Implantable cardiac stimulating device and method for administering synchronized cardioversion shock therapy to provide preemptive depol

What is claimed is:

1. An implantable cardioversion shock therapy system, comprising:

sensing means for sensing intrinsic R-waves;

pulse generating means for generating a cardioversion shock and for delivering the cardioversion shock to cardiac tissue; and

control means, responsive to the sensing means, for analyzing the R-waves to confirm a tachycardia episode, and for controlling the pulse generating means to trigger the cardioversion shock after a predetermined delay corresponding to a period when the cardiac tissue is repolarized, the predetermined delay being synchronized from a last R-wave following the confirmation of the tachycardia episode so that the cardioversion shock preempts a successive R-wave.

2. The system of claim 1, further comprising:

an external programming means for sending system operating parameters to and receiving data from the control means;

telemetry means, coupled to the control means, for receiving the system operating parameters, including the predetermined delay, from the external programming means.

3. The system of claim 1, wherein the control means further comprises:

means for defining the predetermined delay as a predetermined fixed time interval which is expected to occur when the cardiac tissue is repolarized.

4. The system of claim 3, wherein the predetermined fixed time interval is at least greater than 300 msec.

5. The system of claim 3, wherein:

the control means further includes means for determining a tachycardia cycle length; and

the predetermined fixed time interval is a programmable value between 150 ms and the tachycardia cycle length.

6. The system of claim 1, wherein the control means further comprises:

means for determining a tachycardia cycle length; and

means for defining the predetermined delay as a predetermined percentage of the tachycardia cycle length which is expected to occur when the cardiac tissue is repolarized.

7. The system of claim 6, wherein the predetermined percentage of the tachycardia cycle length is in the range of 30-99% of the tachycardia cycle length.

8. An implantable system for administering cardioversion shock therapy to cardiac tissue in accordance with a tachycardia rate of a patient to preempt an expected tachycardia beat, the system comprising:

sensing means for sensing intrinsic R-waves;

pulse generating means for generating a cardioversion shock and delivering the cardioversion shock to the cardiac tissue; and

control means, responsive to the sensing means, for analyzing the intrinsic R-waves to determine a tachycardia cycle length, and for triggering the pulse generating means to deliver the cardioversion shock after a predetermined time, determined as a function of the tachycardia cycle length, has elapsed following a preceding tachycardia beat to elicit a heartbeat that preempts the expected tachycardia beat.

9. The system of claim 8, wherein the function of the tachycardia cycle length is a predetermined percentage of the tachycardia cycle length.

10. The system of claim 9, further comprising:

an external programming means for sending system operating parameters to and receiving data from the control means;

telemetry means, coupled to the control means, for receiving the system operating parameters, including the predetermined percentage, from the external programming.

11. An implantable cardioversion shock therapy system, comprising:

cardiac sensing means for sensing naturally occurring cardiac depolarizations in a selected chamber of a patient's heart;

pulse generator means for providing a cardioversion shock of sufficient energy to terminate a tachycardia episode in the selected chamber of the heart;

a shocking lead, coupled to the pulse generator means, for delivering the cardioversion shock to the selected chamber of the heart; and

processing means, coupled to the cardiac sensing means and the pulse generator means, for confirming a tachycardia episode based on the sensed cardiac depolarizations and for controlling the pulse generator means to provide the cardioversion shock after a predetermined time interval following confirmation of the tachycardia episode to preempt an expected tachycardia beat.

12. The system of claim 11, wherein the processing means further comprises:

control means for defining the predetermined time interval as an alert interval corresponding to an expected alert period of the heart following the refractory period and prior to the next expected tachycardia beat.

13. The system of claim 12, wherein:

the processing means includes means for determining a tachycardia cycle length; and

the control means includes means for defining the predetermined time interval as a predetermined percentage of the tachycardia cycle length following a preceding tachycardia beat.

14. The system of claim 13, wherein the predetermined percentage of the tachycardia cycle length is in the range of 30-99% of the tachycardia cycle length.

15. The system of claim 11, wherein the control means further comprises:

means for defining the predetermined time interval as a predetermined fixed time interval, triggered from a preceding tachycardia beat, which is expected to occur when the cardiac tissue is repolarized.

16. The system of claim 15, wherein:

the control means further includes means for determining a tachycardia cycle length; and

the predetermined fixed time interval is a programmable value between 150 ms and the tachycardia cycle length.

17. An implantable cardioversion shock therapy system, comprising:

means for detecting a tachycardia episode;

means for delivering a cardioversion shock to cardiac tissue in response to the tachycardia episode; and

means for delaying delivery of the cardioversion shock by a predetermined time interval measured from a preceding tachycardia beat and preemptively before an expected tachycardia beat.

