Effect of Extrasystoles on Idioventricular Rhythm

Abstract

Ventricular extrasystoles exert a variable effect on idioventricular rhythm. The postextrasystolic, or the "returning," cycle may be unchanged, or it may be shorter or longer than the dominant cycle. The factors responsible for these different responses have never been investigated. Therefore, the effect of electrically induced extrasystoles was studied by microelectrode techniques in an in vitro model of idioventricular rhythm consisting of spontaneously firing Purkinje fiber bundles with their attached ventricular segments. Late extrasystoles lengthened the Purkinje returning cycle, primarily by transiently increasing the duration of spontaneous phase-4 depolarization. In contrast, early extrasystoles produced shortening of the returning cycle, primarily because of the marked abbreviation of the duration of the extrasystolic action potential. Other factors affecting the ultimate duration of the Purkinje returning cycle were delineated. The effect of extrasystoles on the electrical activity of the ventricular segments was studied by surface electrograms. This effect was complex and was modified by the direction of propagation of the extrasystole with respect to the pacemaker site and by the degree of conduction delay. Ventricular electrical activity usually reflected the electrical events occurring in the pacemaker cells, but frequent exceptions occurred: the ventricular returning cycle could be prolonged while the corresponding Purkinje returning cycle was shortened and vice versa. "Interpolation" of an extrasystole did not necessarily indicate failure of the extrasystole to discharge the pacemaker site. The effects of extrasystoles on idioventricular rhythm which have been described in the earlier literature are easily understood from the findings of this study.