REFRACTORY PERIOD EXTENSION DURING VENTRICULAR PACING AT FIBRILLATORYPACING RATES

Refractory period extension (RPE) has been proposed as a basic mechani sm for defibrillation but it remains unclear if RPE exists at the fast rates associated with ventricular fibrillation. In 7 pentobarbital an esthetized dogs, we measured refractory periods with and without 8 ms rectangular transcardiac shocks at left ventricular pacing rates of 20 0-600 beats/min. To achieve these high rates, an incremental rate paci ng 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 elec trode array(4 x 4 cm) was used to detect the presence, or absence of a n activation sequence sweeping away from the pacing site. At each rate , a control refractory period (RPc) was measured and refractory period s were also measured for 8 and 12 V/cm shocks with coupling intervals of 60% to 90% of RPc. RPc decreased as the rare increased with a minim um 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. RP E/RPc increased with increased coupling interval or higher shock inten sity. We conclude that during ventricular pacing at fibrillatory rates tissue is nearly always in a refractory state; that RPE exists at fib rillatory activation rates; and that RPE/RPc versus shock coupling int erval does not vary strongly with pacing rate. These findings support the hypothesis that RPE contributes to defibrillation.