WO2013186560A1

WO2013186560A1 - Apparatus and method for collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject

Info

Publication number: WO2013186560A1
Application number: PCT/GB2013/051547
Authority: WO
Inventors: Dr Magdi SABA
Original assignee: St George's Hospital Medical School; St George's Healthcare Nhs Trust
Priority date: 2012-06-13
Filing date: 2013-06-12
Publication date: 2013-12-19
Also published as: GB201210491D0

Abstract

An apparatus for collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject. The apparatus comprises a first signal recording unit configured to record signals indicative of temporal changes in electrical data measured at a plurality of electrical sensors positioned at a corresponding plurality of locations on a surface of a living body due to a natural electrical pulse, a second signal recording unit configured to record signals indicative of temporal changes in electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on the surface of the living body due to a stimulated electrical pulse within the living body, a control unit configured to control the apparatus so that both recording units record signals corresponding to substantially the same phase in the subject's respiratory cycle.

Description

APPARATUS AND METHOD FOR COLLECTING DATA TO ASSIST IN THE DETERMINATION OF A SITE OF ORIGIN OF A NATURAL ELECTRICAL PULSE

IN A LIVING SUBJECT

The present invention relates to improving the collection of data that may subsequently be used to determine a site of origin of a natural electrical pulse inside a living body, such as a ventricular tachycardia arrhythmia (VT).

Sudden cardiac death (SCD) afflicts an estimated 450,000 people annually in United States alone. Ninety percent of these events are related to structural heart disease, of which ischemic heart disease represents the majority. Loss of functioning myocardium through infarction leads to a decline in ventricular function and congestive heart failure, and provides the substrate for malignant ventricular tachyarrhythmias.

The recognition that depressed left ventricular systolic function secondary to myocardial infarction dramatically increases the risk of SCD led to the design and execution of several, large, multicenter, randomized trials over the past 15 years the results of which collectively showed a survival benefit conferred by the implantation of an implantable cardioverter- defibrillator (ICD) compared to optimal medical therapy alone. The ICD is now indicated for the primary prevention of SCD in patients with depressed left ventricular systolic function and symptoms of heart failure, and for secondary prevention in patients who have been resuscitated from an episode of SCD.

Ventricular tachycardia (VT) is a frequently-lethal arrhythmia arising from the ventricles that is most commonly associated with cardiac disease, mainly ischemic heart disease and idiopathic cardiomyopathy. With the advent and widespread use of the ICD, many patients are successfully treated for such malignant ventricular tachyarrhythmias, which would have been otherwise fatal. However, as such patients survive these events; both the incidence and prevalence of patients with recurrent ICD shocks for VT are increasing. Strategies to control VT include anti-arrhythmic medications and ablative therapy. The findings of the classic drug trials, specifically CAST, where anti-arrhythmic drugs were administered to suppress complex ventricular ectopy in post-myocardial infarction patients, were disturbing. Such drugs, namely the class I anti-arrhythmic drugs, were associated with increased, not decreased, mortality. It is now contraindicated to use this class of drugs in patients with structural heart disease. Therefore, there is a restricted choice of anti-arrhythmic drugs to use, with limited efficacy and considerable side effect profiles, in an increasing population of patients with VT who are receiving recurrent ICD shocks. Trial results have shown that ICD shocks are associated with increased patient morbidity, hospitalizations, and mortality.

The mechanical interruption of VT circuits in the left ventricular myocardium was first practiced by surgeons guided by cardiac electrophysiologists as subendocardial resection of scarred tissue and aneurysmectomy. Catheter-based techniques soon evolved, due to increasing demand. Currently the ablation of VT is almost solely performed in the electrophysiology laboratory by a cardiac electrophysiologist using a variety of energy sources, such as chemical, thermal, electrical and optical, and mainly by radiofrequency waves and low-temperature (cryo- ablation). However, myriad factors contrive to make catheter ablation of VT the most challenging electrophysiological procedure for a patient to undergo and an electrophysiologist to undertake. In its current state, catheter ablation for VT is indicated as important adjunctive therapy in patients with symptomatic VT in combination with the ICD and anti-arrhythmic drugs.

The most time-consuming step in the VT ablation procedure is the identification of its site of origin (SO). Considerable experience is required to conduct the rapid visual inspection and comparison of multiple electrocardiographs (ECGs) followed by rapid catheter manipulation to successive sites during pace-mapping. In pace-mapping, a stimulated electric pulse is introduced to the myocardium at a specific site using a catheter and the depolarization pulse propagation is monitored on 12 leads of a standard ECG. Automated matching of pace-maps and the VT ECG can be performed by existing software to determine when the myocardium has been stimulated at the VT SO. But, when the myocardium is stimulated at a site other than the VT SO, the matching software provides no data on the VT SO or any guidance as to where to stimulate or otherwise direct attention next to bracket or converge on the VT SO. In such situations, the practitioner must conventionally use their experience to interpret the pace- mapping data before selecting the next site to stimulate. However, even for experienced practitioners this can be difficult.

