Direct and reflex cardiac bradydysrhythmias from small vagal nerve stiumaltions

Alterations in cardiac pacemaker location, its rate of discharge, and A-V conduction patterns were induced in anesthetized adult dogs by electrical stimulation of the thoracic vagi and their small cardiac branches before and after cervical vagotomy. Electrical activity from small, contiguous bipolar silver electrodes was amplified and recorded by an optical oscillograph. The electrodes were located over the SA node, the three internodal pathways, the left atrium, and ventricular epicardium. A hoffman-type plaque electrode was placed over the A-V node to record a His bundle electrogram simultaneously with a Lead II electrocardiogram. Electrical stimulation of the intact left recurrent laryngeal nerve and its cardiac branches before and after vagotomy induced both direct and reflex effects on SA nodal cycle length. Efferent dromotropic effects on the A-V node varied from first- to third-degree heart block during stimulation of individual left recurrent cardiac branches. Stimulation of the right recurrent cardiac nerve induced atrial bradycardia with heart block above the His bundle. Stimulation of individual right vagal branches near the heart induced bradycardia, cardiac asystole, shifts in atrial pacemaker location, or activation of His pacemakers. Establishment of the His rhythm probably indicates selective inhibition of supraventricular but not of the His bundle. Asystole and His rhythms induced during stimulation of the more caudal branches of the right cardiac vagal nerves were generally reflexly mediated and were abolished by cervical vagotomy.