Electrophysiological background of individual variability in electrical defibrillation efficacy

This study was performed to test whether heart-to-heart variability of defibrillation efficacy is attributable to differences in postshock refractory state of nondepolarized myocardium. In 30 anesthetized dogs, a localized potential gradient was created using 1-16 V of stimulus across a pair of platinum plate electrodes on the right ventricle (5-mm interelectrode distance). The postshock recovery interval (PSRI) of the relatively refractory myocardium directly adjacent to the excited area was estimated by measuring the recovery interval after an appropriately timed field stimulus. The transcardiac defibrillation threshold (DFT) was also determined. The results showed that DFT normalized by the weight of the heart was inversely correlated with the PSRI measured with a field stimulus of 6 V (local shock intensity approximately 5 V/cm) or more (6 V: r = -0.502, P < 0.005; 16 V: r = -0.635, P < 0.0005). This observation suggests that variability of defibrillation efficacy in intact hearts is largely due to differences in the postshock refractory state of the nondepolarized myocardium.