Prospective comparison of biphasic and monophasic shocks for implantable cardioverter-defibrillators using endocardial leads

Bidirectional shocks using 2 current pathways have been used in endocardial lead systems for implantable cardioverter-defibrillators, but the optimal shock waveform for endocardial defibrillation is unknown. The clinical efficacy and electrical characteristics of bidirectional monophasic and biphasic shocks for endocardial cardioversion-defibrillation of fast monomorphic or polymorphic ventricular tachycardia (VT), or ventricular fibrillation (VF) were evaluated. Thirty-three patients (mean age 60 ± 12 years, and mean left ventricular ejection fraction 34 ± 13%) were studied. Defibrillation catheter electrodes were located in the right ventricular apex and superior vena cava/right atrial junction. A triple-electrode configuration including the 2 catheter electrodes and a left thoracic patch was used to deliver bidirectional shocks from the right ventricular cathode to an atrial anode (pathway 1) and the thoracic patch (pathway 2). The shock waveforms examined were sequential and simultaneous monophasic, and simultaneous biphasic. The efficacy of 580 V (20 J) shocks for fast monomorphic VT were comparable for the 3 waveforms (73% for sequential monophasic, 73% for simultaneous monophasic, and 100% for simultaneous biphasic). However, for polymorphic VT and VF, 580 V sequential monophasic shocks had a significantly lower efficacy (25%) than did simultaneous monophasic (75%; p = 0.01) or biphasic (89%; p < 0.001) shocks. Single-shock defibrillation thresholds with simultaneous biphasic shocks were significantly lower (9 ± 5 J) than were those with simultaneous monophasic shocks (15 ± 4 J; p < 0.02). Using paired identical shocks ≤550 V (18 J), simultaneous biphasic shocks successfully terminated polymorphic VT and VF in 9 of 11 patients in whom simultaneous monophasic shocks were previously ineffective (p < 0.001). The mean impedance in the right ventricular-atrial pathway was significantly lower than that in the right ventricular-patch pathway for both sequential and simultaneous monophasic shocks. There was no correlation between impedance in each pathway and clinical outcome of shock delivery for this shock pattern. It is concluded that efficacy of bidirectional endocardial shocks varies with shock waveform and induced arrhythmia. Simultaneous biphasic shocks enhance the efficacy and reduce the energy needs of bidirectional shocks, and should be used in preference to monophasic shocks when implantable cardioverter-defibrillators are used with this endocardial lead system.