminal may be blocked by disabling low side switches 182a-c and any other switches (e.g., low voltage output circuit low side switches 236 and 238 shown in FIG. 5) included in therapy delivery circuit 84 that, when turned on, would provide a return path for the test pulse 352 to ground 190 via an electrode terminal. No return path for the test pulse 352 is provided by simultaneously holding all low side switches (e.g., switches 182a-c, 236 and 238 shown in FIG. 5) that are configured to switchably connect a respective electrode terminal to ground 190 in an off, non-conducting state.

[0130] FIG. 8 is a flow chart 400 of a method for performing a leakage current test according to some examples. Control circuit 80 may be configured to perform a leakage current test to enable automatic or user reprogramming of a therapy delivery pathway, ICD replacement by a clinician or other corrective action to be taken. At block 402, control circuit 80 may control therapy delivery circuit 84 to deliver a test pulse to a selected electrode terminal. For the sake of illustration, flow chart 400 is described below with reference to the test pulse being delivered to an electrode terminal that may be coupled to a low impedance electrode, e.g., any of coil electrodes 24/124, 26/126 or housing 15/115. The electrode terminal coupled to a low impedance (e.g., high surface area) electrode may be referred to herein as a “low impedance electrode terminal.” The test pulse can alternatively be delivered to an electrode terminal coupled to a high impedance electrode, e.g., any of electrodes 24 or 26 shown in FIG. 1A or any of electrodes 120, 122, 128, 130, 132, 134 or 136 shown in FIG. 3. An electrode terminal coupled to a high impedance tip or ring electrode may be referred to as a high impedance electrode terminal. During the leakage current test, with continued reference to FIGs. 7A and 7B, control circuit 80 may disable the low side switches 182a-c so that a return path to ground 190 is not provided through a low side switch 182a-c of HV output circuit 160. A return path via any other available electrode terminals may be blocked (e.g., held open) by simultaneously disabling all switches that, when turned on, can provide a return path to ground via a corresponding electrode terminal. For example, in addition to low side switches 182a-c shown in FIG. 5, low side switches 236 and 238 of low voltage output circuit 240 may be simultaneously disabled so that a return path via electrode terminals 228 and 230, respectively, to ground 190 is open.

[0131] To deliver the test pulse at block 402, control circuit 80 may enable a high side switch 180a-c coupled to the low impedance electrode terminal (224, 226 or 215), e.g., by applying a trigger current to the high side switch to bias the high side switch into a conducting state. The test pulse can be delivered via a selected one of the high side switches 180a-c in some examples as generally shown in FIG. 7A. In other examples, however, the test pulse can be delivered to an electrode terminal 224, 226 or 215 as shown in FIG. 7B without requiring biasing a high side switch 180a-c into an ON state. Control circuit 80 may control the therapy delivery circuit 84 to deliver the test pulse, which may be generated by the low voltage therapy circuit 85 (see FIG. 5) to an electrode terminal 224, 226 or 215. With the low side switches 182a-c (and any other switches that couple an electrode terminal to ground, generally referred to herein as “low side switches”) simultaneously disabled during delivery of the test pulse, a return path for the test pulse to ground 190 may only be the leakage current pathway (if it exists). Thus, control circuit 80 may measure a response signal (block 404) to the test pulse to detect a leakage current pathway.

[0132] The test pulse may be a voltage pulse signal, as represented by test pulse 352 in FIGs. 7A and 7B, which may be 2 volts to 10 volts or 5 volts to 8 volts or about 6 to 7.5 volts in amplitude in various examples. The test pulse may be generated by a 2x, 3x or 4x charge pump, e.g., charge pump 234 shown in FIG. 5, to be a multiple of the battery voltage of power source 98 (FIG. 4). The test pulse may be 80 to 250 microseconds or about 150 to 200 microsecond or about 180 microseconds in various examples. The test pulse may be a mono-, bi- or multiphasic signal in various examples.

[0133] The response signal 354 to the test pulse may be measured by control circuit 80 at block 404, for example, by measuring the current flowing through a low impedance resistor 355 (e.g., 50 ohms or less) along the input to the high side switches 180a-c of HV output circuit 160, e.g., as shown in FIG. 7A or in series with the low impedance electrode terminal 224, 226 or 215. The measured current flowing during the test pulse when a return path via any other electrodes is disabled is representative of the leakage current flowing to ground 190 via a leakage pathway, e.g., pathway 210. Low side switches 182a- c and any other switches coupled to electrode terminals that could provide a return path to ground are all disabled so that the test pulse is delivered without enabling a return path to ground 190 via any of the electrode terminals. The delivered test pulse voltage to ground (e.g., the known amplitude of test pulse 352 or the voltage measured from the low impedance electrode terminal 224, 226 or 215) and the measured current flowing during the test pulse may be used to compute a leakage path impedance (e.g., corresponding to RLeak 212 in FIG.7A or RLeak 362 in FIG. 7B) at block 406. In other examples, the test pulse may be delivered as a current pulse and the response signal may be measured as a voltage signal. In this case, the leakage path impedance can be determined from the amplitude of the test current pulse and the measured voltage signal.

