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.
Gill
R M
RM
Department of Electrophysiology Research, Lilly Research Laboratories Division, Eli Lilly and Company, Indianapolis, Indiana 46285, USA.
Sweeney
R J
RJ
Reid
P R
PR
D000818:Animals; D002304:Cardiac Pacing, Artificial / Q000379:methods; D004285:Dogs; D006352:Heart Ventricles; D012032:Refractory Period, Electrophysiological; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy*