On the mechanism of the probabilistic nature of ventricular defibrillation threshold

The probabilistic nature of the ventricular defibrillation threshold (DFT) remains poorly understood. We hypothesized that shock outcome is a function of the amount of myocardium in its vulnerable period (VP). The endocardial surface of five isolated, perfused swine right ventricles was mapped with 477 bipolar electrodes during ventricular fibrillation (VF). Shock parameters and VF cycle length were not significantly different in the successful (S; n = 26) and failed (F; n = 26) trials. At the instant of the shock, the number of sites with 45- to 55-ms recovery was significantly smaller in the S trials than the F trials (P < 0.04). No significant difference in the number of sites with recovery intervals outside the 45- to 55-ms range was seen in S and F shocks. Endocardial action potential showed that a recovery time of 45-55 ms corresponded to the VP spanning -15 to -60 mV in 92% of the regenerative action potentials. We conclude that the probabilistic nature of the DFT is related to the amount of myocardium in its VP.