Effect of shock timing on cardioversion efficacy

Attempts to improve defibrillation efficacy by timing the shock to local or global measures of electrical activity during ventricular fibrillation have been inconclusive, and the proposed mechanisms for such a benefit remain speculative. Using video imaging technology, we recorded transmembrane potential (Vm) from ∼5000 sites over the anterior and posterior ventricular epicardium of 6 Langendorff-perfused rabbit hearts during cardioversion of stable reentrant arrhythmias. Monophasic shocks at three strengths (0.75, 1.5, and 3.0 A) for each polarity were delivered between electrodes inside the right ventricle (RV) and above the left atrium multiple times. Cardioversion efficacy at the three strengths was 21%, 42%, and. 92% for RV+ shocks, which primarily depolarized the epicardium, and 10%, 15%, and 33% for RV- shocks, which primarily hyperpolarized the epicardium. Outcome was dependent on the pre-shock state for both polarities at strengths near the 50% success level. Preceding successful shocks, more epicardium was recovered resulting in more shock-induced excitation and longer post-shock depolarization (for RV+) and faster post-shock elimination of excitable gaps (for RV-) compared to failures. These findings provide a mechanistic link between shock-induced changes in Vm and the effect of polarity and timing on cardioversion efficacy.