Electrophysiologic depression in myocardium by defibrillation-level shocks

The authors investigated the electrophysiological consequences of high currents on isolated strips of cardiac muscle. The objective was to measure the graded changes in intracellular action potentials - amplitude, duration, excitability, conduction velocity - resulting from the various levels of electrical shock typically used in defibrillation, cardioversion, and ablation. Following the shock, reversible injury is observed, whose severity is graded in proportion to shock strength. Smaller shocks (below 500 ma/cm², 10 ms) caused depression in action potential amplitude, duration, and propagation velocity. The degree of depression and the time course for complete recovery (0-50-s duration), followed by an action potential recovery phase similar to that observed for the smaller shocks. These graded effects can in conjunction with the whole heart description, of the current densities during defibrillation, permit an estimation of the total volume of tissue subject to inexcitability, reduced propagation velocity, shortened duration, and depressed action potentials.