WO2007054906A2

WO2007054906A2 - Defibrillator providing a synchronized defibrillation shock

Info

Publication number: WO2007054906A2
Application number: PCT/IB2006/054181
Authority: WO
Inventors: David Snyder; Thomas Lyster
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2005-11-14
Filing date: 2006-11-09
Publication date: 2007-05-18
Also published as: WO2007054906A3; EP1954345A2; JP2009515587A

Abstract

A system and method for synchronizing delivery of shock therapy to a patient from a defibrillator. The period for a patient ECG waveform exhibiting a threshold periodicity is calculated and a time reference during the period of the ECG waveform is determined for delivery of shock therapy to the patient. The defibrillator is prepared for delivery of the shock therapy at the time reference during a subsequent period of the ECG waveform.

Description

DEFIBRILLATOR PROVIDING A SYNCHRONIZED DEFIBRILLATION

SHOCK

The invention relates generally to electrotherapy circuits, and more particularly, to a defibrillator capable of delivering a synchronized defibrillation shock. Defibrillators deliver a high-amplitude current impulse to the heart in order to restore normal rhythm and contractile function in the patients who are experiencing arrhythmia, such as ventricular fibrillation ("VF") or ventricular tachycardia ("VT") that is not accompanied by a palpable pulse. There are several classes of defibrillators, including manual defibrillators, implantable defibrillators, and automatic external defibrillators ("AEDs"). AEDs differ from manual defibrillators in that AEDs can automatically analyze the electrocardiogram ("ECG") rhythm to determine if defibrillation is necessary. In nearly all AED designs, the user is prompted to press a shock button to deliver the defibrillation shock to the patient when a shock is advised by the AED.

Figure 1 is an illustration of a defibrillator 10 being applied by a user 12 to resuscitate a patient 14 suffering from cardiac arrest. In cardiac arrest, otherwise known as sudden cardiac arrest, the patient is stricken with a life threatening interruption to their normal heart rhythm, typically in the form of VF or VT that is not accompanied by a palpable pulse (i.e., shockable VT). In VF, the normal rhythmic ventricular contractions are replaced by rapid, irregular twitching that results in ineffective and severely reduced pumping by the heart. If normal rhythm is not restored within a time frame commonly understood to be approximately 8 to 10 minutes, the patient 14 will die. Conversely, the quicker defibrillation can be applied after the onset of VF, the better the chances that the patient 14 will survive the event. The defibrillator 10 may be in the form of an AED capable of being used by a first responder. The defibrillator 10 may also be in the form of a manual defibrillator for use by paramedics or other highly trained medical personnel.

A pair of electrodes 16 are applied across the chest of the patient 14 by the user 12 in order to acquire an ECG signal from the patient's heart. The defibrillator 10 then analyzes the ECG signal for signs of arrhythmia. IfVF is detected, the defibrillator 10 signals the user 12 that a shock is advised. After detecting VF or other shockable rhythm, the user 12 then presses a shock button on the defibrillator 10 to deliver defibrillation pulse to resuscitate the patient 14.

It is well known in the medical art that certain heart arrhythmias, for example, paroxysmal supraventricular tachycardia, may be treated via delivery of a defibrillation shock to restore sinus rhythm. Furthermore, these shocks should be synchronized to the ECG waveform morphology to minimize the risk of inappropriate induction of VF, a life- threatening arrhythmia. The hemodynamic stability of these rhythms cannot be unambiguously determined from the ECG, and thus, the arrhythmia may or may not be immediately life threatening. Other arrhythmias, for example, VF may also be treated with a synchronized shock, having the benefit of reduced dose requirement (Hsu et al., Circulation 1998;98:808-812; Kidwai et al., J Electrocardiol. 2002;35:235-44).

Conventional shock synchronization has focused on the problem of synchronizing shocks to a broad variety of arrhythmias, both life threatening and non-life threatening. These arrhythmias may be either periodic (complexes repeat at precise time intervals) or non- periodic (interval between complexes is variable). Because of the necessity of accommodating non-periodic arrhythmias, the shocks must be triggered by a real-time morphological feature of the ECG waveform, occurring at some time after initiation of a shock sequence, either by button press or automatically.

Conventional manual synchronization systems which require a skilled operator to determine the urgency of treatment, observe the ECG, and manually adjust the system to identify a morphological feature of the ECG (as indicated on an ECG display) to be used as the synchronization point for delivery of the defibrillation shock. Once the shock sequence is initiated by button press, a similar feature must be identified in real-time as a trigger for delivery of the shock.

