Effects of flecainide, encainide, and clofilium on ventricular refractory period extension by transcardiac shocks

Progressive depolarization: a unified hypothesis for defibrillation and fibrillation induction by shocks

Experimental studies of defibrillation have burgeoned since the introduction of the upper limit of vulnerability (ULV) hypothesis for defibrillation. Much of this progress is due to the valuable work carried out in pursuit of this hypothesis. The ULV hypothesis presented a unified electrophysiologic scheme for linking the processes of defibrillation and shock-induced fibrillation. In addition to its scientific ramifications, this work also raised the possibility of simpler and safer means for clinical defibrillation threshold testing. Recent results from an optical mapping study of defibrillation suggest, however, that the experimental data supporting the ULV hypothesis could instead be interpreted in a manner consistent with traditional views of defibrillation such as the critical mass hypothesis. This review will describe the evidence calling for such a reinterpretation. In one regard the ULV hypothesis superseded the critical mass hypothesis by linking the defibrillation and shock-induced fibrillation processes. Therefore, this review also will discuss the rationale for developing a new defibrillation hypothesis. This new hypothesis, progressive depolarization, uses traditional defibrillation concepts to cover the same ground as the ULV hypothesis in mechanistically unifying defibrillation and shock-induced fibrillation. It does so in a manner consistent with experimental data supporting the ULV hypothesis but which also takes advantage of what has been learned from optical studies of defibrillation. This review will briefly describe how this new hypothesis relates to other contemporary viewpoints and related experimental results.

Refractory period extension during ventricular pacing at fibrillatory pacing rates

Refractory period extension (RPE) has been proposed as a basic mechanism for defibrillation but it remains unclear if RPE exists at the fast rates associated with ventricular fibrillation. In 7 pentobarbital anesthetized dogs, we measured refractory periods with and without 8 ms rectangular transcardiac shocks at left ventricular pacing rates of 200-600 beats/min. To achieve these high rates, an incremental rate pacing method was used to produce pacing train timing sequences requiring 4.5-27 seconds. A variably timed premature stimulus followed the last stimulus in each pacing train. To determine refractoriness, a 128 electrode array (4 x 4 cm) was used to detect the presence, or absence of an activation sequence sweeping away from the pacing site. At each rate, a control refractory period (RPc) was measured and refractory periods were also measured for 8 and 12 V/cm shocks with coupling intervals of 60% to 90% of RPc. RPc decreased as the rate increased with a minimum RPc of 94 ms at a rate of 600 beats/min (100 ms cycle length). RPE/RPc versus shock coupling interval was similar at all pacing rates. RPE/RPc increased with increased coupling interval or higher shock intensity. We conclude that during ventricular pacing at fibrillatory rates tissue is nearly always in a refractory state; that RPE exists at fibrillatory activation rates; and that RPE/RPc versus shock coupling interval does not vary strongly with pacing rate. These findings support the hypothesis that RPE contributes to defibrillation.

Refractory interval after transcardiac shocks during ventricular fibrillation

BACKGROUND

Measurements of refractory period extension by shocks during ventricular pacing at fast rates predict that all tissue should be refractory for a brief interval after shocks during fibrillation. This study experimentally determined whether a refractory interval was present just after a shock during fibrillation.

METHODS AND RESULTS

In pentobarbital-anesthetized dogs, rectangular monophasic (4-ms) or biphasic (2.5/1.5-ms) shocks were followed with a 2-ms postshock stimulus (PSS) delivered to the defibrillation electrodes. We measured the effect of PSS on the shock current (I50) required for 50% defibrillation success. In group 1 (n = 6), a 1.0-A PSS had no effect on I50 when delivered up to 35 ms after monophasic shocks but greatly increased I50 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 shocks. 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 I50.

CONCLUSIONS

We conclude that after both monophasic and biphasic shocks during fibrillation, there is a postshock interval during which the heart is refractory to the refibrillating effect of PSS. The interval is shorter for biphasic than for monophasic shocks with the same duration and defibrillation efficacy. These findings support the refractory period extension hypothesis for defibrillation and suggest that propagating depolarization activity is absent immediately after defibrillation shocks but that it develops again at the end of the refractory interval or later.