Lead System

Background. Shock delivery of an implantable defibrillator may cause a change in the amplitude of endocardial electrograms and impair the detection of ventricular fibrillation. Thus, the effects of shock discharges on the amplitude of endocardial electrograms were evaluated in five patients undergoing implantation of a cardioverter-defibrillator in combination with a new nonthoracotomy lead system. Methods and Results. At implant, bipolar endocardial electrograms were recorded before each shock application, during ventricular fibrillation, during redetection of ventricular fibrillation in case the applied shock was ineffective, and at intervals of 5, 10, 20, 30, 60, and 120 seconds after each shock delivery. The amplitude of the endocardial electrograms decreased from 10.5±3.8 mV during sinus rhythm to 6.3±+1.9 mV during initial ventricular fibrillation and declined to 2.2+1.3 mV during redetection of ventricular fibrillation. After successful termination, the following bipolar electrograms could be obtained at the predetermined intervals: 1.9±+1.2 mV, 3.1±+1.8 mV, 4.5±+1.9 mV, 6.5±2.9 mV, 9.5±3.3 mV, and 10.4±3.8 mV. At predischarge testing, failure of redetection of ventricular fibrillation could be documented in two patients, requiring rescue external defibrillation in both cases to restore sinus rhythm. Conclusions. These findings demonstrate that the implantable cardioverter-defibrillator did not ensure reliable redetection of ventricular fibrillation in patients using the implanted nonthoracotomy lead system. Thus, the potential risk of sudden cardiac death may persist in these patients despite defibrillator therapy. (Circulation 1992;86:1217-1222)