Phasic Responses of SA and AV Nodes to Vagal Stimulation

Abstract

Efferent vagal impulses appear in brief trains at periods that are harmonically related to the cardiac cycle [1,2]. Clustering of vagal discharges in bursts results from feedback circuits mediated by baroreceptor reflexes. When the systolic pulse wave reaches the baroreceptors in the aorta and the carotid sinuses, it triggers, after a delay, a discrete burst in the efferent cardiac vagal fibers [3, 4]. In turn, brief vagal discharges induce a relatively brief hyperpolarization of cells in the sinoatrial region and may result in phase-dependent changes of pacemaker cycle length and entrain the pacemaker to discharge at periods that are briefer or longer than its own intrinsic period [5]. Recent experiments have shown that the response of the atrioventricular (AV) node to brief vagal discharges is also phase-dependent [6–9] but its time course is distinctly separate from that of the sinoatrial response. Indeed, the results of de la Fuente et al. [9] and Levy et al. [6] have convincingly demonstrated that the vagal effects on both systems can be out of phase with each other by as much as 180° depending on the basic cycle length and recent experiments by Jalife et al. [10] indicate that the differential sensitivity of the two nodes could set the stage for vagally induced rhythm alterations that predominate in one or the other system, depending on frequency relations. In this essay we shall discuss the implications of this phasic behavior in terms of 1) the entrainment of the cardiac pacemaker by phasic vagal activity; 2) the changes in atrioventricular conduction induced by the vagal burst; 3) the dynamic interactions between the vagal discharge, the pacemaker cycle and the atrioventricular conduction system; and 4) the possible clinical implications of these interactions.