[0134] Control circuit 80 may be configured to compare the response signal amplitude and/or the determined leakage path impedance to a respective leakage path detection threshold at block 408. The high side switches 180a-c may each have a specified trigger current required to turn the switch on from a non-conducting state, a latching current that prevents the switch from turning off when the trigger current is removed and a holding current that holds the current in a conducting state, as generally described above in conjunction with FIG. 6. The trigger current can be higher than the latching current and the latching current can be higher than the holding current. For example, the high side switches 180a-c may have a specified latching current (e.g., according to manufacturer specification) between 70 and 150 mA or between 80 and 120 mA in various examples. After the latch period, a specified holding current required to maintain a conducting state of the high side switches 180a-c may be 15 to 40 mA or 20 to 30 mA in various examples. If a leakage current pulled through a high side switch 180a, 180b or 180c after the high side switch is enabled by the trigger current is greater than the latching current (during the latch period) or the holding current (after the latch period expires), the high side switch may remain ON in a conducting state even after control circuit 80 disables a low side switch 182a-c of a different leg of the H-bridge.

[0135] As such, control circuit 80 may compare the computed leakage path impedance determined in response to the test pulse to a threshold impedance at block 408. The threshold impedance can be established based on an impedance that is expected to allow a high voltage arc to occur during therapeutic electrical stimulation pulse delivery. In this case, a threshold impedance that is 100 kiloohms or less, as an example, may indicate a possible leakage current path that is detected by control circuit 80.

[0136] In some examples, the threshold impedance can be established based on an expected voltage amplitude of a therapeutic electrical stimulation pulse and the specified holding current of the high side switches 180a-c. A leakage path impedance that is greater than the threshold impedance may allow the high side switch 180a-c to turn off when a low side switch 182a-c is disabled thereby starving the high side switch of the holding current needed to maintain it in a conducting state. However, a leakage path impedance that is less than the threshold impedance may allow enough current to continue flowing through a triggered high side switch 180a, 180b, or 180c after a low side switch is disabled so that shoot through current may flow through the in series low side switch 182a, 182b, or 182c enabled for delivering a second phase of an electrical stimulation pulse. The shoot through current may cause a short circuit shutdown of the HV therapy circuit 84.

[0137] In an illustrative example, if the leakage path impedance is 20 kiloohms or less, when the CV/DF shock delivered by the HV output circuit 160 has an expected voltage amplitude of 400 volts or more, e.g., at the trailing edge of the first phase of a biphasic CV/DF shock, a leakage current of 20 mA may flow through the high side switch. In some instances, this leakage current of 20 mA, alone or in combination with any other inherent internal ICD circuit loads that may be carrying low amplitude currents through the high side switch, may be sufficient to hold the high side switch ON in a conducting state. Accordingly, if the leakage impedance is less than the threshold impedance, control circuit 80 may generate an alert output at block 410 for transmission by telemetry circuit 88 to notify a clinician of a leakage current pathway detection and a possible isolation breach. Additionally, in response to the leakage current pathway detection, control circuit 80 may select a different therapy delivery electrode vector at block 410 that may be less susceptible to the leakage current pathway for delivering electrical stimulation pulses to the therapy delivery load 192. For example, when other therapy delivery electrode vectors are available and a leakage current pathway is detected when a test pulse is delivered to an electrode that is currently programmed to be included in an electrical stimulation pulse delivery electrode vector, control circuit 80 may eliminate the electrode from an electrical stimulation pulse delivery electrode vector, e.g., a CV/DF shock delivery electrode vector or a cardiac pacing pulse electrode delivery vector. A different therapy delivery electrode vector may be selected that could be less susceptible to the leakage current pathway. [0138] In some examples, control circuit 80 may compute a predicted leakage current during CV/DF shock (or other electrical stimulation pulse) delivery based on a programmed shock energy or calculated, measured or estimated trailing edge voltage of a phase of a stimulation pulse and the measured or estimated therapy delivery load 192. If the predicted leakage current during shock delivery is greater than the holding current (or a safety margin less than the holding current, e.g., 70%, 80% or 90% of the holding current), control circuit 80 may generate an alert output for transmission by telemetry circuit 88. The threshold applied to a leakage impedance or a leakage current at block 408 may take into account any inherent current that may flow in the ICD circuitry that could be additive to the leakage current pulling current through a high side switch 180-c during delivery of an electrical stimulation pulse to the therapy delivery load.

[0139] FIG. 9 is a conceptual diagram 500 of a connector assembly 117 coupled to ICD housing 115 according to some examples. The housing 115 encloses the ICD electronic circuitry 510 of ICD 114, e.g., as described above in conjunction with FIGs. 4 and 5. Connector assembly 117 may be configured to receive an RA lead, an RV lead and/or an LV lead as described above in conjunction with FIG. 3. To accommodate multiple medical electrical leads, connector assembly 117 may include multiple bores for receiving the proximal ends of the RA lead, the RV lead and the LV lead. One connector assembly bore 501 for receiving the proximal end of the RA lead, e.g., lead 116 in FIG. 3, may include an RA ring electrode connector (RA R) and RA tip electrode connector (RA T) that are each coupled to electronic circuitry 510 via respective electrical feedthroughs 502 and a respective electrode terminals 530. Another connector assembly bore 503 for receiving the proximal end of the RV lead, e.g., lead 118 in FIG. 3, may include an RV tip electrode connector (RV T), RV ring electrode connector (RV R), RV coil electrode connector (RV C) and SVC coil electrode connector (SVC C) that are each coupled to electronic circuitry 510 via respective electrical feedthroughs 504 and a respective electrode terminals 530. Another connector bore 505 may be configured to receive the proximal end of LV lead 121 and may include an LV tip electrode connector (LV T) and LV ring electrode connectors (e.g., LVR 1, LVR 2, and LVR 3) that can each be coupled to electronic circuitry 510 via respective electrical feedthroughs 506 and a respective one of electrode terminals 530. Antenna 216 may be coupled to the internal telemetry circuit included in electronic circuitry 510 via feedthrough 508.

