Is the Second Phase of a Biphasic Defibrillation Waveform the Defihrillating Phase?

Abstract

Why some biphasic waveforms defibrillate with lower energies than monophasic waveforms of similar duration is unknown. One hypothesis is that the first phase of a biphasic waveform acts as a conditioning, hyperpolarizing prepulse to prepare for defibrillation by a second depolarizing phase. To test whether the second phase of a biphasic waveform is the defibrillating phase, three monophasic waveforms, an ascending ramp (A), a square wave (S), and a descending ramp (D), were compared to three biphasic waveforms with A, S, or D in the first phase (biphasic first phase) and three biphasic waveforms with A, S, or D in the second phase (biphasic second phase). Two defibrillation thresholds for each waveform were performed in 18 open chest pigs and mean defibrillation thresholds were compared. In nine pigs 16-msec monophasic and 16/16-msec biphasic waveforms were ranked by mean current and energy at defibrillation threshold. The ranks were the same for monophasic and biphasic second phase waveforms: for mean current A < S = D and for energy A < S < D. The ranks were different for the biphasic first phase waveforms: for mean current S < A = D and for energy S < A = D. Although ranks for the 16-msec monophasic waveforms matched those for the 16/16-msec biphasic second phase waveforms, the biphasic waveforms had higher mean currents and energies at defibrillation threshold. In nine pigs defibrillation thresholds for 6-msec monophasic and 6/6-msec biphasic waveforms were ranked. For mean current the ranks were monophasic: A < S = D; biphasic first phase: A = S = D; and biphasic second phase: S = D < A. For energy the ranks were monophasic: A = S < D; biphasic first phase: A = S = D; and biphasic second phase: S = D < A. Thus, ranks for the 6-msec monophasic waveforms differed from those for the 6/6-msec biphasic second phase waveforms. For 16/16-msec biphasic waveforms, less effective for defibrillation than corresponding 16-msec monophasic waveforms, these results support the hypothesis that the second phase of a biphasic waveform defibrillates since the defibrillation efficacy of a 16/16-msec biphasic waveform is related to the defibrillation efficacy of its second phase waveshape. However, for clinically useful 6/6-msec biphasic waveforms, more effective for defibrillation than 6-msec monophasic waveforms, the hypothesis is not supported because the ability of a 6/6-msec biphasic waveform to defibrillate is unrelated to the defibrillation efficacy of its second phase waveshape.