Automatic synchronization schemes have also been described (U.S. patent No. 5,507,778 to Freeman; Hsu et al. Circulation. 1998;98:808-812) and implemented in AEDs. Similar to the manually adjusted systems, these also trigger delivery of the shock based on the real-time identification of a particular morphological feature in the patient's ECG. In particular, this morphological feature is typically identified subsequent to initiation of the shock sequence, which occurs either automatically or by button press.

Unfortunately, due to the wide variety of ECG morphology in cardiac arrhythmias, the process of identifying unique morphological synchronization features in real-time is difficult and failure prone. For example, if the desired morphological feature cannot be identified, either an unsynchronized shock is delivered (with attendant risk of VF induction), or the shock is aborted (no treatment delivered). In either case, the risk of patient injury or death is increased.

Therefore, there is a need for an improved system and method for automatic defibrillation shock synchronization that can provide treatment for patients in life-threatening arrhythmias.

One aspect of the invention is a method and system for synchronizing delivery of shock therapy to a patient from a defibrillator. The period for a patient ECG waveform exhibiting a threshold periodicity is calculated and a time reference during the period of the ECG waveform is determined for delivery of shock therapy to the patient. The defibrillator is prepared for delivery of the shock therapy at the time reference during a subsequent period of the ECG waveform.

Another aspect of the invention is a method and system for delivering a defibrillating shock to a patient. The periodicity of an ECG waveform of a patient is analyzed. In response to the ECG waveform exhibiting a threshold periodicity, a time reference at which to deliver the defibrillating shock during a period of the ECG waveform is determined and the defibrillating shock is delivered synchronized with the time reference for a subsequent period of the ECG waveform. In response to the ECG waveform failing to exhibit the threshold periodicity, an unsynchronized defibrillating shock is delivered to the patient.

Another aspect of the invention is a defibrillator having electrodes configured to be electrically coupled to a patient, a high- voltage delivery circuit for generating a defibrillating pulse to be provided through the electrodes, and a defibrillator control circuit. The defibrillator control circuit is coupled to the electrodes and the high-voltage delivery circuit and is configured to analyze a patient ECG waveform and calculate a time during a period of the ECG waveform at which to deliver a defibrillating pulse if the ECG waveform exhibits a threshold periodicity. The defibrillator control circuit controls the high-voltage delivery circuit to deliver the defibrillating pulse to the patient synchronized with the calculated time during a later period of the ECG waveform.

In the drawings:

Figure 1 is an illustration of a defibrillator being applied to a patient suffering from cardiac arrest.

Figure 2 is an illustration of a defibrillator and electrodes in which shock synchronization according to one embodiment of the present invention can be implemented.

Figure 3 is a is a simplified block diagram of the defibrillator of Figure 2.

Figure 4 is a flow diagram of shock synchronization according to an embodiment of the present invention.

Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. Moreover, the particular embodiments of the present invention described herein are provided by way of example and should not be used to limit the scope of the invention to these particular embodiments. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention.

Figure 2 illustrates defibrillator 110 according to an embodiment of the present invention. For purposes of the discussion that follows, the defibrillator 110 is configured as an AED, and is designed for small physical size, light weight, and relatively simple user interface capable of being operated by personnel without high training levels or who otherwise would use the defibrillator 110 only infrequently. In contrast, a paramedic or clinical defibrillator, on the other hand, tends to be larger, heavier, and have a more complex user interface capable of supporting a larger number of manual monitoring and analysis functions. Although the present embodiment of the invention is described with respect to application in an AED, other embodiments include application in different types of defibrillators, for example, manual defibrillators, and paramedic or clinical defibrillators. A pair of electrodes 116 is connected to a connector 126 for insertion into a socket 128 on the defibrillator 110. Located on a top surface of the defibrillator 110 is an on-off switch 118 that activates the defibrillator 110 and begins the process of the prompting the user 12 (Figure 1) to connect the electrodes 116 to the patient 14. A battery condition indicator 120 provides a continual visual indication of the defibrillator status and the available battery charge. A display 122 preferably provides for display of text such as user prompts and graphics such as ECG waveforms. A shock button 124 provides for delivery of the shock to the patient 14 if ECG analysis indicates that a shockable rhythm is present. Administration of defibrillation shocks is done by prompting the user 12 to manually press the shock button 124.

