Optimizing Phase Two of the Biphasic Waveform

The second model assumption is that the goal of the optimal second phase is to remove the charge deposited on myocardial cells by phase one, returning the membrane voltage to zero. The benefit of phase two can be seen in Fig. 8. The closer phase two discharges the residual membrane to zero voltage, the lower the predicted amplitude required for defibrillation in phase one. Retrospective metaanalysis29 and prospective30 data indicate that defibrillation efficacy is inversely related to the residual membrane voltage at the end of phase two. The phase one current required correlates with the square of the calculated residual membrane potential after phase two.

The major predictions of this theory have been supported by animal2'4'30 and human studies.1'46 One such prediction is that the optimal ratio of phase one to phase two is higher for larger values of ts (capacitance, pathway resistance, or both) and lower for lower values of Ts.

Figure 6 illustrates the differences in predicted cell membrane responses to the electrical fields of 140 and 40 |F waveforms of equal stored energy. Phase one of the 140 |F waveform produces a weaker, but longer-lasting, field than phase one of the 40 |F waveform. The cell response to the applied 140 | F waveform is slower and continues longer. For phase two, the leading-edge voltage is a greater fraction of the phase one, leading-edge voltage, resulting in more rapid charge burping for this waveform. In addition, the phase two, negative applied voltage exceeds a minimal absolute value longer for the 140 |F waveform, resulting in a persistent negative residual membrane voltage for high ratios of phase two to phase one. The cell-response curves appear underdamped. In contrast, because the negative applied voltage decays rapidly for the 40 | F waveform, the cell response does not decrease below the relative zero value. The cell-response curves appear overdamped.

The experimentally measured effect on cell response and DFT of changing the ratio of phase one to phase two is shown in Fig. 9. For the 40 |F waveform the charge-burping hypothesis predicts that phase-duration ratios of 0.5 and 1.0 fail to return the cell membrane

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