[0140] Any one or more of the electrode connectors shown in FIG. 9, antenna 216, and/or electrically conductive components of feedthroughs 502, 504, 506 and 508 could become exposed to body fluids due to a breach of the electrically insulating material surrounding the electrode connectors, antenna 216 and feedthroughs 502, 504, 506 and 508 of connector assembly 117. A leakage current pathway 520 (shown conceptually) may conduct current via any exposed electrically conductive connector, antenna, or feedthrough component of connector assembly 117 to ground 190. Control circuit 80 may control the therapy delivery circuit 84 to deliver a test pulse 552 for enabling measurement of a response signal, e.g., the current signal 512. Control circuit 80 may enable a selected one of switches 532 for electrically conducting the test pulse 552 to a respective selected one of electrode terminals 530.

[0141] Low side switches that, when turned on by control circuit 80, couple a respective electrode terminal 530 to ground 190 are not shown for the sake of clarity in FIG. 9. It is to be understood that when test pulse 552 is delivered, however, all low side switches coupling electrode terminals 530 to ground 190 are simultaneously disabled by control circuit 80 (e.g., maintained in an off, non-conducting state) to hold any return paths to ground 190 via any of electrode terminal 530 open. In this way, the only available return path to ground 190 for test pulse 552 that allows a response current signal 512 to be measured is the leakage current pathway 520. It is to be understood that the measured response current signal 512 may include low amplitude current flowing through other inherent internal ICD circuit loads. Other inherent current contributing to the response current signal 512 may be negligible or may be accounted for in the measured response current signal 512.

[0142] For example, an estimated inherent current when no leakage current pathway exists may be subtracted from the measured response current signal 512 when comparing it to a leakage current pathway detection threshold. Alternatively, the leakage current pathway detection threshold compared to the response current signal 512 may be adjusted accordingly to account for any inherent current flow. When the measured response current signal 512 is used to compute the leakage pathway impedance Rleak, the response current signal 512 may be adjusted to account for an estimated inherent current flow and/or an impedance threshold applied to the computed leakage pathway impedance Rleak for detecting the leakage current pathway 520 may be adjusted accordingly.

[0143] Response current signal 512 is conceptually shown being measured across a resistor that is in series with a respective electrode terminal 530 receiving the test pulse 552 when a selected one of switches 532 is enabled for delivering the test pulse 552. The response signal 512 may be measured in any of a number of locations along the test pulse current path, e.g., before or after an electrode terminal 530, for measuring current flowing via the selected one of electrode terminals 530 to ground 190, for detecting a leakage current pathway that may exist through one or more connectors and associated feedthroughs of connector assembly 117. While the response signal 512 is described here as being measured as a response current signal, it is to be understood that a response voltage signal could be measured when test pulse 552 is delivered as a controlled current pulse.

[0144] FIG. 10 is a conceptual diagram 700 of another technique for performing a leakage current test according to some examples. FIG. 11 is a flow chart 800 of a method for performing a leakage current test according to the techniques shown by the diagram of FIG. 10. With reference to FIGs. 10 and 11, control circuit 80 may control therapy delivery circuit 84 to deliver a first test pulse 701 (block 802 of flow chart 800) via a selected low impedance electrode terminal shown as electrode terminal 704 in FIG. 10. As described above, a test pulse 701 can be applied to a low impedance electrode terminal 704 coupled to a corresponding coil electrode, e.g., any of electrodes 24/124, 26/126 or housing 15/115. Control circuit 80 may control therapy delivery circuit 84 to select a relatively high impedance pace/sense electrode 706 as a return electrode for providing a return path to ground for the test pulse 701. When a leakage current pathway exists, the leakage current pathway, having leakage path impedance 710, provides a parallel path to ground 190 for test pulse 701.

[0145] The response signal may be measured as the current signal 720a during delivery of the test pulse 701. The response current signal 720a may be measured in series with the electrode terminal 704, for example. At block 804 of flow chart 800, control circuit 80 may determine the impedance of the parallel return path, Rleak 710 in parallel with Rload2 718, from the delivered voltage of the test pulse 701 and the measured response current signal 720a. Test pulse 701 can be delivered as a constant current or a constant voltage pulse with the corresponding response signal measured between the test pulse source and electrode terminal 704 in some examples though the response signal may be measurable at other locations along the circuitry of the test pulse path.

[0146] At block 806, control circuit 80 may deliver a second test pulse 702. Each of the first test pulse 701 and the second test pulse 702 may be monophasic test pulses in some examples. Both of the test pulses may be generated by the low voltage therapy circuit 85 as described above. Control circuit 80 may control therapy delivery circuit 84 to deliver the second test pulse 702 via the selected high impedance pace/sense electrode 706 and enable a return path via the low impedance coil electrode 704 that was used to deliver the first test pulse 701. The response current signal 720b may be measured as the current flowing to the electrode terminal 706. Control circuit 80 may determine the impedance of the second parallel return path to ground at block 808. The second parallel return path is Rleak 710 in parallel with Rloadl 708, which now includes the low impedance coil electrode 704 instead of the high impedance pace/sense electrode 706 associated with Rload2 718 as shown in the left side of diagram 700.