Figure 3 is a simplified block diagram of the defibrillator 110 (Figure 2) according to an embodiment of the present invention. An ECG front end 202 is connected to the pair of electrodes 116 that are connected across the chest of the patient 14. The ECG front end 202 operates to amplify, buffer, filter and digitize an electrical ECG signal generated by the patient's heart to produce a stream of digitized ECG samples. The digitized ECG samples are provided to a controller 206 that performs an analysis to detect VF, shockable VT or other shockable rhythm. If a shockable rhythm is detected, the controller 206 sends a signal to HV delivery 208 to charge-up in preparation for delivering a shock. Pressing the shock button 124 then delivers a defibrillation shock from the HV delivery 208 to the patient 14 through the electrodes 116.

The controller 206 is coupled to further receive input from a microphone 212 to produce a voice strip. The analog audio signal from the microphone 212 is preferably digitized to produce a stream of digitized audio samples which may be stored as part of an event summary 130 in a memory 218. A user interface 214 may consist of the display 122, an audio speaker (not shown), and front panel buttons such as the on-off button 118 and shock button 124 for providing user control as well as visual and audible prompts. A clock 216 provides real-time clock data to the controller 206 for time-stamping information contained in the event summary 130. The memory 218, implemented either as on-board RAM, a removable memory card, or a combination of different memory technologies, operates to store the event summary 130 digitally as it is compiled over the treatment of the patient 14. The event summary 130 may include the streams of digitized ECG, audio samples, and other event data, as previously described.

Figure 4 illustrates a process 400 for delivering a synchronized defibrillation shock if analysis of the ECG indicates a synchronized defibrillation shock should be administered. As previously discussed, the controller 206 (Figure 3) performs analysis to detect VF, shockable VT or other shockable rhythm. At step 410, if a shockable rhythm is detected, the controller 206 commands the HV delivery 208 to prepare for delivery of a defibrillation shock to the patient 14.

The controller 206 performs further analysis of the ECG to determine if a synchronized defibrillation shock is to be delivered, and if so, calculate the timing for delivering a synchronized defibrillation shock to the patient 14 in response to the shock button 124 being pressed. The ECG is analyzed by the controller 206 at step 414 to determine whether the ECG exhibits a suitable level of periodicity. Preferably, the threshold level of periodicity is set to identify ECG signals exhibiting a relatively high periodicity. That is, the threshold level of periodicity should provide sufficient confidence that delivering a synchronized defibrillation shock will be beneficial to the patient 14. Such a determination is well within the skill of those ordinarily skilled in the art. For determining the periodicity of the ECG, a process using an autocorrelation function can be applied by the controller 206 to determine whether the ECG is periodic. In alternative embodiments, the periodicity of the ECG can be determined by applying alternative algorithms and processes which identify a recurrent characteristic of the ECG waveform. Suitable autocorrelation functions and alternative algorithms are known, and those ordinarily skilled in the art have sufficient knowledge to apply these functions and algorithms for determining periodicity of the ECG waveform. Consequently, a more detailed discussion regarding the determination of ECG periodicity has been omitted herein in the interest of brevity.

At step 414, if the controller 206 determines that the ECG is not periodic, the defibrillation shock will be delivered to the patient 14 immediately in response to the shock button 124 being pressed at steps 418 and 422. In contrast, where the ECG exhibits high periodicity, there are significant benefits to synchronizing delivery of a defibrillation shock with a morphological feature of the ECG. Consequently, if the controller 206 determines that the ECG is periodic at step 414, the controller 206 performs further processes to synchronize delivery of the defibrillation shock when the shock button 124 is pressed. As part of the process, the period T of the ECG is calculated by the controller 206 at step 426. Where an autocorrelation function is used at step 414 by the controller 206 to determine periodicity of the ECG waveform, as previously discussed, the period T can also be determined using the same function, as is known. Alternative algorithms or processes can also be used to determine the period T of the ECG as well.

At step 430, the controller further analyzes the periodic ECG to identify a time reference to during a period T of the ECG. The time reference to represents a time during a period T of the ECG at which delivery of a defibrillating shock is synchronized. The time reference t₀ can be based on a morphological feature or an ECG waveform characteristic, such as an amplitude maximum, a first derivative maximum, or a second derivative maximum. In alternative embodiments, the time reference t₀ is selected based on zero crossings of the ECG waveform. In still other embodiments the time reference t₀ is selected based on characteristics other than, or in addition to, a morphological feature of the ECG. In the embodiment shown in Figure 4, the time reference t₀ is identified by analyzing data representing the most recent period T of the ECG. However, it will be appreciated that the data for one or more previous periods T of the ECG can be analyzed by the controller 206 to select a time reference to.

