EFFECTS OF ATRIAL DEFIBRILLATION SHOCKS ON THE VENTRICLES IN ISOLATEDSHEEP HEARTS

Background-The effects of cardioversion of atrial fibrillation on the activation sequence of the ventricles have not been previously studied . In this study we examined the events in the ventricle that follow th e application of atrial defibrillatory shocks. Methods and Results-We used video imaging technology to study the sequence of activation on t he surface of the ventricles in the Langendorff-perfused sheep heart. We recorded transmembrane potentials simultaneously from over 20 000 s ites on the epicardium before and after biphasic shocks applied by a p rogrammable atrial defibrillator. The first epicardial activation afte r the shock depended on both the voltage and timing of the shock. Duri ng ventricular diastole shocks as low as 10 V produced ventricular exc itation, although the time between the shock and the first epicardial activation (latency) was approximate to 30 ms. As the shock voltage wa s increased to 120 V, latency decreased to zero and the entire epicard ium was depolarized within 30 ms. For 120-V shocks delivered late in s ystole, the depolarization sequence produced by the shock was similar to the previous repolarization sequence. Shocks of 120 V applied 150 t o 300 ms after the previous ventricular excitation induced ventricular fibrillation. Ventricular fibrillation was induced by multiple focal beats after the shock, which produced waves that propagated but broke down into reentry within regions of high repolarization gradients. Con clusions-These results demonstrate that atrial defibrillation shocks e xcite the ventricles even at low shock voltages. In addition, ventricu lar fibrillation can be induced by shocks given in the vulnerable peri od by producing focal patterns that break down into reentrant waves.