An arrhythmia is an abnormal heart beat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With atrial tachycardia (AT), the atria of the heart beat abnormally fast. With ventricular tachycardia (VT), the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, some types of tachycardia, particularly VT, can trigger ventricular fibrillation wherein the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs. Ventricular fibrillation, if not terminated, is fatal. Hence, it is highly desirable to prevent or terminate arrhythmias, particularly arrhythmias of the type that can lead to a ventricular fibrillation.
For patients prone to arrhythmias, cardiac stimulation devices, such as pacemakers or ICDs can be implanted in the patient to detect the arrhythmias and deliver appropriate electrical therapy to the heart of the patient. Pacemakers typically recognize arrhythmias such as bradycardia and tachycardia and deliver electrical pacing pulses to the heart in an effort to terminate the arrhythmias and cause the heart to revert to a normal sinus rhythm. ICDs additionally recognize atrial fibrillation and ventricular fibrillation and deliver electrical shocks to terminate the fibrillation. To detect the arrhythmias, cardiac stimulation devices carefully monitor characteristics of the heart, particularly the heart rate. The heart rate is tracked by the device by examining electrical signals that result in the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is a result of the atrial depolarization or electrical activation of the atrial tissue manifested as a P-wave in a surface electrocardiogram (ECG). The IEGM is a recording of the electrical signal from within the heart and in the case of the atrium, is referred to as an atrial IEGM. The contraction of ventricular muscle tissue follows the electrical depolarization of the ventricle, which is manifest on the ECG by an R-wave (sometimes referred to as the “QRS complex”) and inside the heart as sharp deflection within a ventricular IEGM termed the intrinsic deflection. Recovery of the cardiac electrical potential is manifest as a T-wave on the ECG. With the T-wave, the active cardiac contraction ceases and the ventricle begins to relax and dilate allowing the ventricle to expand and fill with blood in preparation for the next cardiac contraction or heartbeat. A similar phase involving the atrial tissue exists but usually does not result in a detectable signal on the ECG because it is a smaller signal proportional to the P-wave amplitude and both coincides with and is obscured by the QRS complex. The sequence of electrical events that represent P-waves, followed by R-waves (or QRS complexes), followed by T-waves can be detected within IEGM signals sensed using pacing leads implanted inside the heart. Once electrical signals corresponding to P-waves, R-waves, and T-waves are detected within the IEGM signals, an examination of these (and possibly other electrical signals from the heart) is used to detect any arrhythmias.
As noted, the terms P-waves, R-waves and T-waves typically refer to features of the ECG. Herein, however, for the sake of clarity and brevity, the terms will be used more generally to also refer to the corresponding signals as sensed internally. More specifically, the term P-wave will be used to refer to electrical signals representative of the depolarization of the atria regardless of where the signals are sensed. Of course, the particular shape of the P-wave will vary depending upon the sensing locations of the leads and on the particular type of sensing leads (such as unipolar or bipolar). Hence, a P-wave sensed within the left ventricle may differ in shape from a P-wave sensed within the right ventricle. The term R-wave will be used herein to refer to electrical signals representative of the depolarization of the ventricles regardless of where the signals are sensed. Where needed, a distinction will be drawn between left ventricular (LV) R-waves and right ventricular (RV) R-waves. The term LV R-wave refers to electrical signals representative of the depolarization of the left ventricle regardless of where the signals are sensed. The term RV R-wave refers to electrical signals representative of the depolarization of the left ventricle regardless of where the signals are sensed. The term T-wave will be used herein to refer to electrical signals representative of the repolarization of the ventricles as sensed by one or more leads placed within the heart. Again, where needed, a distinction and may be drawn between LV T-waves and RV T-waves. Finally, where an electrical signal is generated in one chamber but sensed in another, it will be referred to, where needed, as a far field signal. Hence, a P-wave sensed in the ventricles is a far field P-wave. An LV R-wave sensed in the right ventricle is a far field LV R-wave.
Once an arrhythmia has been detected, the implantable cardiac simulation device provides the appropriate electrical therapy to the heart, typically using the same leads used to sense the IEGM signals. With single-chambered pacemakers, only a single lead is provided for pacing and sensing at a single location within only one of the chambers of the heart, typically the right ventricle. With dual-chambered pacemakers, two leads are typically provided such that pacing and sensing can be performed in two chambers of the heart, typically the right atrium and right ventricle. With biventricular pacemakers, an additional lead is provided into the left ventricle such that pacing and sensing can be performed in both ventricles. Biventricular pacemakers also usually have a lead mounted in the right atrium as well. Hence, biventricular pacemakers typically receive three sets of electrical signals sensed separately in the right atrium and the left and right ventricles. These electrical signals are processed within the pacemaker on separate channels (a right atrial channel, a left ventricular channel and a right ventricular channel), and signals corresponding to P-waves, R-waves, and T-waves can be identified, depending upon the programming of the implantable device, within the separate channels. Biventricular pacing is particularly advantageous because it permits the timing of contractions of the left and right ventricles to be synchronized as needed to achieve optimal pacing therapy. In particular, biventricular pacemakers have shown the ability to increase the performance of patients with congestive heart failure (CHF) by synchronizing the contraction between the left and right ventricles.
