Inappropriate Shock Delivery and Biventricular Pacing Cardiac Defibrillators

Abstract

In the United States, physicians adapt currently available defibrillators to accommodate leads for biventricular pacing in those congestive heart failure patients who might benefit from cardiac resynchronization and who are additionally at risk for sudden cardiac death. The adaptation of the lead system of available defibrillators to also allow them to function as biventricular pacemakers presents occasions in which inappropriate shocks are delivered due to double counting of the right and left ventricular depolarizations by the implantable cardiac defibrillator.

We reviewed a series of inappropriate shock deliveries that occurred after the implantation of biventricular pacing cardiac defibrillators at our institution; all of these shocks were related to ventricular double counting. Each had different underlying causes and management strategies. Complications such as these emphasize the importance of attentiveness to ventricular channel electrograms and to device sensing with the use of biventricular pacing cardiac defibrillators. In addition, a thorough working knowledge of pacemaker and defibrillator operation is essential for the prediction and correction of inappropriate therapies. (Tex Heart Inst J 2003;30:45–9)

Biventricular pacing therapy for congestive heart failure and mechanical dyssynchrony has been shown to be effective as a new therapy for patients with moderate-to-severe congestive heart failure associated with intraventricular conduction delay. ¹ The U.S. Food and Drug Administration has approved the use of biventricular pacemakers for cardiac resynchronization. However, many patients who are ideal candidates for biventricular pacing therapy also fall into the category of those who would benefit from implantable cardiac defibrillators (ICDs)—namely, those with severely depressed ejection fractions and ventricular tachycardia (VT). However, lead systems for biventricular pacing with cardiac defibrillators are not yet commercially available in the United States. Therefore, physicians in this country adapt currently available defibrillators to accommodate leads for biventricular pacing. The adaptation of available defibrillators to also function as biventricular pacemakers presents occasions in which inappropriate shocks are delivered due to double counting of the right and left ventricular depolarizations by the ICD. This series of cases demonstrates situations in which various forms of ventricular double counting in a defibrillator configured for biventricular pacing can result in the delivery of inappropriate shocks.

Case Reports

Patient 1

In November 2001, a 52-year-old man with idiopathic dilated cardiomyopathy was referred to our service for episodes of VT associated with pulmonary edema. During the previous few months, he had experienced progressively worsening dyspnea on exertion (New York Heart Association functional class [NYHA FC] III–IV). Coronary angiography revealed no significant coronary artery disease and an ejection fraction of 0.12. Despite therapy with amiodarone, the patient developed documented, nonsustained, monomorphic VT. A 12-lead electrocardiogram showed left bundle branch block with a QRS duration of 236 msec.

We decided to implant a biventricular pacing cardiac defibrillator (GEM® III DR, Medtronic, Inc.; Minneapolis, Minn). Under local anesthesia in the left pectoral area, a left ventricular (LV) lead was placed (Attain™ Model 2187, Medtronic, Inc.) through the coronary sinus into a posterolateral left ventricular branch. The LV pacing threshold was 0.2 V at a 0.5-msec pulse width with an impedance of 782 ohms. The QRS duration during LV pacing was 252 msec. The LV electrogram amplitude was 13.1 mV. Subsequently, the right ventricular (RV) lead (Sprint Quattro Secure™ Model 6947, Medtronic, Inc.) was positioned in the RV apex. The pacing threshold was 0.9 V at a 0.5-msec pulse width with an impedance of 652 ohms. The QRS duration during RV pacing was 256 msec. The RV electrogram amplitude was 20.9 mV. The left and right ventricular leads were attached via a Y-adaptor (Model 2872, Medtronic, Inc.), and testing revealed a biventricular pacing threshold of 1.2 V at a 0.5-msec pulse width with an impedance of 468 ohms. The QRS duration during biventricular pacing was 168 msec. The biventricular electrogram amplitude was 16.7 mV. Finally, the right atrial lead (CapSureFix® Novus Model 5076, Medtronic, Inc.) was positioned in the right atrial appendage with a pacing threshold of 1.4 V at a 0.5-msec pulse width with an impedance of 468 ohms. The atrial electrogram amplitude was 2.9 mV. No oversensing on the atrial or biventricular leads was detected. The ICD was programmed as follows: ventricular fibrillation (VF) interval zone of 320 msec (188 beats/min) with biventricular pacing in DDDR mode, rate 70–120 beats/min, paced AV interval of 170 msec, sensed AV interval of 150 msec, and postventricular atrial refractory period of 250 msec.

Ventricular fibrillation was induced by T wave shock, and 20- and 30-J defibrillation shocks were unsuccessful. Therefore, external cardioversion with 200 J was required. The polarity of the defibrillation shock was reversed, and VF was again induced by T wave shock. Defibrillation thresholds were found to be less than 20 J. On the day after implantation, examination revealed proper functioning of the device without evidence of oversensing or other problems.