18. A method of providing cardioversion shock therapy to cardiac tissue, comprising the steps of:

detecting a tachycardia episode; and

delivering a cardioversion shock after a predetermined delay period corresponding to a period between a preceding tachycardia beat and an expected tachycardia beat when the cardiac tissue is non-refractory following detection of the tachycardia episode.

19. The method of claim 18, wherein the delaying step comprises the step of:

delivering the cardioversion shock after a predetermined fixed time interval following the preceding tachycardia beat has elapsed.

20. The method of claim 18, wherein the delaying step comprises the steps of:

determining a tachycardia cycle length; and

delivering the cardioversion shock after a length of time corresponding to a predetermined percentage of the tachycardia cycle length has elapsed following the preceding tachycardia beat.

21. The method of claim 18, wherein the detecting step comprises the step of:

detecting a ventricular tachycardia episode.

22. The method of claim 18, wherein the detecting step comprises the step of:

detecting an atrial tachycardia episode.

23. A method of administering cardioversion shock therapy to cardiac tissue to preempt an expected tachycardia beat, comprising the steps of:

detecting an occurrence of a tachycardia episode;

determining a tachycardia cycle length of the tachycardia episode; and

delivering a cardioversion shock to the cardiac tissue after a length of time, determined as a function of preceding tachycardia beat so that the cardioversion shock is delivered when the cardiac tissue is repolarized.

24. The method of claim 23, wherein the function of the tachycardia cycle length is a predetermined percentage of the tachycardia cycle length.

25. An implantable cardioversion shock therapy system, comprising:

sensing means for sensing intrinsic P-waves;

pulse generating means for generating a cardioversion shock and for delivering the cardioversion shock to cardiac tissue; and

control means, responsive to the sensing means, for analyzing the P-waves to confirm an atrial tachycardia episode, and for controlling the pulse generating means to trigger the cardioversion shock after a predetermined delay, corresponding to a period when the cardiac tissue is repolarized, the predetermined delay being synchronized from a last P-wave following the confirmation of the atrial tachycardia episode so that the cardioversion shock preempts a successive P-wave.

26. The system of claim 25, further comprising:

an external programming means for sending system operating parameters to and receiving data from the control means;

telemetry means, coupled to the control means, for receiving the system operating parameters, including the predetermined delay, from the external programming means.

27. The system of claim 25, wherein the control means further comprises:

means for defining the predetermined delay as a predetermined fixed time interval which is expected to occur when the cardiac tissue is repolarized.

28. The system of claim 27, wherein the predetermined fixed time interval is at least greater than 300 msec.

29. The system of claim 27, wherein:

the control means further includes means for determining a tachycardia cycle length; and

the predetermined fixed time interval is a programmable value between 150 ms and the tachycardia cycle length.

30. The system of claim 25, wherein the control means further comprises:

means for determining a tachycardia cycle length; and

means for defining the predetermined delay as a predetermined percentage of the tachycardia cycle length which is expected to occur when the cardiac tissue is repolarized.

31. The system of claim 30, wherein the predetermined percentage of the tachycardia cycle length is in the range of 30-99% of the tachycardia cycle length.

Description Submit all comments and votes

FIELD OF THE INVENTION

The present invention relates to implantable cardiac stimulation devices that provide cardioversion shock therapy for interrupting episodes of tachycardia. More particularly, the present invention relates to specific improvements to such devices that (i) reduce the potential for lethal acceleration, (ii) increase the probability that the treatment will safely terminate the arrhythmia, and (iii) allow cardioversion therapy to use a lower energy electrical stimulation pulse, thereby lowering power consumption of the devices and reducing discomfort to patients.

BACKGROUND OF THE INVENTION

One form of cardiac arrhythmia with serious consequences is tachycardia. Tachycardia is a condition where an abnormally high heart rate severely affects the ability of the heart to pump blood. The higher the heart rate, the more dangerous the condition. In ventricular tachycardia (VT), the QRS complexes defining the heart rate are abnormally broad and occur at a rate in the range from about 100 to about 250 beats per minute. Sustained episodes of VT are particularly dangerous because they may deteriorate into ventricular fibrillation (VF), the most life-threatening cardiac arrhythmia. VF is the result of disordered, rapid stimulation of ventricular cardiac tissue, which prevents the ventricles from contracting in a coordinated fashion. VF may cause a severe drop in cardiac output and death if not quickly reverted.

Tachycardia is often the result of electrical feedback within the heart--a natural beat results in the feedback of an electrical stimulus which prematurely triggers another beat. Tachycardia control is frequently achieved by applying electrical stimulation to the heart. The application of electrical stimulation disrupts the stability of the feedback loop, thereby returning the heart to normal sinus rhythm.