[0147] At block 810, control circuit 80 may compare the first and second parallel path impedances. When a leakage current pathway is not present, the first and second parallel path impedance measurements will be nearly equal (because the parallel path via Rleak 710 is not present or shunts negligible or no current to ground 190). If Rleak 710 is very high (e.g., effectively an open circuit due to no significant isolation breach), the first impedance measured for the first test pulse 701 from a low impedance coil electrode 704 to a high impedance pace/sense electrode 706 will be about equal to the second impedance measured for the second test pulse 702 from the high impedance pace/sense electrode 706 to the low impedance coil electrode 704. When a leakage current pathway exists, however, the first and second parallel path impedance measurements will be different due to the parasitic resistance of the leakage pathway having a greater influence on the overall impedance of the first parallel path than the overall impedance of the second parallel path. [0148] The impedance measurement of the first test pulse 701 can be equal to the resistance Rloadl 708 including the lower impedance of the coil electrode 704 plus the parallel combination of Rleak 710 and Rload2 718 (which includes the high impedance pace/sense electrode 706). The impedance measurement of the second test pulse 702 can be equal to the resistance Rload2 718 including the higher impedance of the pace/sense electrode 706 plus the parallel combination of Rleak 710 and Rloadl 708 (which includes the impedance of low impedance coil electrode 704). Because the coil electrode 704 is a low impedance electrode (e.g., less than 100 ohms or approximately 20 to 50 ohms in some examples) compared to the pace/sense electrode 706 (e.g., greater than 100 ohms or approximately 200 to 700 ohms or about 500 ohms in some examples), Rleak 710 has less impact on the impedance measurement during the second test pulse 702 than during the first test pulse 701. As a result, as Rleak 710 decreases (e.g., due to an isolation breach of any of the connector assembly connectors, antenna or feedthroughs in ICD 14/114, e.g., as described in conjunction with FIG. 9), a greater difference in the first and second impedance measurements will exist.

[0149] At block 812, control circuit 80 may determine if the first and second impedance measurements are within a threshold range of each other. If not, control circuit 80 may detect a leakage current pathway that may be associated with an isolation breach at block 814. The difference between the first and second impedance measurements may be compared to a threshold difference for detecting the leakage current pathway. In other examples, the first impedance measurement may be compared to a threshold based on the second impedance measurement for detecting the leakage current pathway. Control circuit 80 may generate an alert that can be transmitted by telemetry circuit 88. In some examples, control circuit 80 may select a different therapy delivery electrode vector in an attempt to mitigate the effects of the leakage current pathway during electrical stimulation pulse delivery, as described above. If the first and second impedance measurements are within a threshold range of each other at block 812, no leakage current pathway is detected by control circuit 80 at block 820. Any leakage current pathways that may exist have a high enough impedance and low enough leakage current that a therapeutic electrical stimulation pulse can be effectively delivered. Control circuit 80 may wait for the next scheduled leakage current test and return to block 802 to start the next test. It is noted that, in some examples, a leakage current pathway detection threshold stored in memory 82 that is applied to the first and second impedances, e.g., applied as a threshold difference to the difference between the first and second impedance, may be set to a value that is based on a holding current required to maintain a high side switch of therapy delivery circuit 84 in a conducting state, e.g., any of switches 180a-c shown in FIG. 5). For example, a value of the difference threshold may be determined and stored in memory 82 based on the minimum impedance of Rleak 710 that would shunt a leakage current that is a safety margin less than the holding current required to maintain a high side switch in a conducting state. The leakage current pathway detection threshold may be determined based on a minimum impedance Rleak 710 of the leakage current pathway that prevents shoot through current in a high side switch.

[0150] The method of FIGs. 10 and 11 may be repeated using different combinations of a low impedance coil electrode and a high impedance pace/sense electrode. For example, either of the ring electrodes 26 or 28 may be used in combination with any of the coil electrodes 24 or 26 or with housing 15 in the example of the extracardiac lead 16 coupled to ICD 14 as shown in FIGs. 1A-2C. In the example of the transvenous ICD system 100 of FIG. 3, any of coil electrodes 124 or 126 or housing 115 may be selected, corresponding to the low impedance electrode terminal 704 shown in FIG. 10. Any of RA tip electrode 20, RA ring electrode 22, RV tip electrode 28, RV ring electrode 30, or LV electrodes 134 or 136 may be selected as the relatively high impedance pace/sense electrode corresponding to high impedance electrode terminal 706. The leakage current test method of FIGs. 10 and 11 that involves delivering two test pulses between a high impedance electrode terminal 706 (coupled to a high impedance pace/sense electrode) and a low impedance electrode terminal 704 (coupled to a low impedance CV/DF electrode) may be repeated multiple times using multiple different combinations of the available low impedance coil electrodes (or ICD housing) paired with any of the available high impedance pace/sense electrodes. However, each test can be performed with one low impedance coil electrode (or housing) and one relatively high impedance pace/sense electrode (e.g., ring or tip electrode) so that when a leakage current pathway is present the difference of the parallel return path impedances can be detected by control circuit 80. As described above, when a leakage current pathway is detected, an electrode terminal used in the leakage current test may be eliminated from a therapy delivery electrode vector by control circuit 80 when other therapy delivery electrode vectors are available. When another therapy delivery electrode vector is not available, delivery of an electrical stimulation therapy may be disabled by control circuit 80 in some instances, e.g., when the therapy is not treating a life-threatening condition.