As previously discussed, the time reference to is used to calculate a time at which to deliver a defibrillation shock during a later period T of the ECG in the event the shock button 124 is pressed. Using the time reference to and the calculated period T, occurrence of the time reference to during a following period T of the ECG can be predicted. Generally, the predicted time at which to deliver the defibrillation shock is (t_o+nT), where n is an integer value greater than zero. Thus, identifying a time reference t₀ in a current period T of the ECG (step 430) corresponding to the time at which a synchronized defibrillation shock should be delivered to the patient 14 can be used for actually delivering a synchronized defibrillation shock at a predicted time during a subsequent period T of the ECG. As a result, delivery of the defibrillation shock can be synchronized with the morphological event represented by the time reference t₀. A time offset can also be added to the predicted time to provide the HV delivery 208 with enough time to be activated and triggered to actually deliver the defibrillation shock at the predicted time.

At step 434, until the shock button 124 is pressed, the controller 206 continues to analyze data for the most recent period T of the ECG to identify a time reference t₀ and calculate a predicted time to deliver the defibrillation shock in the event the shock button 124 is pressed. Thus, in response to the shock button 124 being pressed, delivery of the defibrillation shock to the patient 14 will be synchronized with the predicted occurrence of the morphological feature represented by the time reference to in a following period T of the ECG waveform.

By calculating a predicted time at which to deliver the defibrillation shock using data for a prior period T of the ECG, delivery of a synchronized defibrillation shock during a subsequent period T does not rely on real-time ECG waveform analysis. As previously discussed, real-time analysis of the ECG presents can be subject to various uncertainties that may ultimately result in delivering an unsynchronized defibrillation shock or aborting the delivery of a defibrillation shock altogether. For example, conventional systems may have difficulty detecting a trigger event in the ECG in real-time to deliver a synchronized shock for borderline cases. As previously discussed, if the trigger event is not detected by the conventional system, a shock may be delivered, but at the end of a time-out period, or the defibrillation shock may be simply aborted. In contrast, where the ECG exhibits sufficient periodicity a synchronized shock will be delivered at the predicted time following the pressing of the shock button. In the event a period cannot be resolved for the ECG, the system determines that the ECG is not periodic, and delivers the shock immediately. In either case, a shock is delivered sooner than for a conventional system performing real-time analysis and struggling with the borderline ECG to detect a triggering event, which delivers a shock only after the time-out period expires. Embodiments of the present invention address the problem of synchronization failure by focusing on treatment of life-threatening arrhythmias possessing a high degree of periodicity. There are additional benefits resulting from the fact that many VF, especially those associated with short arrest durations, also exhibit high periodicity. Synchronized shocks delivered to these VF rhythms may reduce defibrillation thresholds and therefore dose requirements.

The predictive method previously described can be used in both automatic defibrillators as well as manual mode defibrillators. In manual mode systems, where a shock will be delivered if the shock button is pressed, the predictive method will be able to determine the appropriate time following the pressing of the shock button to deliver the shock for synchronized delivery.

In summary, embodiments of the present invention include a system that monitors a patient's ECG and determines a degree of periodicity, as well as the corresponding rhythm period (interval of ECG complex repetition). A morphological feature of the ECG is identified in a periodic waveform for time reference. If a shock is deemed beneficial (via separate arrhythmia detection system), and a shock sequence is initiated either automatically or by button press, the shock delivery time is based on the periodicity of the ECG. That is, for rhythms with low periodicity, an unsynchronized shock is delivered immediately and for rhythms with high periodicity, the shock is delivered at a time equal to the morphological time reference, plus an integer multiple of the rhythm period, both previously determined.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For instance, in the illustrated sequence of Figure 4, ECG periodicity (step 414) may be performed before the shock advised step 410. After ECG periodicity has been analyzed the process would determine whether a shock is advised. Depending upon the result of the ECG periodicity analysis the subsequent shock advised step may use a particular algorithm. For instance, when high ECG waveform periodicity is determined the shock advised analysis may consider an identified morphological feature of the ECG. Accordingly, the invention is not limited except as by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for synchronizing delivery of shock therapy to a patient from a defibrillator, comprising: calculating a period of a patient ECG waveform exhibiting periodicity; determining a time reference during the period of the ECG waveform for delivery of shock therapy to the patient; and preparing the defibrillator for delivery of the shock therapy at the time reference during a subsequent period of the ECG waveform.

2. The method of claim 1 wherein calculating a period of the ECG waveform comprises applying a process using autocorrelation to determine the period of the ECG waveform.

3. The method of claim 1, further comprising: in response to a manual input, delivering the shock therapy to the patient at the time reference during a period of the ECG waveform subsequent to the manual input.