WO 2007/109406 discloses a cardiographic apparatus in combination with a respiratory gate. This allows a cardiographic dataset limited to identified end-expiration periods to be generated. However, the application does not consider pace-mapping. According to the present invention there is provided, in one aspect, an apparatus for collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject, the apparatus comprising: a natural electrical pulse signal recording unit configured to record signals indicative of temporal changes in electrical data measured at a plurality of electrical sensors positioned at a corresponding plurality of locations on a surface of a living body due to a natural electrical pulse, a stimulated electrical pulse signal recording unit configured to record signals indicative of temporal changes in electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on the surface of the living body due to a stimulated electrical pulse within the living body, a control unit configured to control the apparatus so that both recording units record signals corresponding to substantially the same phase in the subject's respiratory cycle. Preferably the apparatus is a pace-mapping apparatus.

According to this aspect, the invention provides an apparatus that can collect data with reduced variation due to the subject's respiratory cycle. That is, conventional apparatuses collect data without regard to the subject's respiratory cycle, and so changes in the data can be introduced due to the subject breathing, which can make subsequent analysis of the data less accurate. The invention reduces the impact of the subject's breathing on the quality of the data, by synchronising the data collection to occur at a predetermined stage in the subject's respiratory cycle. As such, the collected data corresponds substantially to a single physical position of the heart within the patient' s chest cavity and can thus be more easily interpreted.

In addition, compared to WO 2007/109406 for example, there are additional benefits to using respiratory gating in connection with pace-mapping in particular rather than passively collected ECGs. The use of respiratory gating on a passive ECG may provide clearer signals for a passively collected ECG, as it allows data to be collected at times when the heart in the same position. However, there is an additional benefit when using respiratory gating in pace-mapping, because the signal noise introduced by the relative movement of the heart to the probe providing the stimulus is also reduced. That is, the probe may move with respect to the heart during the respiratory cycle (even if the clinician is attempting to maintain a constant position of the probe), and this will lead to the location of the stimulus being provided to slightly varying positions on the heart over the course of the respiratory cycle. The use of respiratory gating in pace-mapping therefore not only improves the signal quality by collecting data when the heart is in the same position, but it also improves the signal quality by collecting data when the relative position of the probe with respect to the heart is in the same position.

In some embodiments, the signal recording units can be separate units, whilst in other embodiments the signal recording units can be the same unit.

Optionally, the apparatus can be configured to collect data to assist in the determination of a site of origin of a ventricular tachycardia arrhythmia. The electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on a surface of the living body can comprise electrocardiograph (ECG) lead data for a plurality of ECG leads. The natural electrical pulse recording unit can be a recording unit configured to record a ventricular tachycardia template. The stimulated electrical pulse recording unit a recording unit configured to record pace-mapping data. According to these embodiments, the apparatus may be used to improve the quality of data collected during a pace-mapping procedure. As such, the apparatus can compensate for motion of the heart within the chest cavity as the subject breathes, which would conventionally lead to degraded data.

Optionally, the apparatus can further comprise a respiratory phase measurement unit configured to detect the respiratory phase of the living subject. The respiratory phase measurement unit can be configured to detect the respiratory phase via chest-wall impedance or lung volume measurements. The control unit can control one or both of the recording units based on the respiratory phase detected by the respiratory phase measurement unit. According to these features, the apparatus can incorporate a means for detecting a subj ect' s inhalation and exhalation, and that information can be used to control the subsequent collection of data. For example, the control could be performed by implementing the collection of data at specific times that correspond to a particular point in the subject's respiratory cycle.

Optionally, the apparatus can further comprise a device for providing the stimulated electrical pulse. The device for providing the stimulated electrical pulse can comprise a probe. The control unit can be configured to control the provision of the stimulated electrical pulse based on the respiratory phase detected by the respiratory phase measurement unit.

According to another aspect, the invention provides a method of collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject, the method comprising: a first recording step of recording signals indicative of temporal changes in electrical data measured at a plurality of electrical sensors positioned at a corresponding plurality of locations on a surface of a living body due to a natural electrical pulse, a second recording step of recording signals indicative of temporal changes in electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on the surface of the living body due to a stimulated electrical pulse within the living body, and wherein both recording steps record signals corresponding to substantially the same phase in the subject' s respiratory cycle.

According to this aspect, a non-invasive method that pertains to data collection and handling is provided.

The method can be a method of collecting data to assist in the determination of a site of origin of a ventricular tachycardia arrhythmia. The electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on a surface of the living body can comprise electrocardiograph (ECG) lead data for a plurality of ECG leads. The first recording step can comprise recording a ventricular tachycardia template. The second recording step can comprise recording pace-mapping data. The method can further comprise a step of determining the respiratory phase of the living subject. The respiratory phase can be determined via chest-wall impedance or lung volume measurements. The method can further comprise controlling the timing of the first and/or the second recording step based on the respiratory phase detected by the respiratory phase measurement unit.

The method can further comprise a step of initiating the stimulated electrical pulse. The method can further comprise controlling the timing of the initiation of the stimulated electrical pulse based on the detected respiratory phase detected.

Optionally, the method can further comprise using the data recorded in the first and second recording steps to determine the site of origin of the natural electrical pulse. The method can further comprise after determining the site of origin of the natural electrical pulse, applying treatment to the site of origin.

According to another aspect, there is provided a computer program capable of execution by an apparatus for collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject, the computer program being arranged, on execution, to cause the apparatus to perform the method of the preceding aspect.

