Postshock Recovery Interval of Relatively Refractory Myocardium as a Possible Explanation for Disparate Defibrillation Efficacy Between Monophasic and Biphasic Waveforms

We investigated the electrophysiological background for the waveform related variability of defibrillation efficacy. In 22 open-chest dogs, a localized potential gradient was created using an 8-V or 16-V field stimulus across a pair of plate electrodes separated by 5 mm. The post shock recovery interval of the nondepolarized myocardium adjacent to the excited area was estimated by the residual refractory period after an appropriately timed field stimulus. The postshock recovery interval and the defibrillation threshold were compared among six different waveforms but with the same total duration of 12 ms (n = 11) or 16 ms (n = 11). Six defibrillation thresholds in individual hearts showed a significant inverse correlation with postshock recovery intervals in most dogs (8/11) tested with a total pulse duration of 12 ms (8 V stimulus: r = -0.80 +/- 0.20 [n = 11]). In contrast, waveforms with a total duration of 16 ms failed to reveal this distinct relationship. We conclude that the waveform related variability of defibrillation efficacy is associated with the refractoriness of relatively refractory myocardium when the total pulse duration is within a certain range. However, the mechanisms responsible for waveform performance may vary as the total pulse duration changes.