4. The method of claim 1 , further comprising: automatically delivering the shock therapy to the patient at the time reference during a subsequent period of the ECG waveform.

5. The method of claim 1 wherein determining the time reference at which to deliver the shock therapy comprises identifying a morphological feature of the ECG waveform.

6. The method of claim 5 wherein determining the time reference at which to deliver the shock therapy comprises analyzing at least one of an amplitude maximum, a first derivative maximum, and a second derivative maximum of the ECG waveform.

7. The method of claim 5 wherein determining the time reference at which to deliver the shock therapy comprises analyzing the occurrence of zero crossings in the ECG waveform.

8. The method of claim 1 , further comprising: determining from the ECG waveform whether delivering shock therapy to the patient is advised.

9. A method for delivering a defibrillating shock to a patient, comprising: analyzing periodicity of an ECG waveform of a patient; in response to the ECG waveform exhibiting a threshold periodicity, determining a time reference at which to deliver the defibrillating shock during a period of the ECG waveform and delivering the defibrillating shock synchronized with the time reference for a subsequent period of the ECG waveform; and otherwise delivering an unsynchronized defibrillating shock to the patient.

10. The method of claim 9 wherein delivering the unsynchronized defibrillating shock and delivering the synchronized defibrillating shock are in response to a manual input, the unsynchronized defibrillating shock delivered immediately in response to the manual input and the synchronized defibrillating shock delivered at the time reference for a period of the ECG waveform following receipt of the manual input.

11. The method of claim 9 wherein delivering the unsynchronized defibrillating shock and delivering the synchronized defibrillating shock are delivered automatically.

12. The method of claim 9 wherein analyzing periodicity of the ECG waveform of the patient comprises applying a process using autocorrelation.

13. The method of claim 12, further comprising determining a period of the patent ECG waveform in response to the ECG waveform exhibiting periodicity.

14. The method of claim 9 wherein determining the time reference at which to deliver the defϊbrillating shock comprises identifying a morphological feature of the ECG waveform.

15. The method of claim 9 wherein determining the time reference at which to deliver the defϊbrillating shock comprises analyzing at least one of an amplitude maximum, a first derivative maximum, and a second derivative maximum of the ECG waveform.

16. The method of claim 9 wherein determining the time reference at which to deliver the defϊbrillating shock comprises analyzing the occurrence of zero crossings of the ECG waveform.

17. The method of claim 9, further comprising: determining from the ECG waveform whether a defibrillating shock should be delivered to the patient.

18. A defibrillator, comprising: electrodes configured to be electrically coupled to a patient; a high-voltage delivery circuit coupled to the electrodes and configured to generate a defibrillating pulse and provide the same through the electrodes; and a defibrillator control circuit coupled to the electrodes and the high- voltage delivery circuit, the defibrillator controller configured to analyze a patient ECG waveform, calculate a time during a period of the ECG waveform at which to deliver a defibrillating pulse if the ECG waveform exhibits a threshold periodicity, and control the high-voltage delivery circuit to deliver the defϊbrillating pulse to the patient synchronized with the calculated time during a later period of the ECG waveform.

19. The defibrillator of claim 18, further comprising a user input device coupled to the defibrillator control circuit and wherein the defibrillator control circuit is configured to control the high-voltage delivery circuit to deliver the defϊbrillating pulse synchronized with the calculated time for a later period of the ECG waveform in response to receiving user input.

20. The defibrillator of claim 18 wherein the defibrillator control circuit is configured to calculate the time at which to deliver the defibrillating pulse by identifying a morphological feature of the ECG waveform.

21. The defibrillator of claim 18 wherein the defibrillator control circuit is configured to calculate the time at which to deliver the defibrillating pulse by determining at least one of an amplitude maximum, a first derivative maximum, a second derivative maximum and the occurrence of zero crossings of the ECG waveform.

22. The defibrillator controller of claim 18 wherein the defibrillator control circuit is further configured to analyze the patient ECG waveform for periodicity.

23. The defibrillator of claim 22 wherein the defibrillator control circuit is configured to analyze the patient ECG waveform for periodicity by applying a process using autocorrelation.

24. The defibrillator of claim 22 wherein the defibrillator control circuit is further configured to deliver an unsynchronized defibrillating pulse in response to the ECG waveform failing to exhibit periodicity.

25. The defibrillator of claim 18 wherein the defibrillator control circuit is further configured to analyze the patient ECG waveform and determine whether delivery of a defibrillating pulse is advised.