[0151] Any of the leakage current tests described herein may be repeated once per hour, once per day, once per week or at another scheduled interval. The leakage current tests described herein may be triggered in response to a short circuit shutdown by therapy delivery circuit 84 or in response to detecting a high current flow during electrical stimulation pulse delivery or a faster than expected voltage drop of the delivered electrical stimulation pulse. The leakage current tests may be performed using a selected one of the available low impedance electrode terminals coupled to a high side switch of the HV output circuit and a CV/DF electrode (or ICD housing) or repeated using a different one or combination of two of the low impedance electrode terminals.

[0152] Furthermore, any of the leakage current test methods disclosed herein may be performed in combination for detecting a leakage current pathway. For example, the leakage current test described in conjunction with FIG. 7A or 7B may be performed one or more times by applying the test pulse to one or more electrode terminals with no return path to ICD electrical ground enabled via another electrode terminal of ICD 14/114. Additionally or alternatively, the leakage current test described in conjunction with FIGs. 10 and 11 may be performed one or more times using different low and high impedance electrode terminal combinations in which a first response signal is measured when a test pulse is applied to a first electrode terminal with a first return path via a second electrode terminal to device ground enabled, and a second response signal is measured when a test pulse is applied to the second electrode terminal with a second return path enabled via the first electrode terminal. In this way, control circuit 80 may obtain multiple response signals measured for each of multiple test pulses applied with and/or without return current paths to electrical ground enabled via a low side switch coupled to an electrode terminal, according to the leakage current test method. A leakage current pathway detection threshold may be determined to be met (or not) based on one method (e.g., the method of FIGs. 7A or 7B). A different leakage current pathway detection threshold may be determined to be met (or not) based on another method (e.g., the method of FIGs. 10 and 11). When at least one or both methods result in a leakage current pathway detection threshold being met, control circuit 80 may detect a leakage current pathway. In this way, false positive and/or false negative detections of a leakage current pathway can be avoided by using two different methods for detecting the leakage current pathway.

[0153] Further disclosed herein is the subject matter of the following examples:

[0154] Example 1. A medical device including a plurality of electrode terminals, a therapy delivery circuit and a control circuit. The therapy delivery circuit being configured to deliver therapeutic electrical stimulation pulses via the plurality of electrode terminals. The therapy delivery circuit comprising one or more low side switches that can each be selectively enabled for providing a return current path from at least one of the plurality of electrode terminals to an electrical ground of the medical device during delivery of a therapeutic electrical stimulation pulse by the therapy delivery circuit. The control circuit being configured to simultaneously hold each of the one or more low side switches in a non-conducting state and control the therapy delivery circuit to deliver a test pulse to a first electrode terminal of the plurality of electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open. The control circuit may be further configured to measure a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state, determine that a leakage current pathway detection threshold is met based on at least the response signal and generate an alert in response to the leakage current pathway detection threshold being met. The medical device may further include a communication circuit configured to transmit the alert.

[0155] Example 2. The medical device of example 1 wherein the first electrode terminal is coupleable to a cardioversion/defibrillation electrode, and the therapy delivery circuit delivers the test pulse to the cardioversion/defibrillation electrode via the first electrode terminal.

[0156] Example 3. The medical device of any one of examples 1 or 2 wherein the therapy delivery circuit further comprises a high voltage output circuit comprising a high side switch coupled to the first electrode terminal, and the control circuit is further configured to control the therapy delivery circuit to deliver the test pulse by applying a trigger current to the high side switch while each of the one or more low side switches are simultaneously held in the non-conducting state.

[0157] Example 4. The medical device of example 1, wherein the first electrode terminal is coupleable to one of a ring electrode or a tip electrode and the therapy delivery circuit is configured to deliver the test pulse to the one of the ring electrode or the tip electrode via the first electrode terminal.

[0158] Example 5. The medical device of any one of examples 1 - 4 wherein the control circuit is further configured to determine a leakage path impedance based on the response signal and compare the leakage path impedance to the leakage current pathway detection threshold. The control circuit may generate the alert in response to the leakage path impedance being less than the leakage current pathway detection threshold.

[0159] Example 6. The medical device of any one of examples 1 - 5 wherein the therapy delivery circuit further comprises a high voltage output circuit comprising a high side switch coupled to one of the plurality of electrode terminals, wherein the high side switch can be held in a conducting state by a holding current, the leakage current pathway detection threshold being based on the holding current. The control circuit being further configured to measure the response signal as a current signal and determine that the current signal meets the leakage current pathway detection threshold that is based on the holding current.

[0160] Example 7. The medical device of any one of examples 1 - 6 wherein the therapy delivery circuit further comprises a high voltage output circuit comprising a high side switch coupled to one of the plurality of electrode terminals, wherein the high side switch can be held in a conducting state by a holding current. The medical device further includes a memory configured to store a leakage current pathway detection threshold as a threshold impedance that is based on the holding current and an expected voltage amplitude of an electrical stimulation pulse delivered by the therapy delivery circuit. The control circuit being further configured to determine a leakage path impedance based on the response signal, determine that the leakage path impedance is less than the threshold impedance; and generate the alert in response to the leakage path impedance being less than the threshold impedance.

[0161] Example 8. The medical device of any one of examples 1 - 7 wherein the control circuit is further configured to select and a therapy delivery electrode vector excluding the first electrode terminal in response to detecting the leakage current pathway.

[0162] Example 9. The medical device of any one of examples 1 - 8 wherein the control circuit is further configured to disable an electrical stimulation therapy in response to detecting the leakage current pathway.

