Effects of sympathetic stimulation on refractory periods of ischemic canine ventricular myocardium.

We studied the difference in effect of sympathetic stimulation on refractory periods of ischemic and non ischemic myocardium in eight dogs, and the effect of sympathetic stimulation on dispersion of refractory periods in ischemic myocardium in seven additional dogs. In the first group of dogs, refractory periods of ischemic sites averaged 164 +/- 2.2 msec (M +/- SEM) and those at non ischemic sites averaged 193 +/- 1.8 msec. Sympathetic stimulation shortened refractory periods at non ischemic sites to an average of 183 +/- 2.0 msec and prolonged refractory periods at ischemic sites to an average of 171 +/- 2.2 msec. As a result of the different effects of sympathetic stimulation on refractory periods of ischemic and non ischemic myocardium, refractory periods between ischemic and non ischemic areas were more uniform during sympathetic stimulation than during coronary occlusion alone. In the second group of dogs in which the effects of sympathetic stimulation on dispersion of refractory periods were studied, pooled variances in refractory periods were calculated. There was no statistically significant difference in the pooled variance of refractory periods during control periods and during sympathetic stimulation alone. Coronary occlusion alone significantly increased the variance in refractory periods, but there was no statistically significant difference in the variance of refractory periods during coronary occlusion alone and during coronary occlusion and sympathetic stimulation. Our findings suggest that at some times during the course of myocardial infarction the effects of high sympathetic tone may be partially protective with respect to arrhythmias by reducing inequalities of recovery between ischemic and non ischemic tissue.