A model of dynamic vagus-sinoatrial node interactions.

Computer simulations of dynamic vagus-sinoatrial (SA) node interactions were performed using an empirical model. The phasic effects of single vagal trains on pacemaker cycle length obtained experimentally in isolated preparations were summarized in phase response curves (PRCs). These PRCs were used to stimulate the interactions of the sinoatrial pacemaker with single or with repetitive vagal input. For single stimuli, the triphasic inhibitory curve describing the time course of a brief vagal burst (G. Brown and J. Eccles. J. Physiol. London 82: 211-241, 1934; and J. Jalife and G. K. Moe. Circ. Res. 45: 595-607, 1979) was used to predict the PRC at any given spontaneous pacemaker cycle length. In simulations of repetitive vagal input the model predicted the entrainment of the pacemaker. The patterns of interaction were dependent on the shape and amplitude of the PRC as well as on the relationship between the spontaneous pacemaker period and the vagal cycle length. At certain vagal frequencies, stable entrainment of the pacemaker occurred, and the entrained pacemaker period held harmonic relations to the vagal input (i.e., 1:1, 2:1, and so on). At other frequencies, zones of instability were found in which arrhythmic patterns developed. These predictions of the model matched the experimental results very closely. Under some conditions, during simulations with fixed sinovagus coupling intervals, the model generated patterns of sinus activity similar to those occurring experimentally or in patients with apparent sinoatrial block. The model was also capable of generating patterns similar to those obtained in cases of isorhythmic atrioventricular dissociation. The study of these interactions may have important bearing on the understanding of the dynamic control of heart rate by the parasympathetic nervous system and may be used to explain certain cardiac dysrhythmias.