Delivery of noncommitted shocks for nonsustained ventricular arrhythmias by a new implantable cardioverter defibrillator with abortive shock capability.

INTRODUCTION: To describe the delivery of noncommitted implantable cardioverter defibrillator (ICD) shocks despite self-termination of ventricular arrhythmias. Abortive shock capability should eliminate the delivery of shocks for self-terminating ventricular arrhythmias. The delivery of noncommitted shocks despite abortive shock capability is, therefore, unexpected and previously unreported.

METHODS AND RESULTS: Among 118 patients who received the Transvene nonthoracotomy lead system and the Jewel ICD (model 7219D), three patients (1.7%) experienced supurious, noncommitted shocks for self-terminating arrhythmias. Only one detection zone (i.e., ventricular fibrillation) had been programmed in the defibrillator in each patient. In all three patients, the ventricular arrhythmias self-terminated during the charging period. One patient received seven shocks during periods of asystole, and the other two patients received one shock each. Two different mechanisms for shock delivery in this setting were identified: one occurring in the absence of electrical activity at the end of the bradycardia escape interval (i.e., associated with bradyarrhythmias), and the other when two sensed electrical events (i.e., escape beats) occurred during the so-called "synchronization" window of the defibrillator.

CONCLUSIONS: In rare patients with the Jewel defibrillator, shocks may be delivered for self-terminating arrhythmias despite abortive shock capability. Patients who are dependent upon pacing from their implanted defibrillator are at particular risk for shock in the aftermath of self-terminating ventricular arrhythmias. Defibrillator programming strategies aimed at eliminating or diminishing the incidence of this problem are discussed.