Relation between repolarization and refractoriness in the human ventricle: cycle length dependence and effect of procainamide.

The cycle length dependence of the action potential duration and the effective refractory period of the right ventricular endocardium were investigated in 24 patients undergoing electrophysiologic studies for suspected ventricular tachycardia. The action potential duration at 90% repolarization and the effective refractory period at twice diastolic threshold strength were measured at the same catheter site at steady state cycle lengths of 350 to 600 ms. Both measurements decreased linearly with decreasing cycle length, maintaining a parallel relation. When the relation between action potential duration and effective refractory period was expressed as the effective refractory period-action potential duration difference, nearly constant values (range -12 to -15 ms) were obtained at all cycle lengths. To determine whether sodium channel blocking drugs influence the effective refractory period-action potential duration relation in humans, measurements of these two variables were obtained in 15 patients before and during the infusion of procainamide. Procainamide prolonged the action potential duration at each cycle length by a near constant amount over baseline values (p less than 0.001). Procainamide also increased the effective refractory period at each cycle length but with a greater incremental increase at the shorter cycle lengths. The rate-dependent increase in the effective refractory period-action potential duration difference became significant at cycle lengths less than or equal to 400 ms; at these high rates, the effective refractory period-action potential duration difference became positive (1.6 ms, p less than 0.01 compared with baseline). Thus, in the human ventricle, the action potential duration and the effective refractory period have a close relation that remains fixed over a wide range of cycle lengths.(ABSTRACT TRUNCATED AT 250 WORDS)