According to another aspect, there is provided a computer-readable medium carrying one or more sequences of instructions, wherein execution of the one or more sequences of instructions by one or more processors causes the one or more processors to perform the method described above:

The present invention is described below, by way of non-limiting example, with reference to the accompanying Figures, in which:

Fig. 1 is a block diagram that illustrates an example system for determining VT SO in a living subject, according to an embodiment;

Fig. 2 is a diagram that illustrates leads and placement of electrodes for standard electrocardiograph (ECG) measurements;

Fig. 3 is an example ECG trace;

Fig. 4 is a graph that illustrates example stimulated signals for pace mapping a ventricle, according to an embodiment;

Fig. 5 is a graph that illustrates example measurements of a natural VT, according to an embodiment;

Fig. 6 is a graph that illustrates example measurements of how ECG traces vary with breathing; Fig. 7 is a schematic diagram representing how a subject's respiratory cycle can be measured;

Fig. 8 is a flow diagram that illustrates a method of data collection;

Fig. 9 is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented.

The present invention has arisen from the recognition that the interpretation of electrocardiograph (ECG) data during pace-mapping to locate a ventricular tachycardia (VT) site of origin is hampered by the quality of the data. In particular, the ECG data can vary with the phase of the subject' s respiratory cycle. This can be problematic if a VT template is captured during a different phase of a subject's respiratory cycle to that of subsequent pace-maps, because there will be differences in the data that are attributable to the collection during different phases, as well as differences due to selection of the pace-mapping site. Conventionally, practitioners have not been aware of the differences that can be introduced by the subject' s breathing, and the practitioner must rely on their experience to interpret the data to identify a VT SO. However, the present invention has identified that the data collection can be improved, to make it easier for the practitioner to interpret pace-mapping data. As such, the present invention relates to the technical implementation of the data collection.

In the following description, techniques are described for determining the site of origin for a natural electrical pulse inside a living body. For the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.

Fig. 1 is a block diagram that illustrates an example system 100 for assisting in the determination of a site of origin of a natural electrical pulse in a living subject, such as a pace- mapping system. The system 100 includes an electrocardiograph (ECG) system 120, a probe system 140 and a computer system 150. The system 100 operates on a patient 190, who is a living subject, such as an animal or human. Although depicted for purposes of illustration, the patient 190 is not part of the system 100.

ECG system 120 includes lead electrodes 122 that provide electrically conducting contact to a surface of a living body. The lead electrodes are connected by electrically conducting wires to an ECG recorder 124. The ECG recorder 124 records traces (on paper called

electrocardiograms, or in digital files, or both) that indicate electrical signals received at or between the lead electrodes 122. The ECG recorder functions as a natural electrical pulse signal recording unit and as a stimulated electrical pulse recording unit, as discussed below. A standard ECG system generates twelve traces, called leads, based on six uni-polar lead electrodes 122 and three bi-polar lead electrodes 122. A bipolar lead determines a difference in electrical voltage between two electrodes. By convention, a positive electrode is one in which the ECG records a positive (upward) deflection when the measured electrical impulse flows toward it and a negative (downward) deflection when it flows away from it. For a uni-polar lead, the electrical potential at an exploring electrode is compared to a reference point that averages electrical activity, rather than to that of another electrode. The single electrode of a uni-polar lead, termed the exploring electrode, is the positive electrode. In some embodiments, one or more steps of ECG recorder 124 are performed by an ECG process, not shown, on computer system 150.

The support table 1 10 supports the patient 190. The patient 190 includes a heart ventricle 192 part of a heart in the patient 190.

Probe system 140 includes a probe 142, a catheter 143 and a probe controller 144. In the illustrated embodiment, the probe system 140 includes probe position sensor 146a and probe position sensor 146b (collectively referenced hereinafter as probe positions sensors 146), and probe measurement process 154 on computer system 150.

The probe 142 is any device that is inserted into a living body for any reason, such as an ablating electrophysiological tip, well known in the art, for measuring voltage in the heart and generating lesions in the heart to change electrical conductance associated with arrhythmia. For example, the probe 142 is depicted in the heart ventricle 192 of patient 190. The probe 142 includes a probe electrode for introducing an electrical stimulation signal to tissue in contact with the probe electrode. An electrical pulse propagates from the probe in response to such a stimulation signal. For example, a direction of pulse propagation 193 as a result of a stimulation signal from probe 142 in contact with a wall of the heart ventricle 192 is depicted in Fig. 1.

The probe controller 144 is any device that is used to control operation of the probe, such as hand held manipulators that control the movement of the probe and control probe operations, such as stimulation, measurement and ablation.

The catheter 143 is a tube inserted into a lumen of the living subject, such as a blood vessel, through which the probe is passed to a particular location in the patient. Inside the catheter 143 are one or more control lines for connecting the probe to the probe controller 144. In other embodiments, the catheter is replaced by any tether that ties the probe to a device located outside the living subject and used to control the probe. In some embodiments the catheter is replaced by a wireless communication link between the probe 142 inside the patient and the probe controller 144 outside the patient. In some embodiments, the probe system includes one or more probe positioning sensors, such as probe positioning sensors 146. Probe positioning sensors 146 determine the three dimensional position of probe 142 using any method known in the art, such as measuring strength of electromagnetic induction from an electrical source in the probe 142. A probe positioning process, such as a process executing on probe controller 144 or computer system 150, uses triangulation or other algorithms to deduce probe position from the measurements made at position sensors 146. Well known probe positioning systems for an electrophysiological catheter tip include CARTO™ provided by Biosense Webster, Inc. of Diamond Bar, California and NAVX™ provided by St. Jude Medical of Sylmar, California.