Three weeks later, while working in the yard, the patient experienced an inappropriate shock delivery. Examination of the device (Fig. 1) revealed that the shock had been initiated by a run of atrial tachycardia. In the presence of the tachycardia, the sensed atrial events were occurring too soon after the sensed ventricular events and were classified as refractory. Because the device was no longer encountering atrial-sensed events, no biventricular pacing was initiated, which in turn allowed the patient's intrinsic QRS complex to emerge. However, the baseline left bundle branch block resulted in double counting of the RV and LV depolarizations. This double counting met the detection criteria for VF and resulted in a shock delivery. With the application of cardioversion, the atrial tachycardia reverted to sinus rhythm, and the device resumed the initially intended biventricular pacing without ventricular double counting.

Fig. 1 Patient 1. Electrogram from the event leading to inappropriate shock delivery: atrial channel (top line), ventricular channel (middle line), and marker channel (bottom line). Atrial tachycardia occurs with atrial events falling within the postventricular atrial refractory period. Inherent ventricular activity emerges with double counting of ventricular electrical activity that meets the rate detection criteria for ventricular fibrillation. With defibrillation, the atrial tachycardia is aborted and biventricular pacing resumes; the QRS narrows and the ventricular double counting resolves.

Double counting persisted despite the shortening of the postventricular atrial refractory period. We then reprogrammed the device and changed the ventricular sensitivity to 0.45 mV from 0.3 mV, and the problem resolved. In addition, we decreased the paced and sensed AV delays to 140 msec and 120 msec, respectively. The patient has received no further inappropriate shock delivery.

Patient 2

In January 2002, an 81-year-old man presented with a history of coronary artery disease and bypass surgery, congestive heart failure (NYHA FC III–IV) with an ejection fraction of 0.20, and paroxysmal atrial fibrillation. He had been admitted to the hospital multiple times for intractable heart failure despite maximal medical therapy. Multiple episodes of nonsustained VT were documented. A baseline electrocardiogram revealed normal sinus rhythm with a prolonged PR interval, and left bundle branch block with a QRS duration of 196 msec.

A biventricular pacing cardiac defibrillator (GEM® III AT, Medtronic, Inc.) was implanted. The technique of device implantation and testing was similar to that described for the previous patient. At the time of implantation, double counting of the patient's native QRS complex was noted despite multiple attempted lead locations and sensitivity adjustments. With medical therapy and the device set to a short AV delay, the patient remained in sinus rhythm with biventricular pacing. The patient, however, returned approximately 2 weeks after discharge from the hospital, having experienced 4 inappropriate shocks during a 3-day period. Otherwise, his functional status and heart failure were markedly improved. Intracardiac electrograms revealed all episodes of inappropriate shocks to be initiated by the conduction of a native QRS complex and double counting of the ventricular electrical activation (Fig. 2). As in the 1st patient, subsequent atrial events occurred within the postventricular atrial refractory period, which resulted in the continuation of native ventricular conduction and double counting. The additional sensed ventricular events met VF detection criteria and led to the delivery of an inappropriate shock.

Fig. 2 Patient 2. Electrogram from the event leading to inappropriate shock delivery: atrial tip-to-ventricular ring electrogram (top line) and marker channel (bottom line). After a native complex is double counted, all subsequent atrial events fall within the refractory period and are not tracked, which allows continuation of the native rhythm and double counting of ventricular activity.

Further attempts at limiting ventricular oversensing were unsuccessful, despite multiple changes in the device settings while the patient remained hospitalized. After discussion with the patient and his family, we decided to perform AV node ablation in order to prevent the patient's intrinsic QRS complex from manifesting and to correct the ventricular double counting. The patient tolerated the procedure well and has received no further inappropriate shocks.

Patient 3

In December 2001, a 78-year-old man presented with runs of nonsustained VT and symptomatic congestive heart failure (NYHA FC III–IV). He had dilated cardiomyopathy with an ejection fraction of less than 0.20. A baseline electrocardiogram revealed normal sinus rhythm with a QRS duration of 196 msec.

A biventricular pacing cardiac defibrillator (GEM® III DR) was implanted. The technique of implantation and testing of the device was similar to that described for the previous patients. At the time of implantation and before the patient was discharged from the hospital, the device was functioning properly, with no double counting. The defibrillator was set to deliver shocks upon detection of VF, but therapy for VT was limited to antitachycardia pacing.

About 2 months after discharge from the hospital, the patient returned, having received 1 inappropriate shock. He noted that he wasn't feeling as energetic as usual that day but otherwise did not relate any worsening symptoms of congestive heart failure or syncope. Intracardiac electrograms revealed an hour-long episode of VT with a cycle length of approximately 340 msec that did not respond to antitachycardia pacing. Near the end of this episode, the morphology of the VT changed and resulted in double counting of the QRS complex at a rate satisfying VF detection criteria. A shock successfully terminated the VT, and normal atrial and biventricular pacing resumed (Fig. 3). The patient's medical therapy was increased to help eliminate episodes of VT. The patient has had no further inappropriate shocks delivered.

