Intra-SA-nodal pacemaker shifts induced by autonomic nerve stimulation in the dog

Pacemaker shifts in the canine heart were inferred during stimulation of thoracic cardiac nerves and following norepinephrine from changes in the initial site of activation of bipolar electrodes sutured over the rostral, middle, and caudal regions of the sinus node, over the internodal pathways, and His bundle. During control periods, pacemaker activity was localized within the sinoatrial (SA) node 87% of the time, with the middle electrode most frequently showing initial activation. Stimulation of the right-sympathetic nerves enhanced sinus node pacemaker dominance, shifting it rostrally within the node. Right-vagal stimulation shifted the pacemaker caudally within the SA node, to nonnodal sites, and to the lower atrioventricular node and His bundle. Left-sympathetic stimulation shifted the pacemaker caudally within the sinus node and enhanced pacemaker activity in the vicinity of the internodal pathway electrodes and His bundle. Dispersion of pacemaker activity was particularly apparent during stimulation of the ventrolateral cervical cardiac nerve. Stimulation of the left-vagal nerves produced effects similar to those of the left-sympathetic nerves. Norepinephrine enhanced pacemaker activity particularly in the rostral region of the sinus node. Slight shifts in pacemaker activity within the sinus node produced changes in pattern of atrial excitation.