A probe measurement process, such as probe measurement process 154 on computer system 150, determines conditions in patient 190 based on measurements made by probe 142. In some embodiments, probe measurement process 154 includes the probe positioning process, described above. For example, in some embodiments, probe measurement process 154 determines the action potential on an inner surface of the heart based on voltage measurements made over one or more heart cycles at probe 142, a probe position determined based on sensors 146, patient position (e.g., based on markers attached to the patient) and a model of the heart of patient 190 based on generic features or pre-operative internal scans of the patient. In some embodiments, such action potential is stored as a three dimensional (3D) electro-anatomic map of all or a portion of the heart and is presented as a coloured area on a cartoon representation of a heart in a two dimensional screen image displayed to a human operator of probe controller 144. The probe position relative to the model heart is estimated using any of several estimation processes that are well known in the art.

Fig. 2 is a block diagram that illustrates leads and placement of electrodes for standard electrocardiograph (ECG) measurements. For reference, a patient 290 is indicted by a drawing with a mid-clavicular line 291, an anterior axillary line 292 and a mid- axillary line 293.

Electrodes for bipolar leads are placed at the upper right arm (RA) 210a, the upper left arm (LA) 210b and the left foot (LF) 210c. These same electrodes are also processed as uni-polar leads, as described below. Electrodes for uni-polar leads are placed at six locations on the chest indicated by VI 210d, V2 210e, V3 21 Of, V4 210g on mid-clavicular line 291, V5 21 Oh on anterior axillary line 292 and V6 210i on the mid-axillary line 293. In some embodiments, the surface electrodes are placed as depicted in Fig. 2. In other embodiments, more or fewer electrodes are placed at zero or more of the positions depicted in Fig. 2.

The standard 12-lead ECG provides spatial information about the heart's electrical activity in 3 approximately orthogonal directions: patient right to left; patient head to toe (superior to inferior); and patient front to back (anterior to posterior). Bipolar lead I is based on the difference between electrode RA 210a and electrode LA 210b; and indicates the propagation 21 la of pulses from patient right to left. Bipolar lead II is based on the difference between electrode RA 210a and electrode LF 210c; and indicates the propagation 21 lb of pulses from superior to inferior (with minor influence for right to left). Bipolar lead III is based on the difference between electrode LA 210b and electrode LF 210c; and indicates the propagation 21 lc of pulses from superior to inferior (with minor influence for left to right). Augmented unipolar limb leads (frontal plane) are designated lead aVR, lead aVL and lead aVF; and, are based on average measurements at RA 210a, LA 210b and LF 210c. Lead aVR indicates the rightward propagation 21 Id of pulses perpendicular to lead III. Lead aVL indicates the leftward propagation 21 le of pulses perpendicular to lead II. Lead aVF indicates the inferior- ward propagation 21 If of pulses perpendicular to lead I. The positive uni-polar chest leads indicate propagation from the heart in a cross-sectional (horizontal) plane through the heart. Leads VI, V2, V3 from electrodes VI 21 Od, V2 21 Oe, V3 21 Of, respectively, indicate propagation in the posterior to anterior direction (negative changes indicate the opposite direction). Leads V4, V5, V6 from electrodes V4 210g, V5 21 Oh, V6 210i, respectively, indicate propagation in the lateral right to left direction (negative changes indicate the opposite direction).

Actual measurements at the standard 12 lead configuration of electrodes vary from patient to patient, depending on the location and direction of the electrical pulses inside the patient, and the size and location and electrical properties of the tissues in the patient.

Fig. 3 shows an example ECG trace, indicating the peaks and troughs P, Q, R, S and T that are conventionally used to describe an ECG trace. In an ECG of a normal patient, heart beat (pulse rate) lies between 60 and 100 beats/minute. Rhythm is regular except for minor variations with respiration. A P-R interval is the time required for completion of aerial depolarization, conduction through the heart tissue, and arrival at the ventricular myocardial cells. The normal P-R interval is 0.12 to 0.20 seconds. The QRS interval represents the time required for ventricular cells to depolarize. The normal duration is 0.06 to 0.10 seconds. The Q-T interval is the time required for depolarization and repolarisation of the ventricles. The time required is proportional to the heart rate. The faster the heart rate, the faster the repolarisation, and therefore the shorter the Q-T interval. With slow heart rates, the Q-T interval is longer. The Q-T interval represents about 40% of the total time between the QRS complexes. In most cases, the Q-T interval lasts between 0.34 and 0.42 seconds.

Ventricular tissue is capable of spontaneous depolarization. When this occurs, a premature ventricular contraction (PVC) is initiated. Because the depolarization wave arises in the myocardium, it usually does not follow the normal path of ventricular depolarization. Therefore, the QRS complex is prolonged and unusual in shape. Ventricular Tachycardia (VT) is defined as a run of 3 or more PVCs.