[0163] Example 10. The medical device of any one of examples 1-9 wherein the plurality of electrode terminals include one or more low impedance electrode terminals each associated with a corresponding low impedance electrode and one or more high impedance electrode terminals each associated with a corresponding high impedance electrode. The control circuit may be further configured to control the therapy delivery circuit to deliver a first test pulse to a first low impedance electrode terminal of the one or more low impedance electrode terminals, enable a first return current path via a first high impedance electrode terminal of the one or more high impedance electrode terminals during the first test pulse and measure a first response signal while the first return current path is enabled. The control circuit may be further configured to control the therapy delivery circuit to deliver a second test pulse to the first high impedance electrode terminal, enable a second return current path via the first low impedance electrode terminal during the second test pulse and measure a second response signal while the second return current path is enabled. The control circuit may detect the leakage current pathway based on a difference between the first response signal and the second response signal.

[0164] Example 11. The medical device of any one of examples 1-10 wherein the communication circuit comprises an antenna, a transceiver having an antenna input and an impedance matching circuit coupled between the antenna and the transceiver and configured to match the impedance of the antenna to the antenna input of the transceiver. The communication circuit may further include a high voltage rated capacitor electrically coupled between the antenna and the impedance matching circuit for filtering electrical stimulation pulses generated by the therapy delivery circuit and received by the antenna. [0165] Example 12. A method comprising delivering a therapeutic electrical stimulation pulse via selected ones of a plurality of electrode terminals when at least one of one or more low side switches of a medical device is selectively enabled for providing a return current path from at least one of the plurality of electrode terminals to an electrical ground of the medical device. The method further includes simultaneously holding each of the one or more low side switches in a non-conducting state and delivering a test pulse to a first electrode terminal of the plurality of electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open. The method may include measuring a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state and determining that a leakage current pathway detection threshold is met based on the response signal. The method may include generating an alert in response to the leakage current pathway detection threshold being met and transmitting the alert.

[0166] Example 13. The method of example 12 further comprising delivering the test pulse to a cardioversion/defibrillation electrode via the first electrode terminal.

[0167] Example 14. The method of any one of examples 12 or 13 further comprising delivering the test pulse by applying a trigger current to a high side switch coupled to the first electrode terminal while each of the one or more low side switches are simultaneously held in the non-conducting state.

[0168] Example 15. The method of example 12 further comprising delivering the test pulse to one of a ring electrode or a tip electrode via the first electrode terminal.

[0169] Example 16. The method of any one of examples 12 - 15 further comprising determining a leakage path impedance based on the response signal and comparing the leakage path impedance to the leakage current pathway detection threshold. The method may further include generating the alert in response to the leakage path impedance being less than the leakage current pathway detection threshold.

[0170] Example 17. The method of any one of examples 12 - 16 further comprising measuring the response signal as a current signal and determining that the current signal meets the leakage current pathway detection threshold, the leakage current pathway detection threshold being based on a holding current required to hold a high side switch in a conducting state, the high side switch being coupled to one of the plurality of electrode terminals.

[0171] Example 18. The method of any one of examples 12 - 17 further comprising storing the leakage current pathway detection threshold as a threshold impedance that is based on a holding current and an expected voltage amplitude of an electrical stimulation pulse delivered by the therapy delivery circuit, the holding current being required to hold a high side switch coupled to one of the plurality of electrode terminals in a conducting state. The method may include determining a leakage path impedance based on the response signal, determining that the leakage path impedance is less than the threshold impedance and generating the alert in response to the leakage path impedance being less than the threshold impedance. [0172] Example 19. The method of any one of examples 12 - 18 further comprising selecting a therapy delivery electrode vector excluding the first electrode terminal in response to detecting the leakage current pathway.

[0173] Example 20. The method of any one of examples 12 - 19 further comprising disabling an electrical stimulation therapy in response to detecting the leakage current pathway.

[0174] Example 21. The method of any one of examples 12 - 20 further comprising delivering a first test pulse to a low impedance electrode terminal of the plurality of electrode terminals, the low impedance electrode terminal being associated with a corresponding low impedance electrode and enabling a first return current path during the first test pulse via a high impedance electrode terminal of the plurality of electrode terminals, the high impedance electrode terminal being associated with a corresponding high impedance electrode. The method further including measuring a first response signal while the first return current path is enabled. The method may further include delivering a second test pulse to the high impedance electrode terminal and enabling a second return current path via the low impedance electrode terminal during the second test pulse. The method may include measuring a second response signal while the second return current path is enabled and determining a difference between the first response signal and the second response signal. The method may further include detecting the leakage current pathway based on the difference between the first response signal and the second response signal.

[0175] Example 22. The method of any one of examples 12 - 21 further comprising transmitting the alert by a transceiver via an impedance matching circuit coupled between an antenna and the transceiver and a high voltage rated capacitor electrically coupled between the antenna and the impedance matching circuit.

[0176] Example 23. A non-transitory computer readable medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to deliver a therapeutic electrical stimulation pulse via selected ones of a plurality of electrode terminals of the medical device when at least one of one or more low side switches of the medical device is selectively enabled for providing a return current path from at least one of the plurality of electrode terminals to an electrical ground of the medical device. The instructions further cause the medical device to simultaneously hold each of the one or more low side switches in a non-conducting state and deliver a test pulse to a first electrode terminal of the plurality of electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open. The instructions may further cause the medical device to measure a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state. The instructions may further cause the medical device to determine that a leakage current pathway detection threshold is met based on the response signal, generate an alert in response to the leakage current pathway detection threshold being met and transmit the alert.