Although cardiac stimulation devices are generally quite effective in detecting and terminating arrhythmias such as tachycardia, in rare cases the stimulation device actually causes tachycardias to occur within the patient, typically as a result of misidentification of P-waves, R-waves, or T-waves. These induced tachycardias are referred to as pacemaker mediated tachycardias. PMTs can arise, for example, within dual-chambered pacemakers because of “retrograde conduction” wherein the depolarization of the ventricles propagates backwards into the atria, causing the atria to depolarize prematurely. As noted, atrial depolarization is manifest by the occurrence of a P-wave, frequently referred to in this particular context as a “retrograde P-wave”. A retrograde P-wave appears within an IEGM substantially the same as a natural P-wave except that it occurs much too soon after a ventricular contraction. (A “natural” P-wave results from the natural AV synchrony of the heart as set by the heart's natural sinus rhythm, and is hereafter referred to as a “sinus” P-wave.) Various techniques have been provided for detecting and preventing PMTs that arise from retrograde conduction with dual chambered pacemakers. One particularly effective technique is described in the U.S. Pat. No. 5,074,308 to Sholder et al., entitled “System and Method for Recognizing Pacemaker-Mediated Tachycardia”.
PMTs are particularly problematic within biventricular pacemakers because of the risk of the detecting the electrical signals associated with the depolarization of one chamber within other chambers. For example, the electrical depolarization of the right ventricle may be detected within the left ventricular channel and vice versa. Likewise, the electrical depolarization of either the right or left ventricle may be detected on the atrial channel. Hence, there is generally a greater chance of misidentification of electrical signals within biventricular system than in single or dual-chambered systems and so there is a generally a greater risk of onset of PMT.
At least one technique has been developed for detecting PMT within a biventricular system so that biventricular pacing can then be suspended. See U.S. Patent Application U.S. 2001/0005790 to Ripart, published Jun. 28, 2001, which describes a technique for detecting PMT primarily based on changes in heart rate so that, for example, pacing in one of the ventricles can then be suspended to thereby break the PMT. Although the technique of Ripart may be capable of detecting certain types of PMT once it has occurred, it would be far preferable to provide techniques for actually preventing the onset of PMT within biventricular systems so that biventricular pacing need not be suspended but instead can be performed more or less continuously.
Accordingly, it would be desirable to provide techniques for reducing the risk of onset of PMT within biventricular pacing systems and it is to this end that aspects of invention are generally directed. In addition, to the extent that the technique of Ripart detects PMT once it has already occurred, it appears to do so primarily based on detection of a high heart rate in combination with a sudden rate increase. Hence, it may not be effective in detecting certain types of PMT, such as relatively lower rate PMT or PMT that is not associated with any sudden rate increase. Accordingly, it would also be desirable to provide improved techniques for detecting and terminating PMT once it has already occurred within a biventricular pacing system and it is to this end that other aspects of invention are directed.
Insofar as the prevention of PMT is concerned, because of the additional sensing channels used in biventricular systems, techniques that are effective for preventing PMT within a dual-chambered pacemaker may not work effectively and so various types of PMTs may nevertheless arise. For example, PMT can occur within a biventricular pacing system as a result of T-waves from the ventricles being detected on the atrial channel and being interpreted by the dual chamber pacemaker as an intrinsic P-wave, which in turn triggers a premature V-pulse in the ventricles. More specifically, whenever an intrinsic P-wave is detected on the atrial channel, the pacemaker is programmed to wait a predetermined amount of time for detection of an R-wave on the ventricular channels. If no R-wave is detected, the logic of the biventricular pacing system concludes that the ventricles failed to depolarize properly and a pair of V-pulses are delivered to the left and right ventricles, synchronized as needed. However, because the signal detected on the atrial channel was not actually an intrinsic P-wave, the ventricles will not likely depolarize within the expected period of time and so no R-wave will be detected on ventricular channels within the period of time. Accordingly, premature V-pulses will be delivered to the left and right ventricles, triggering another T-wave that likely causes another false detection of a P-wave on the atrial channel, thus triggering yet another pair of premature V-pulses. This process can continue indefinitely causing the heart to beat at the rate determined by the rate of the premature V-pulses and, hence, PMT occurs.
Conventionally, within dual-chambered devices, to prevent this form of PMT, a post ventricular atrial refractory period (PVARP) is applied to the atrial channel immediately following the delivery of a V-pulse to the right ventricle. During the PVARP, the device does not respond to any electrical events sensed on the atrial channel and so the device does not misinterpret a far field T-wave as an intrinsic P-wave. However, within a biventricular pacing system, the use of a PVARP is problematic. If the PVARP is initiated simultaneously with the first of the two ventricular pulses, the PVARP may have already expired before the T-wave propagates into the atria. Hence, this signal may be detected and misinterpreted as an intrinsic P-wave, thus triggering PMT. This is also referred to as T-wave oversensing.
Accordingly, it would be particularly desirable to provide techniques for preventing the onset of PMT within biventricular pacing systems by preventing false detection of intrinsic P-waves on the atrial channel and it is to this end that further aspects of invention are directed.
In another example of PMT within biventricular pacing systems, a T-wave associated with a V-pulse delivered to the right ventricle is erroneously detected on the left ventricular channel as an R-wave. Biventricular-triggered pacing systems are typically programmed to deliver a V-pulse to the left ventricle a fixed period of time (e.g. 20 milliseconds (ms)) following detection of an R-wave on the right ventricular channel to better synchronize the left and right ventricles. A refractory period (typically 300 ms) is then applied to the right ventricular channel. However, the resulting T-wave may be large in magnitude and fall outside the refractory period where it is then sensed on the right ventricular channel and misinterpreted as an intrinsic R-wave. If so, the device then delivers another V-pulse to the left ventricle shortly thereafter. The pacing pulse delivered to the left ventricle will eventually trigger another T-wave, which will also probably be misinterpreted as an R-wave on the right ventricular channel, triggering yet another V-pulse in the left ventricle and PMT thereby ensues in an endless loop.
Accordingly, it would also be particularly desirable to provide techniques for preventing the onset of PMT within biventricular pacing systems by preventing false detection of intrinsic R-waves on the ventricular channels and it is to this and that still other aspects of invention are directed.