Adequate sensing of ventricular tachycardia (VT) and ventricular fibrillation (VF) is necessary for proper functioning of an implantable cardioverter defibrillator (ICD). Several ICDs currently undergoing investigation have programmable fixed gain sensitivity for tachycardia detection. If intracardiac electrogram amplitude decreases below the programmed sensitivity during VT or VF, detection of a ventricular arrhythmia may be delayed or missed. The mean amplitude of intracardiac electrograms (ICEGM) recorded with bipolar epicardial or transvenous sensing leads was measured in 63 patients during induced VT and VF recorded in the operating room at the time of ICD implantation. The mean amplitude of the ICEGM during 41 episodes of VF in 15 patients decreased from 14.9 +/- 0.9 mV during sinus rhythm to 8.8 +/- 0.7 mV at 1 second, 9.7 +/- 0.7 mV at 5 seconds, and 9.4 +/- 0.7 mV at 10 seconds (p < 0.0001 vs sinus rhythm ICEGM) with endocardial leads. The mean amplitude of the ICEGM recorded during 173 episodes of VF in 43 patients with epicardial leads decreased from 10.4 +/- 0.3 mV in sinus rhythm to 7.8 +/- 0.3 mV at 1 second, 8.3 +/- 0.3 mV at 5 seconds and 8 mV at 10 seconds (p <0.0001 vs sinus rhythm ICEGM). The mean amplitude of epicardial and transvenous ICEGMs recorded during 34 episodes of monomorphic VT decreased from 18.5 +/- 1.8 mV (epicardial) and 14.4 +/- 2.0 mV (transvenous) during sinus rhythm (p = 0.15, epicardial vs transvenous) to 16.0 +/- 1.7 mV (epicardial) and 13.7 +/- 1.9 mV (transvenous) at 10 seconds (< 10% of baseline amplitude).(ABSTRACT TRUNCATED AT 250 WORDS)