[0177] Example 24. A medical device comprising a plurality of electrode terminals including one or more low impedance electrode terminals each associated with a corresponding low impedance electrode and one or more high impedance electrode terminals each associated with a corresponding high impedance electrode. The medical device further comprising a therapy delivery circuit configured to deliver electrical stimulation pulses via the plurality of electrode terminals and a control circuit configured to control the therapy delivery circuit to deliver a first test pulse to a first low impedance electrode terminal of the one or more low impedance electrode terminals. The control circuit further configured to enable a first return current path via a first high impedance electrode terminal of the one or more high impedance electrode terminals during the first test pulse and measure a first response signal while the first return current path is enabled. The control circuit may further control the therapy delivery circuit to deliver a second test pulse to the first high impedance electrode terminal and enable a second return current path via the first low impedance electrode terminal during the second test pulse. The control circuit may measure a second response signal while the second return current path is enabled. The control circuit may detect a leakage current pathway based on the first response signal and the second response signal and generate an alert in response to detecting the leakage current pathway. The medical device may further include a communication circuit configured to transmit the alert.

[0178] Example 25. The medical device of example 24 wherein the control circuit is further configured to detect the leakage current pathway based on the first response signal and the second response signal by determining a difference between the first response signal and the second response signal, determining that the difference between the first response signal and the second response signal is greater than a difference threshold and detecting the leakage current pathway in response to the difference between the first response signal and the second response signal being greater than the difference threshold. [0179] Example 26. The medical device of any one of examples 24 or 25 wherein the control circuit is further configured to detect the leakage current pathway by determining a first impedance based on the first response signal, determining a second impedance based on the second response signal, determining a difference between the first impedance and the second impedance and determining that the difference between the first impedance and the second impedance is greater than a difference threshold. The control circuit may detect the leakage current pathway in response to the difference between the first impedance and the second impedance being greater than the difference threshold.

[0180] Example 27. A method comprising delivering a first test pulse to a low impedance electrode terminal associated with a corresponding low impedance electrode, enabling a first return current path via a high impedance electrode terminal associated with a corresponding high impedance electrode during the first test pulse and measuring a first response signal while the first return current path is enabled. The method may further include delivering a second test pulse to the high impedance electrode terminal, enabling a second return current path via the low impedance electrode terminal during the second test pulse, measuring a second response signal while the second return current path is enabled and detecting a leakage current pathway based on the first response signal and the second response signal. The method may include generating an alert in response to detecting the leakage current pathway and transmitting the alert.

[0181] Example 28. The method of example 27 wherein detecting the leakage current pathway based on the first response signal and the second response signal comprises determining a difference between the first response signal and the second response signal, determining that the difference between the first response signal and the second response signal is greater than a difference threshold and detecting the leakage current pathway in response to the difference between the first response signal and the second response signal being greater than the difference threshold.

[0182] Example 29. The method of any one of examples 27 or 28 wherein detecting the leakage current pathway comprises determining a first impedance based on the first response signal, determining a second impedance based on the second response signal, determining a difference between the first impedance and the second impedance, determining that the difference between the first impedance and the second impedance is greater than a difference threshold and detecting the leakage current pathway in response to the difference between the first impedance and the second impedance being greater than the difference threshold.

[0183] Example 30. A non-transitory computer readable medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to deliver a first test pulse to a low impedance electrode terminal associated with a corresponding low impedance electrode, enable a first return current path via a high impedance electrode terminal associated with a corresponding high impedance electrode during the first test pulse and measure a first response signal while the first return current path is enabled. The instructions may further cause the medical device to deliver a second test pulse to the high impedance electrode terminal, enable a second return current path via the low impedance electrode terminal during the second test pulse and measure a second response signal while the second return current path is enabled. The instructions may further cause the medical device to detect a leakage current pathway based on the first response signal and the second response signal, generate an alert in response to detecting the leakage current pathway and transmit the alert.

[0184] It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device.

[0185] In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware -based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

[0186] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements.

[0187] Thus, a medical device has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.

Claims

WHAT IS CLAIMED IS:

1. A medical device comprising: a plurality of electrode terminals; a therapy delivery circuit configured to deliver therapeutic electrical stimulation pulses via the plurality of electrode terminals, the therapy delivery circuit comprising one or more low side switches that can each be selectively enabled for providing a return current path from at least one of the plurality of electrode terminals to an electrical ground of the medical device during delivery of a therapeutic electrical stimulation pulse by the therapy delivery circuit; a control circuit configured to: simultaneously hold each of the one or more low side switches in a nonconducting state; control the therapy delivery circuit to deliver a test pulse to a first electrode terminal of the plurality of electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open; measure a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state; determine that a leakage current pathway detection threshold is met based on at least the response signal; and generate an alert in response to the leakage current pathway detection threshold being met; and a communication circuit configured to transmit the alert.

2. The medical device of claim 1 wherein: the first electrode terminal is coupleable to a cardioversion/defibrillation electrode; and the therapy delivery circuit delivers the test pulse to the cardioversion/defibrillation electrode via the first electrode terminal.

