Effect of Cycle-Length Alteration upon the Configuration of the Canine Ventricular Action Potential

Action potentials recorded from canine ventricular fibers displayed characteristic alterations in configuration upon abrupt change in the cycle length. After a relative prolongation of the diastolic interval, abbreviation of phase 2 and slowing of phase 3 were consistently observed, while a relative shortening of diastole was terminated by an action potential displaying prolongation of phase 2 and a steeper phase 3. Neither circumstance was attended by a consistent alteration in action potential duration. The more conventional view, directly relating duration of phase 2 to the length of the preceding diastole, clearly does not obtain immediately following an abrupt alteration of cycle length. This cycle-length-dependent phenomenon may account, in part, for the post-extrasystolic T-wave alterations observed in the human electrocardiogram. It is also consistent with the clinical observation that such T-wave changes may occur without premature contractions, requiring only a relative prolongation of the R-R interval. Evidence is also presented which relates electrical alter-nans of the T wave to this interval-dependent phenomenon. This characteristic change in action potential configuration occurs under the same temporal circumstances that others have associated with potentiation of contractility. It is therefore suggested that these inotropic phenomena and changes in action potential configuration may relate to alterations in membrane permeability.