Relationship between "extension of refractoriness" and probability of successful defibrillation

The "extension of refractoriness" hypothesis, which suggests that the shock halts fibrillation by extending the refractory period, has not been directly tested. Defibrillation (5 isolated rabbit hearts; 111 episodes) was attempted by 8-ms pulses (65% tilt) delivered through epicardial patches. Monophasic action potentials were recorded in a low current density region (6.3 V/cm at 90% success). Fifty shocks failed to convert; 61 shocks successfully defibrillated. Postshock response duration (from shock to repolarization) was significantly longer for successful type A (with no postshock activations) defibrillation (102.3 +/- 7.5 ms) than for unsuccessful defibrillation (47.6 +/- 4.3 ms; P < 0.0001) for shocks occurring during the last 40% of the fibrillation action potential. Probability of success and postshock response duration both increased with current intensity. However, at each intensity, response durations for successful defibrillation were significantly longer than those for unsuccessful defibrillation. A minimum prolongation of 75 ms was associated with type A defibrillation. These results suggest that shock-induced response duration correlates with successful defibrillation and that a response of 75 ms is required to completely block fibrillation wavefronts.