3. The medical device of any one of claims 1 or 2 wherein: the therapy delivery circuit further comprises a high voltage output circuit comprising a high side switch coupled to the first electrode terminal; and the control circuit is further configured to control the therapy delivery circuit to deliver the test pulse by applying a trigger current to the high side switch while each of the one or more low side switches are simultaneously held in the non-conducting state.

4. The medical device of claim 1, wherein: the first electrode terminal is coupleable to one of a ring electrode or a tip electrode; and the therapy delivery circuit is configured to deliver the test pulse to the one of the ring electrode or the tip electrode via the first electrode terminal.

5. The medical device of any one of claims 1 - 4 wherein the control circuit is further configured to: determine a leakage path impedance based on the response signal; compare the leakage path impedance to the leakage current pathway detection threshold; and generate the alert in response to the leakage path impedance being less than the leakage current pathway detection threshold.

6. The medical device of any one of claims 1 - 5 wherein: the therapy delivery circuit further comprises a high voltage output circuit comprising a high side switch coupled to one of the plurality of electrode terminals, wherein the high side switch can be held in a conducting state by a holding current, the leakage current pathway detection threshold being based on the holding current; and the control circuit is further configured to: measure the response signal as a current signal; and determine that the current signal meets the leakage current pathway detection threshold that is based on the holding current.

7. The medical device of any one of claims 1 - 6 wherein: the therapy delivery circuit further comprises a high voltage output circuit comprising a high side switch coupled to one of the plurality of electrode terminals, wherein the high side switch can be held in a conducting state by a holding current; and the medical device further comprising a memory configured to store a leakage current pathway detection threshold as a threshold impedance that is based on the holding current and an expected voltage amplitude of an electrical stimulation pulse delivered by the therapy delivery circuit; the control circuit being further configured to: determine a leakage path impedance based on the response signal; determine that the leakage path impedance is less than the threshold impedance; and generate the alert in response to the leakage path impedance being less than the threshold impedance.

8. The medical device of any one of claims 1 - 7 wherein the control circuit is further configured to select and a therapy delivery electrode vector excluding the first electrode terminal in response to detecting the leakage current pathway.

9. The medical device of any one of claims 1 - 8 wherein the control circuit is further configured to disable an electrical stimulation therapy in response to detecting the leakage current pathway.

10. The medical device of any one of claims 1-9 wherein: the plurality of electrode terminals include: one or more low impedance electrode terminals each associated with a corresponding low impedance electrode; and one or more high impedance electrode terminals each associated with a corresponding high impedance electrode: the control circuit is further configured to: control the therapy delivery circuit to deliver a first test pulse to a first low impedance electrode terminal of the one or more low impedance electrode terminals; enable a first return current path via a first high impedance electrode terminal of the one or more high impedance electrode terminals during the first test pulse; measure a first response signal while the first return current path is enabled; control the therapy delivery circuit to deliver a second test pulse to the first high impedance electrode terminal; enable a second return current path via the first low impedance electrode terminal during the second test pulse; measure a second response signal while the second return current path is enabled; and detect the leakage current pathway based on a difference between the first response signal and the second response signal.

11. The medical device of any one of claims 1-10 wherein the communication circuit comprises: an antenna; a transceiver having an antenna input; an impedance matching circuit coupled between the antenna and the transceiver and configured to match the impedance of the antenna to the antenna input of the transceiver; and a high voltage rated capacitor electrically coupled between the antenna and the impedance matching circuit for filtering electrical stimulation pulses generated by the therapy delivery circuit and received by the antenna.

12. A method comprising: delivering a therapeutic electrical stimulation pulse via selected ones of a plurality of electrode terminals when at least one of one or more low side switches of a medical device is selectively enabled for providing a return current path from at least one of the plurality of electrode terminals to an electrical ground of the medical device; simultaneously holding each of the one or more low side switches in a nonconducting state; delivering a test pulse to a first electrode terminal of the plurality of electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open; measuring a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state; determining that a leakage current pathway detection threshold is met based on the response signal; generating an alert in response to the leakage current pathway detection threshold being met; and transmitting the alert.

13. The method of claim 12 further comprising: determining a leakage path impedance based on the response signal; comparing the leakage path impedance to the leakage current pathway detection threshold; and generating the alert in response to the leakage path impedance being less than the leakage current pathway detection threshold.

14. The method of any one of claims 12 - 13 further comprising: storing the leakage current pathway detection threshold as a threshold impedance that is based on a holding current and an expected voltage amplitude of an electrical stimulation pulse delivered by the therapy delivery circuit, the holding current being required to hold a high side switch coupled to one of the plurality of electrode terminals in a conducting state; determining a leakage path impedance based on the response signal; determining that the leakage path impedance is less than the threshold impedance; and generating the alert in response to the leakage path impedance being less than the threshold impedance.

15. A non-transitory computer readable medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to: deliver a therapeutic electrical stimulation pulse via selected ones of a plurality of electrode terminals of the medical device when at least one of one or more low side switches of the medical device is selectively enabled for providing a return current path from at least one of the plurality of electrode terminals to an electrical ground of the medical device; simultaneously hold each of the one or more low side switches in a non-conducting state; deliver a test pulse to a first electrode terminal of the plurality of electrode terminals while each of the one or more low side switches are simultaneously held in the non-conducting state so that a return current path for the test pulse via the plurality of electrode terminals is open; measure a response signal while the test pulse is delivered and each of the one or more low side switches are simultaneously held in the non-conducting state; determine that a leakage current pathway detection threshold is met based on the response signal; generate an alert in response to the leakage current pathway detection threshold being met; and transmit the alert.