Effect of premature depolarization on the duration of action potentials in Purkinje and ventricular fibers of the moderator band of the pig heart. Role of proximity and the duration of the preceding action potential.

Abstract

We compared premature and nonpremature action potentials in Purkinje and ventricular fibers from the moderator band of the pig heart to determine if the duration of premature action potentials depended on factors other than preceding cycle length. In Purkinje fibers, the duration of premature action potentials was cycle-length dependent in responses originating more than 100 msec after the preceding repolarization, but the duration of earlier responses was less than the cycle-length-dependent duration. This cycle-length-independent shortening of premature responses increased with greater proximity to the preceding repolarization and increasing duration of the preceding action potential. In ventricular fibers, the duration of premature action potentials was greater than the cycle-length-dependent duration. This cycle-length-independent lengthening increased as the duration of the preceding action potential increased; it also depended on proximity, being greatest when proximity ranged between 26 and 275 msec. The difference between the durations of simultaneously recorded Purkinje and ventricular action potentials decreased as prematurity increased, but the earliest premature Purkinje action potential was consistently shorter than the simultaneously recorded ventricular action potential. Thus, premature stimulation produced different effects in Purkinje and ventricular fibers. However, in both fibers, the deviation of the duration of premature action potentials from the cycle-length-dependent duration was determined, at least in part, by the duration of the preceding action potential and proximity.