To determine the source of VT, a probe is used to stimulate the heart once per heartbeat for one or more heartbeats at each of several locations in the ventricle of interest. This process is called pace-mapping. The 12-lead ECG of the VT (also referred to as a VT template) is compared to each pace-mapped 12 lead ECG (also referred to as a pace-map). When a match is found, it is determined that the stimulated site is the VT SO. When there is no match, however, a new site must be selected for pace-mapping, using whatever information the practitioner can infer from the previous pace-map(s) and the VT template. It can take an electrophysiologist tens of pace-mapping locations and several hours to find the VT SO.

Fig. 4 is a graph that illustrates example stimulated signals for pace mapping a ventricle, according to an embodiment. The horizontal axis 302 indicates time, with the large tick marks separated by 0.1 seconds and the small tick marks by 0.01 seconds. Figure 4 includes plots of multiple traces, each offset vertically by a different amount to avoid confusion, and all sharing the same horizontal time axis 302. Vertical axis 304 indicates the change in a measurable physical phenomenon, such as voltage, pressure, from some fixed value.

Trace 310, at the bottom, indicates a stimulation signal input to a probe, e.g., probe 142, to cause a depolarization at a location on a ventricle wall. The stimulation pulse is repeated at a rate indicated by beat interval 333.

Trace 31 1 indicates patient blood pressure during the stimulation. Horizontal line 312 provides a vertical origin for the blood pressure trace 311.

Trace 313 indicates electrical voltage measured at the probe tip, e.g. at the tip of probe 142. Horizontal Line 314 indicates a voltage measured at a proximal bipolar electrode. Trace 313 indicates that the ventricle wall is depolarized upon stimulation and then gradually re- establishes polarization after a few tenths of a second.

Traces 315 are the 2 local bipolar electrogram channels from the right ventricular chamber-a distal pair at the tip of the probe and a more proximal pair father up on the shaft of the catheter (e.g., on catheter 143 father from the probe 142).

The remaining traces indicate the 12 standard lead measurements. Traces 320a, 320b, 320c, 320d, 320e, 320f, 320g, 320h, 320i, 320j, 320k, 3201 (collectively referenced hereinafter as traces 320) depicted voltage measurements at leads I, II, III, aVR, aVL, aVF, VI, V2, V3, V4, V5, V6, respectively, of a standard 12-lead ECG.

The time of the stimulated pulse is indicated by vertical line to 330a. Also depicted is a time tl 330b, shortly after time to 330a. In the illustrated embodiment, time tl 330b is 0.08 seconds after time to 330a. It can be seen that in the interval from time to time tl, some leads present a large increase in voltage (e.g., lead VI 320g), some leads present a large decrease in voltage (e.g., leads V2 320h, V3 320i and V4 320j) and some leads express little change (e.g., lead II 320b and lead aVF 320f).

Fig. 5 is a graph that illustrates example measurements of a natural VT (or a 'VT template'), according to an embodiment. The horizontal axis 402 indicates time, with the large tick marks separated by 0.1 seconds and the small tick marks by 0.01 seconds. Figure 5 includes plots of multiple traces, each offset vertically by a different amount to avoid confusion, and all sharing the same horizontal time axis 402. Vertical axis 404 indicates the change in a measurable physical phenomenon, such as voltage, from some fixed value. The natural heart beat is indicated by beat interval 433.

The traces indicate the 12 standard lead measurements for the natural VT. Traces 420a, 420b, 420c, 420d, 420e, 420f, 420g, 420h, 420i, 420j, 420k, 4201 (collectively referenced hereinafter as traces 420) depicted voltage measurements at leads I, II, III, aVR, aVL, aVF, VI, V2, V3, V4, V5, V6, respectively, of a standard 12-lead ECG

The time of the QRS start is indicated by vertical line to 430a. Also depicted is a time tl

430b, shortly after time tl 430a. In the illustrated embodiment, time tl 430b is 0.08 seconds after time to 430a. It can be seen that in the interval from time tO to time tl, some leads present a large increase in voltage (e.g., lead aVL 420e), some leads present a large decrease in voltage (e.g., leads aVF 420f, V2 420h and V3 420i) and some leads express little change (e.g., lead aVR 420d). These expressions differ at several leads from those expressed in Fig. 4.

Because the two 12-lead ECGs of FIGs 4 and 5 do not match, the site of the pace map for traces 320 is unlikely to be the VT SO. However, some differences in the traces may be due to the two ECGs being taken at different stages in the subj ect's respiratory cycle.

During inhalation, a patient' s diaphragm contracts, causing the diaphragm to flatten and thereby increase the volume of the thoracic cavity. As the lungs fill, the patient's chest also swells, such that a reclining patient' s chest will be seen to rise. During exhalation, the opposite processes occur: the diaphragm relaxes and domes back into the thoracic cavity, whilst the lungs empty and the patient' s chest deflates.