One type of electrical stimulation therapy that is known for interrupting tachycardia is cardioversion shock therapy. Cardioversion shock therapy is performed by applying an electrical shock to cardiac tissue in order to depolarize the ventricular myocardium. This allows the site of fastest spontaneous discharge, typically the sinus node, to regain pacing control, thereby terminating the tachycardia episode. In known cardioversion systems, the cardioversion shock is electronically synchronized to fire at the R-wave following confirmation of the arrhythmia. Because of electronic delays which are the result of device switching and charging requirements, the shock is generally provided shortly after the confirming R-wave. Nevertheless, the shock is often administered while significant portions of cardiac tissue are still refractory from the confirming depolarization. A successful cardioversion shock that is administered while significant portions of cardiac tissue are refractory may require a higher energy content than would otherwise be the case. Also, an improperly timed cardioversion shock can cause an afterdepolarization which can prolong the arrhythmia and even lead to a lethal acceleration.

Another type of electrical stimulation therapy known for interrupting tachycardia is antitachycardia pacing. This type of therapy is typically provided by a pacemaker that delivers antitachycardia pacing pulses to cardiac tissue in a manner intended to revert the tachycardia episode. Antitachycardia pacing pulses are of much lower energy than cardioversion shocks, typically between about 25 .mu.joules and about 30 .mu.joules and accordingly, and such pacing pulses should be delivered when the heart is most responsive to external stimulation. More particularly, antitachycardia pacing pulses should be delivered when the heart is non-refractory.

Unfortunately, there is usually no way of knowing exactly when the refractory period associated with the preceding R-wave ends. In recent years, pacemakers that provide antitachycardia pacing therapy have employed various techniques in an attempt to deliver antitachycardia pacing pulses to the portion of the cardiac cycle most likely to lead to termination of the tachycardia episode. For example, U.S. Pat. No. 4,280,502 (Baker et al.) refers to a pacemaker which, after confirmation of a tachycardia episode, automatically initiates a search routine consisting of a sequence of stimulation pulses. The pulses are provided within a predetermined time interval after a confirmation tachycardia beat. The refractory period is estimated from the experimental results of the search routine, and the search is terminated when a normal heartbeat is detected. If the first search routine is unsuccessful at terminating the tachycardia episode, a second pulse is then applied following the previously determined refractory interval by a second interval which is also experimentally determined by a second search routine.

U.S. Pat. No. 4,390,021 (Spurrell et al.) refers to a pacemaker which generates a sequence of two electrical stimulation pulses to terminate tachycardia. The delay of the first stimulation pulse and the coupled delay between the first and the second pulse are each scanned through 16 discrete steps. Successful time delay parameters are permanently stored, and on the next confirmed episode of tachycardia, scanning begins with the most recent successful synchronization parameters.

U.S. Pat. No. 4,587,970 (Holley et al.) refers to a pacemaker which uses experimental data taken from a general sample population to determine a function estimating the relationship between refractory period and heart rate. A sequence of pacing pulses is generated at intervals defined by the predetermined function in an attempt to synchronize the pacing pulses to a time shortly after the end of the refractory period. If the tachycardia episode is not terminated, another sequence of pulses is generated. The rate of the new sequence is decreased or increased depending upon whether an unevoked heartbeat was sensed during the preceding sequence.

U.S. Pat. No. 4,398,536 (Nappholz et al.) refers to a programmable pacemaker which automatically increases the pulse rate. A burst of pulses is generated after the last heartbeat used to confirm tachycardia. The initial time interval between the last heartbeat used to confirm tachycardia and the first pulse in the sequence is equal to a measured heartbeat cycle less a fixed decrement. If tachycardia persists, another pulse burst is generated at a higher rate. After exceeding the maximum rate, the scanning resumes during the next cycle at the minimum rate. The last burst rate which is successful in terminating tachycardia is stored in the pacemaker and is used for the first burst generated following the next tachycardia confirmation.

The features described above have enabled pacemakers capable of providing antitachycardia pacing therapy to interrupt tachycardia. However, this technology is not applicable to implantable cardiac stimulating devices that provide cardioversion shock therapy. Indeed, the aforedescribed approaches taken with respect to antitachycardia pacing devices would be inappropriate for cardioversion shock therapy systems, because they require the generation and delivery of a "sequence" or "burst" of pulses. A sequence or burst of cardioversion shocks, which are typically several orders of magnitude greater in energy content than antitachycardia pacing pulses, would rapidly deplete limited energy reserves, and could possibly cause great discomfort to the patient.

Prior art cardioversion shock systems either provide no synchronization or they synchronize the stimulation pulses to fire immediately after an R-wave (that is, when the tissue is generally refractory), rather than in the period at which the heart is fully responsive to external stimulation (i.e., when the heart is in a repolarized state, or non-refractory state). During the refractory period of the heart, the cardiac muscle is insensitive to restimulation and cannot respond to a stimulus until after most of the repolarization process is completed.

