The Effect of Phase Separation on Biphasic Waveform Defibrillation

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Abstract
It has been hypothesized that the defibrillafion efficacy of a biphasic shock is caused hy the large change in voltage between the two phases. This study examined the effects of separating the two phases in time thus splifting in half the rapid voltage change at phase reversal. The study was performed in three parts each using six dogs. Part I determined defibrillation thresholds (DFTs) for two exponentially truncated biphasic waveforms (3.5/2 msec and 6/6 msec) with interphase time delays of 0, 1, 2, 3, 4, 6, 8, and 10 msec. In Part II, probability of success curves were generated using an up down method with 15 shocks for each delay for the 3.5/2 msec biphasic waveform with inferphase delays of 0, 2, 3, 4, and 5 msec. In Part III, DFTs were determined using a 3.5/2 msec and 6/6 msec biphasic as well as a fhird waveform that consisted of two sequential 6‐msec pulses of the same polarity with interphase delays of 0, 5, 10, 15, 20, 25. 50, and 100 msec. In all three parts the defibrillating cathode was a 6.17 cm2 transvenous spring electrode positioned in the RV apex and the anode was a 113 cm2 cutaneous left chest wall eiectrode patch. With all waveforms, the trailing edge voitage of the first phase was equal to the negative of the leading edge voltage of the second phase. There was no statistical difference in DFTs or in 50% successful defibriliation points for phase separations from 0 to 6 msec and 0 to 5 msec for Parts I and II, respectively. In Part I there was a significant increase in DFTs for phase separations of 8 and 10 msec compared to a phase separation of 0 msec. In Part III there was no significant difference for seporations of 0 and 5 msec; however, there was a significant increase in DFT requirements for separations from 5 to 50 msec, which then decreased with a separation of 100 msec for ail three waveforms tested. In conciusion, defibriilation efficacy was unchanged with phase separations up to 6 msec. With phase separation, the rapid voltage change during phase reversal is split in half and. thus, cannot explain the improved efficacy of biphasic waveforms.