REFRACTORY INTERVAL AFTER TRANSCARDIAC SHOCKS DURING VENTRICULAR-FIBRILLATION

Background Measurements of refractory period extension by shocks durin g ventricular pacing at fast rates predict that all tissue should he r efractory for a brief interval after shocks during fibrillation. This study experimentally determined whether a refractory interval was pres ent just after a shock during fibrillation. Methods and Results In pen tobarbital anesthetized dogs, rectangular monophasic (4-ms) or biphasi c (2.5/1.5-ms) shocks were followed with a 2-ms postshock stimulus (PS S) delivered to the defibrillation electrodes. We measured the effect of PSS on the shock current (I-50) required for 50% defibrillation suc cess. In group 1 (n=6), a 1.0-A PSS had no effect on I-50 when deliver ed up to 35 ms after monophasic shocks but greatly increased I-50 when delivered at 50 to 90 ms. A 0.5-A PSS had no effect at any timing. In group 2 (n=6), we compared 1.0-A PSSs after monophasic and biphasic s hocks. The effect of PSS after monophasic shocks was similar to group 1. After biphasic shocks, PSS at the same timings had similar effects but caused even greater increases in I-50. Conclusions We conclude tha t after both monophasic and biphasic shocks during fibrillation, there is a postshock interval during which the heart is refractory to the r efibrillating effect of PSS. The interval is shorter for biphasic than for monophasic shocks with the same duration and defibrillation effic acy. These findings support the refractory period extension hypothesis for defibrillation and suggest that propagating depolarization activi ty is absent immediately after defibrillation shocks but that it devel ops again at the end of the refractory interval or later.