Within the thoracic cavity, the heart also moves during the respiratory cycle, as it is partly supported by the moving diaphragm and is surrounded by the inflating and deflating lungs. In fact, as the patient inhales, the heart moves downwards with the diaphragm and also moves further away from the surface of the chest, as the patient' s chest swells. As the patient exhales, the heart moves back up within the thoracic cavity and closer to the surface of the chest. As the heart moves, the relative position of the probe with respect to the heart can also move. The present invention has identified that these movements affect the ECG readings that are obtained during pace-mapping. This is because the ECG leads are applied to positions on the surface of the body, including the patient' s chest. Therefore, the relative position of those leads to the heart will vary during inhalation and exhalation, as the heart moves within the chest. This, in turn, causes variations in the electrical signal detected by the ECG leads which are not due to variations in the signals generated by the heart during pace-mapping, but which are simply due to the different path the signals have travelled between the heart and the ECG lead. Also, as the position of the probe with respect to the heart moves, slightly different signals will be recorded anyway, irrespective of the motion of the heart in the chest. As such, differences in ECG traces are made more difficult to interpret because the differences being looked for are at least partly masked by differences introduced by the heart' s motion and the probe's motion.

Fig. 6 shows an example of how breathing can change an ECG reading. The data shows traces from pace-mapping under two different conditions. Whilst maintaining the probe in contact with the same location in the ventricle, pacing was performed during normal breathing and then during deep inspiration. A visually appreciable difference in QRS morphology can be seen. This difference has not previously been appreciated, and makes it difficult to accurately interpret pace-map data.

To overcome this problem, ECG data can be collected only during a certain phase of the respiratory cycle. By only collecting data at the same stage of the respiratory cycle, differences in received ECG data due to the heart's motion and the probe's motion are minimised because the heart and probe are in substantially the same position whenever data is collected. Whilst any phase of the respiratory cycle can be used, it is preferable to collect data at the point of deepest exhalation, as this will be the point at which the heart is physically closest to the surface of the patient' s chest (and therefore also closest to the ECG leads connected to the chest).

The collection of the data at only a specific stage of the respiratory cycle can be implemented in many ways. For example, the data received by the ECG controller 124 can be 'gated', so that only data from a pre-defined stage in the respiratory cycle is recorded.

Alternatively, the probe 142 can be controlled to produce the electrical stimulation at a predetermined stage of respiration. In some cases a combination of methods might be used, for example by gating the data received by the controller 124 when collecting the ECG template, and controlling the production of the electrical stimulation by the probe 142 during the pace- mapping.

The control of the data collection process is implemented by a control unit, which can be a specific controller, such as probe controller 144, or can be part of the computer system 150 for example. However, in order to implement the control, information regarding the patient' s respiratory cycle is needed. This can be obtained by any suitable method, such as chest wall impedance for example. Fig.7 illustrates how chest wall impedance data collection may be performed. For the sake of clarity, the ECG system is not shown, but would also be present (for example, as depicted in Fig. 1). Sensors 510 are attached to the front and back of the chest of the patient 190. In the depicted example, the sensors are connected to the computer system 150, which acts as the control unit, controlling the probe and/or the ECG recorder. By measuring changes in current transmitted through the patient between sensors 510, the patient' s state of inhalation or exhalation can be determined. The greatest impedance (or resistance to passing current) is measured at the point of maximum inhalation, and the lowest impedance is measured at the point of maximum exhalation. Therefore, changes in impedance can be used to infer changes in the patient' s respiratory state. The impedance data collection and analysis is performed, in this example, by computer system 150. The analysis can then be used to control the collection of ECG data.

The determination of the respiratory phase does not have to be performed using impedance, according to the present invention. For example, if a patient is under anaesthesia, their breathing will be monitored and controlled and measured lung volumes may provide an alternative source of data indicative of the patient' s respiratory cycle. Another alternative would be the use of magnetic field interference to measure the change in position of the patient' s chest.

Fig. 8 shows a flow chart illustrating the method of ECG and pace-mapping data collection. In step 520, the patient' s respiratory cycle is monitored. This allows the collection of further data to be controlled based on the stage of respiratory cycle. The monitoring in step 520 can be performed by any suitable means, as previously discussed.

In step 530, the initial ECG template, based upon the heart' s natural rhythm is recorded. The data is collected at a predetermined stage in the patient' s respiratory cycle. As discussed above, this could be any stage in the cycle, but is preferably at the point of deepest exhalation.

In step 540, pace-mapping data is collected, which data is indicative of an artificial heart stimulation at a particular point in the heart. The data is collected for the same predetermined stage in the respiratory cycle as in step 530. As such, variations in the collected data due to changes in the heart' s position are reduced.

Step 550 is an optional step that is not required to perform the invention. The invention primarily resides in the method and apparatus for collecting an improved data set, in which variations due to external factors are reduced. Nonetheless, the data can subsequently be used to assist in identifying the position of a VT, and the improved data set allows for the VT to be more easily identified. Fig. 8 is a block diagram that illustrates a computer system 700 upon which an embodiment of the invention may be implemented. Computer system 700 includes a

communication mechanism such as a bus 710 for passing information between other internal and external components of the computer system 700. Information is represented as physical signals of a measurable phenomenon, typically electric voltages, but including, in other embodiments, such phenomena as magnetic, electromagnetic, pressure, chemical, molecular atomic and quantum interactions. For example, north and south magnetic fields, or a zero and non-zero electric voltage, represent two states (0, 1) of a binary digit (bit). A sequence of binary digits constitutes digital data that is used to represent a number or code for a character. A bus 710 includes many parallel conductors of information so that information is transferred quickly among devices coupled to the bus 710. One or more processors 702 for processing information are coupled with the bus 710. A processor 702 performs a set of operations on information. The set of operations include bringing information in from the bus 710 and placing information on the bus 710. The set of operations also typically include comparing two or more units of information, shifting positions of units of information, and combining two or more units of information, such as by addition or multiplication. A sequence of operations to be executed by the processor 702 constitutes computer instructions.

