EFFECT OF CHANGE IN CYCLE LENGTH ON VENTRICULAR ACTION POTENTIAL IN MAN

Electrophysiologic studies of human papillary fibers disclosed a characteristic alteration in the configuration of action potential, associated with abrupt change in the cycle length. After a relative prolongation of the cycle, phase 2 was abbreviated and phase 3 lengthened. Conversely, a relative abbreviation of the cycle was terminated by an action potential with a more sustained phase 2 and a more precipitous phase 3. In either case the total duration of the action potential remained unchanged. The physiologic mechanism underlying these changes in action potential, although obscure, may represent a primary alteration in repolarization possibly caused by time-dependent changes in membrane permeability to K+. This pattern of change in action potential, associated with abrupt alteration in rate, is clearly at variance with the conventional concept of a direct relation between duration of phase 2, the total action potential and the length of the preceding diastole. The latter relation appears to hold true only if the preparation is driven at a constant rate. It is suggested that this interval-dependent alteration in repolarization may account for the so-called change in post-extrasystolic T wave that is occasionally observed in the surface electrocardiogram. The absence of such T wave changes in normal subjects suggests that these changes in repolarization may be of insufficient magnitude in the normal subject to be recorded in the surface electrocardiogram. A temporal relation has been observed between the magnitude of contractility and morphology of the human action potential. Similar observations have been made in the dog. Whether the parallel between morphology of action potential and potentiation of contractility reflects a causal relation between inotropic phenomena and membrane permeability remains to be seen. © 1968.