INCREASING FIBRILLATION DURATION ENHANCES RELATIVE ASYMMETRICAL BIPHASIC VERSUS MONOPHASIC DEFIBRILLATOR WAVE-FORM EFFICACY

Biphasic waveforms reduce defibrillation threshold compared with corresponding monophasic waveforms. However, effects of fibrillation duration and relative efficacy of monophasic and biphasic waveforms are unknown. This study used a newly developed defibrillation model, the isolated right- and left-sided working rabbit heart, with epicardial defibrillation electrodes, to compare threshold for a monophasic waveform (5 msec rectangular) and an asymmetrical biphasic waveform (5 msec each pulse, V2 = 50% V1). Mean voltage defibrillation threshold (V50) was determined from sigmoidal probability of successful defibrillation versus shock intensity curves after 5, 15, and 30 seconds of fibrillation in a paired study with 10 hearts. Results showed that biphasic waveforms had significantly lower voltage and energy thresholds at all fibrillation durations and that their relative efficacy improved with increasing fibrillation duration. Biphasic voltage threshold was 38.2 ± 2.2, 44.7 ± 4.8, and 46.6 ± 3.2 V after 5, 15, and 30 seconds of fibrillation compared with monophasic waveforms of 51.7 ± 4.4 (p < 0.002), 63.0 ± 7.6 (p < 0.05), and 72.1 ± 3.9 V (p < 0.005). Biphasic waveform energy threshold was 0.67 that for the monophasic waveform after 5 seconds of fibrillation (0.12 ± 0.01 versus 0.18 ± 0.03 J, p < 0.05). The ratio between biphasic waveform threshold and monophasic waveform threshold (B/M) decreased to 0.62 at 15 seconds. At 30 seconds, B/M was 0.52 (0.17 ± 0.02 versus 0.33 ± 0.04 J, p < 0.02). This study also showed that biphasic waveform threshold was a nonlinear function of monophasic waveform threshold so that improved biphasic defibrillator waveform efficacy was greatest for hearts having higher monophasic thresholds. These results are consistent with the hypothesis that biphasic waveforms lower defibrillation threshold by forcing recovery of excitation channels in the depolarized, partially refractory ventricular cells found during fibrillation.