Fig. 3 Patient 3. A) Electrogram from the event leading to inappropriate shock therapy: atrial channel (top line), ventricular channel (middle line), and marker channel (bottom line). Ventricular tachycardia occurs with a cycle length of approximately 340 msec. No double counting occurs with this morphology of the ventricular tachycardia. B) Electrogram from the event leading to inappropriate shock delivery: atrial channel (top line), ventricular channel (middle line), and marker channel (bottom line). The morphology of the ventricular tachycardia has changed, although it still has a cycle length of approximately 340 msec. The double counting fulfills ventricular fibrillation detection criteria and leads to inappropriate shock delivery. The shock results in resumption of normal atrial and biventricular pacing with cessation of double counting.

Discussion

Advances in cardiac pacing and defibrillating technologies have the potential for improving patients' lives. However, the cases presented here demonstrate that the new technologies, under certain situations, can result in inappropriate shock delivery.

Many instances of oversensing in single ventricular lead systems have been reported. ^4–6 To our knowledge, there are only 2 previous reports ^2,3 of inappropriate shock delivery in biventricular pacing cardiac defibrillators as a result of oversensing. The 1st case involved inappropriate sensing of atrial electrical activity by the ventricular leads, in addition to double counting of right and left ventricular electrical activations during sinus rhythm. This difficulty was promptly resolved by shortening of the AV delay, which resulted in constant biventricular pacing. ² In the 2nd case, double detection involving a run of ventricular premature beats resulted in undesired defibrillation therapy. Double counting was not corrected by changes in sensitivity. Therefore, prolongation of the detection delays and activation of the stability detection algorithm were implemented. In addition, reduction of the AV delay, increased ventricular pacing rate after mode switch, and rate smoothing were implemented to help reduce spontaneous ventricular rhythm. These changes eliminated any further inappropriate shocking and oversensing, as determined at follow-up. ³

In our 1st patient, proper function and operation of the device was well documented initially after implantation. Oversensing of ventricular activity was not present at that time. This case is unique in that inappropriate shocks occurred only during episodes of atrial tachycardia, wherein the pacemaker reverted to the patient's inherent ventricular activity rather than to biventricular pacing, which in turn resulted in double counting from the RV and LV leads.

The 2nd patient presented an interesting therapeutic challenge, because the device showed oversensing of ventricular activity at the time of implantation, but adjustments to medications and device settings allowed maintenance of sinus rhythm and biventricular pacing. However, in this instance, the conduction of a single native QRS complex led to double counting and subsequent detection of atrial activity within the postventricular atrial refractory period; these events terminated ventricular pacing.

Our 3rd patient experienced unexpected shock delivery for VT, during which the QRS complexes were double counted. In retrospect, this therapy may have been fortunate for the patient clinically, due to the length of his episode of sustained arrhythmia. However, this case again demonstrates an unforeseen outcome of biventricular defibrillator therapy.

One alternative to the adaptation of currently available defibrillators for biventricular pacing is the use of 2 separate devices. However, this option has inherent shortcomings, including increased iatrogenic risks, patient discomfort, and overall costs. In addition, the placement of a 2nd device adds the potential for unforeseen device–device interactions and complications.

The use of adaptors to allow both biventricular pacing and defibrillation raises the likelihood of ventricular double counting as the result of a widely split right and left ventricular electrical activity. The therapeutic implications are complicated, given that in certain instances adjustments made to optimize biventricular pacing therapies and to reduce oversensing may compromise the function of the defibrillator. For example, decreasing the sensitivity may reduce ventricular oversensing and double counting but may also reduce the efficacy of the defibrillator's algorithms for detecting ventricular arrhythmias.

At least one of the previously reported cases from Europe, where biventricular pacing cardiac defibrillators have been released for marketing (Contak™ CD Congestive Heart Failure Device, Guidant Corporation; St. Paul, Minn), also involved the use of a Y-adapter for connecting the right and left ventricular leads. ² These adaptation problems with double counting center mainly on the currently available configurations, which result in combined sensing from both ventricular leads even though current algorithms for defibrillators are designed for sensing from only 1 lead. In fact, future biventricular pacing defibrillators are expected to sense from the right ventricle alone and to have separate inputs for the right and left ventricular leads (Medtronic Insync® Cardiac Resynchronization System, Medtronic, Inc.). Even so, studies with single ventricular lead systems have revealed episodes of inappropriate shocks in 16% of patients with such defibrillators. ⁴

Conclusion

The interventricular conduction delay that is often seen in patients with biventricular pacing defibrillators can result in widely split ventricular electrical activation, which can be sensed as 2 separate ventricular events. Complications such as those that occurred in our 3 patients emphasize the importance of attentiveness to ventricular channel electrograms and to device sensing. In addition, a thorough working knowledge of pacemaker and defibrillator operation is essential for the prediction and correction of inappropriate therapies.

Addendum

Since this paper was submitted for publication, biventricular pacing defibrillators have been approved for use in the United States. It remains to be seen whether further advances in device technology, such as individual ventricular lead sensing or improved arrhythmia-sensing algorithms, will result in fewer such complications with the use of biventricular pacing defibrillators.