Computer system 700 also includes a memory 704 coupled to bus 710. The memory 704, such as a random access memory (RAM) or other dynamic storage device, stores information including computer instructions. Dynamic memory allows information stored therein to be changed by the computer system 700. RAM allows a unit of information stored at a location called a memory address to be stored and retrieved independently of information at neighbouring addresses. The memory 704 is also used by the processor 702 to store temporary values during execution of computer instructions. The computer system 700 also includes a read only memory (ROM) 706 or other static storage device coupled to the bus 710 for storing static information, including instructions, that is not changed by the computer system 700. Also coupled to bus 710 is a non-volatile (persistent) storage device 708, such as a magnetic disk or optical disk, for storing information, including instructions, that persists even when the computer system 700 is turned off or otherwise loses power.

Information, including instructions, is provided to the bus 710 for use by the processor from an external input device 712, such as a keyboard containing alphanumeric keys operated by a human user, or a sensor. A sensor detects conditions in its vicinity and transforms those detections into signals compatible with the signals used to represent information in computer system 700. Other external devices coupled to bus 710, used primarily for interacting with humans, include a display device 714, such as a cathode ray tube (CRT) or a liquid crystal display (LCD), for presenting images, and a pointing device 716, such as a mouse or a trackball or cursor direction keys, for controlling a position of a small cursor image presented on the display 714 and issuing commands associated with graphical elements presented on the display 714.

In the illustrated embodiment, special purpose hardware, such as an application specific integrated circuit (IC) 720, is coupled to bus 710. The special purpose hardware is configured to perform operations not performed by processor 702 quickly enough for special purposes.

Examples of application specific ICs include graphics accelerator cards for generating images for display 714, cryptographic boards for encrypting and decrypting messages sent over a network, speech recognition, and interfaces to special external devices, such as robotic arms and medical scanning equipment that repeatedly perform some complex sequence of operations that are more efficiently implemented in hardware.

Computer system 700 also includes one or more instances of a communications interface 770 coupled to bus 710. Communication interface 770 provides a two-way communication coupling to a variety of external devices that operate with their own processors, such as printers, scanners and external disks. In general the coupling is with a network link 778 that is connected to a local network 780 to which a variety of external devices with their own processors are connected. For example, communication interface 770 may be a parallel port or a serial port or a universal serial bus (USB) port on a personal computer. In some embodiments, communications interface 770 is an integrated services digital network (ISDN) card or a digital subscriber line (DSL) card or a telephone modem that provides an information communication connection to a corresponding type of telephone line. In some embodiments, a communication interface 770 is a cable modem that converts signals on bus 710 into signals for a communication connection over a coaxial cable or into optical signals for a communication connection over a fibre optic cable. As another example, communications interface 770 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN, such as Ethernet. Wireless links may also be implemented. Carrier waves, such as acoustic waves and electromagnetic waves, including radio, optical and infrared waves travel through space without wires or cables. Signals include man-made variations in amplitude, frequency, phase, polarization or other physical properties of carrier waves. For wireless links, the communications interface 770 sends and receives electrical, acoustic or electromagnetic signals, including infrared and optical signals that carry information streams, such as digital data.

The term computer-readable medium is used herein to refer to any medium that participates in providing information to processor 702, including instructions for execution. Such a medium may take many forms, including, but not limited to, non- volatile media, volatile media and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 708. Volatile media include, for example, dynamic memory 704. Transmission media include, for example, coaxial cables, copper wire, fibre optic cables, and waves that travel through space without wires or cables, such as acoustic waves and

electromagnetic waves, including radio, optical and infrared waves.

Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, a magnetic tape, or any other magnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD) or any other optical medium, punch cards, paper tape, or any other physical medium with patterns of holes, a RAM, a programmable ROM (PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memory chip or cartridge, a carrier wave, or any other medium from which a computer can read.

Network link 778 typically provides information communication through one or more networks to other devices that use or process the information. For example, network link 778 may provide a connection through local network 780 to a host computer 782 or to equipment 784 operated by an Internet Service Provider (ISP). ISP equipment 784 in turn provides data communication services through the public, world-wide packet-switching communication network of networks now commonly referred to as the Internet 790. A computer called a server 792 connected to the Internet provides a service in response to information received over the Internet. For example, server 792 provides information representing video data for presentation at display 714.

The computer system 700 can be used for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system 700 in response to processor 702 executing one or more sequences of one or more instructions contained in memory 704. Such instructions, also called software and program code, may be read into memory 704 from another computer-readable medium such as storage device 708. Execution of the sequences of instructions contained in memory 704 causes processor 702 to perform the method steps described herein. In alternative embodiments, hardware, such as application specific integrated circuit 720, may be used in place of or in combination with software to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware and software.

The signals transmitted over network link 778 and other networks through

communications interface 770, carry information to and from computer system 700. Computer system 700 can send and receive information, including program code, through the networks 780, 790 among others, through network link 778 and communications interface 770. In an example using the Internet 790, a server 792 transmits program code for a particular application, requested by a message sent from computer 700, through Internet 790, ISP equipment 784, local network 780 and communications interface 770. The received code may be executed by processor 702 as it is received, or may be stored in storage device 708 or other non-volatile storage for later execution, or both. In this manner, computer system 700 may obtain application program code in the form of a signal on a carrier wave.