Furthermore, the cardioversion shocks provided by some "synchronized" systems are administered at approximately the same time relative to the confirming R-wave regardless of the patient's heart rate. Since the timing of the repolarization period relative to the previous R-wave is a function of heart rate, these prior art systems that are heart rate independent do not accurately synchronize delivery of cardioversion shocks to the repolarization period. Consequently, these systems may administer shocks while the heart is generally refractory, may require higher than necessary energy content, and may require additional shocks, thereby increasing the possibility of discomfort to the patient and reducing the useful life of the implanted cardioverter.

What is needed, therefore, is an implantable cardiac stimulating device that attempts to administer a cardioversion shock during the period of the cardiac cycle when the heart is primarily repolarized, so that a lower energy cardioversion shock can effectively interrupt a tachycardia episode because the tissue is responsive to an electrical stimulation pulse. A cardioversion shock properly administered when the heart is responsive to an external stimulus, would optimize the chance of eliciting a heartbeat that preempts the next expected tachycardia beat.

SUMMARY OF THE INVENTION

The present invention provides an implantable cardiac stimulating device and a method for synchronizing cardioversion shocks to the heart during a predetermined time interval when the heart is expected to be repolarized.

The implantable cardiac stimulating device of the present invention administers therapy for terminating a tachycardia more effectively and safely than known "synchronous" cardioversion shock therapy systems, which simply synchronize the shocks to the refractory period of the previous R-wave that is used to confirm episodes of tachycardia. In general, the present invention delays the shocks until late in the cardiac cycle, when the heart is expected to be repolarized, thereby optimizing the chance for the vast majority of the ventricular myocardium to be non-refractory.

This invention relates not only to the implantable cardiac stimulating device itself, including the manner in which cardioversion shocks are delivered to the heart, but also to a method of synchronizing the cardioversion shocks to the repolarization period of the cardiac cycle. More particularly, the present invention is directed to providing preemptive cardioversion shock therapy shortly before the tachycardia beat which follows the confirmation so that it falls in the repolarization period of the cardiac cycle.

In a preferred embodiment of the present invention, a cardioversion shock is synchronized to be administered during the repolarization period which follows the refractory period after a confirmation R-wave. While it is possible to determine the end of the refractory period, it would require extra stimuli and a delay in the time-to-termination of the arrhythmia. Instead, the present invention proposes determining a delay period which has a high probability of falling in the repolarization period of the cardiac cycle. Typically, the physician will characterize the patient's various tachycardia rhythms at implant and can easily determine a window which will preempt the various tachycardias. For example, if the patient has tachycardias at a multiplicity of rates, the physician may desire a delay period which also varies as a function of the tachycardia rate, e.g., using a percentage of the measured tachycardia rate. Alternatively, it may be desirable to select a fixed delay, say 300 or 400 ms, after the last R-wave to insure that the cardioversion shock does not occur during a T-wave and accelerate the arrhythmia.

Such a properly synchronized cardioversion shock has a high probability of creating an action potential that is conducted through the heart. If successful in creating an action potential, the cardioversion shock will elicit a heartbeat that preempts the next anticipated tachycardia beat. In this way, the tachycardia episode may be efficiently and safely reverted.

The preferred embodiment of the present invention includes a sensor for monitoring the cardiac signals; an analog-to-digital converter for converting the analog data from the sensor into conventional digital signals; a microprocessor for analyzing incoming sensor data, processing a sequence of stored instructions, and controlling the generation and delivery of shocks; nonvolatile memory for storing control instructions and program data; an internal telemetry stage for sending and receiving data from an external programmer; and a pulse generator for generating cardioversion shocks under control of the microprocessor. The cardioversion shocks are delivered to cardiac tissue by a shocking lead, which may also be used as part of the electrophysiology sensor. Although the invention is described in the context of an implantable device, the principles disclosed herein may also be applied to external cardioversion shock therapy systems.

The present invention provides numerous advantages over prior art cardioversion systems. Potentially less energy is required to successfully revert the tachycardia to sinus rhythm if the cardioversion shock is properly synchronized to the repolarization period of the cardiac cycle. Also, fewer cardioversion shocks may be necessary. In fact, the preferred embodiment of this invention uses only a single cardioversion shock to interrupt a tachycardia episode. The use of fewer, lower amplitude shocks both reduces the discomfort to the patient and increases the useful life of the implantable cardiac stimulating device by using less energy.

In the preferred embodiment, the approach to cardioversion therapy described herein is heart rate dependent. More particularly, the patient's tachycardia rate is measured and utilized to synchronize the cardioversion shock to the repolarization period of the cardiac cycle. This is necessary for proper synchronization, since the timing of the rep