The Effect of Changes in Rate and Rhythm on Supernormal Excitability in the Isolated Purkinje System of the Dog

Abstract

Microelectrode techniques were used to investigate the effects of rate and rhythm changes upon the period of supernormal excitability during repolarization in isolated canine right and left bundle branch and Purkinje fibers. The same microelectrode was used for both intracellular stimulation and recording. Excitability curves (strength-interval curves) were measured as the minimum depolarizing current required to re-excite the fiber during repolarization of a conducted action potential. During the supernormal period it required an average of 17.0 ± 4.6 SD % less than diastolic current to re-excite the fibers throughout the left and right bundle branch-Purkinje system. The supernormal period of excitability in the bundle branch-Purkinje system was voltage dependent, reaching minimum current requirements at 74.3 ± 5.8 SD mV. Excitability curves of conducted action potentials were determined at basic cycle lengths ranging from 1000 to 300 msec and following single and multiple premature beats. The decrease in action potential durations associated with shorter cycle lengths was not accompanied by a corresponding shortening of the supernormal period of excitability. Therefore, at shorter cycle lengths the supernormal period encompassed a greater proportion of the total action potential; in some cases as much as 50% of the total action potential duration exhibited a period of supernormal excitability. The supernormal period of excitability could be eliminated by perfusing the tissues in elevated potassium (5.0 and 7.5 mM) Tyrode's solution. The possible implications for the generation of re-entrant arrhythmias are discussed.