Various forms of computer readable media may be involved in carrying one or more sequence of instructions or data or both to processor 702 for execution. For example, instructions and data may initially be carried on a magnetic disk of a remote computer such as host 782. The remote computer loads the instructions and data into its dynamic memory and sends the instructions and data over a telephone line using a modem. A modem local to the computer system 700 receives the instructions and data on a telephone line and uses an infra-red transmitter to convert the instructions and data to a signal on an infra-red a carrier wave serving as the network link 778. An infrared detector serving as communications interface 770 receives the instructions and data carried in the infrared signal and places information representing the instructions and data onto bus 710. Bus 710 carries the information to memory 704 from which processor 702 retrieves and executes the instructions using some of the data sent with the instructions. The instructions and data received in memory 704 may optionally be stored on storage device 708, either before or after execution by the processor 702.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made by the skilled addressee. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims

An apparatus for collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject, the apparatus comprising:

a natural electrical pulse signal recording unit configured to record signals indicative of temporal changes in electrical data measured at a plurality of electrical sensors positioned at a corresponding plurality of locations on a surface of a living body due to a natural electrical pulse,

a stimulated electrical pulse signal recording unit configured to record signals indicative of temporal changes in electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on the surface of the living body due to a stimulated electrical pulse within the living body,

a control unit configured to control the apparatus so that both recording units record signals corresponding to substantially the same phase in the subject' s respiratory cycle.

An apparatus according to claim 1, wherein the apparatus is configured to collect data to assist in the determination of a site of origin of a ventricular tachycardia arrhythmia.

An apparatus according to claim 1 or 2, wherein the electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on a surface of the living body comprises electrocardiograph (ECG) lead data for a plurality of ECG leads.

An apparatus according to any one of the preceding claims, wherein the natural electrical pulse recording unit is a recording unit configured to record a ventricular tachycardia template.

An apparatus according to any one of the preceding claims, wherein the stimulated electrical pulse recording unit a recording unit configured to record pace-mapping data.

An apparatus according to any one of the preceding claims, further comprising a respiratory phase measurement unit configured to detect the respiratory phase of the living subject.

An apparatus according to claim 6, wherein the respiratory phase measurement unit is configured to detect the respiratory phase via chest-wall impedance or lung volume measurements.

8. An apparatus according to claim 6 or 7, wherein the control unit is configured to control one or both of the recording units based on the respiratory phase detected by the respiratory phase measurement unit.

9. An apparatus according to any one of the preceding claims, further comprising a device for providing the stimulated electrical pulse.

10. An apparatus according to claim 9, wherein the device for providing the stimulated electrical pulse comprises a probe.

11. An apparatus according to claim 9 or 10, wherein the control unit is configured to control the provision of the stimulated electrical pulse based on the respiratory phase detected by the respiratory phase measurement unit.

12. A method of collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject, the method comprising:

a first recording step of recording signals indicative of temporal changes in electrical data measured at a plurality of electrical sensors positioned at a corresponding plurality of locations on a surface of a living body due to a natural electrical pulse,

a second recording step of recording signals indicative of temporal changes in electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on the surface of the living body due to a stimulated electrical pulse within the living body, and

wherein both recording steps record signals corresponding to substantially the same phase in the subject's respiratory cycle.

13. A method according to claim 12, wherein the method is a method of collecting data to assist in the determination of a site of origin of a ventricular tachycardia arrhythmia.

14. A method according to claim 12or 13, wherein the electrical data measured at the plurality of electrical sensors positioned at the corresponding plurality of locations on a surface of the living body comprises electrocardiograph (ECG) lead data for a plurality of ECG leads.

15. A method according to any one of claims 12 to 14, wherein the first recording step comprises recording a ventricular tachycardia template

16. A method according to any one of the claims 12 to 15, wherein the second recording step comprises recording pace-mapping data.

17. A method according to any one of claims 12 to 16, further comprising a step of determining the respiratory phase of the living subject.

18. A method according to claim 17, wherein the respiratory phase is determined via chest-wall impedance or lung volume measurements.

19. A method according to claim 17 or 18, further comprising controlling the timing of the first and/or the second recording step based on the respiratory phase detected by the respiratory phase measurement unit.

20. A method according to any one of the claims 12 to 19, further comprising a step of initiating the stimulated electrical pulse.

21. A method according to claim 20, further comprising controlling the timing of the initiation of the stimulated electrical pulse based on the detected respiratory phase detected.

22. A method according to any one of claims 12 to 21, further comprising using the data

recorded in the first and second recording steps to determine the site of origin of the natural electrical pulse.

23. A method according to claim 22, further comprising, after determining the site of origin of the natural electrical pulse, applying treatment to the site of origin.

24. A computer program capable of execution by an apparatus for collecting data to assist in the determination of a site of origin of a natural electrical pulse in a living subject, the computer program being arranged, on execution, to cause the apparatus to perform the method of any one of claims 12 to 22

25. A computer-readable medium carrying one or more sequences of instructions, wherein

execution of the one or more sequences of instructions by one or more processors causes the one or more processors to perform the method of any one of claims 12 to 22: