Effect of Efferent Vagal Stimulation on Coronary Circulation in Atrioventricular Block

In order to differentiate the effect of vagal stimulation from that of heart rate on the coronary circulation, the effect of efferent vagal stimulation was investigated in 26 dogs by keeping the heart rate constant by means of an atrioventricular block. 1. In the initial phase of the vagal stimulation in sinus rhythm the coronary blood flow increased with a rise in systolic pressure and slight decrease in the mean blood pressure; coronary flow then declined gradually with depression of blood pressure. The calculated coronary vascular resistance decreased in all experiments, and averaged -22%. 2. In complete A-V block no increase in coronary flow was observed during vagal stimulation. Both the coronary blood flow and blood pressure declined slightly, and the coronary vascular resistance remained unchanged. This indicates that bradycardia may play a major role in reduction of the coronary vascular resistance during vagal stimulation in sinus rhythm. 3. The effect of atrial contraction was observed in the instantaneous tracing of coronary blood flow in the experiment on A-V block. With each atrial contraction the coronary blood flow decreased synchronously, presumably owing to increase in intraventricular pressure and in extravascular pressure to intramural coronary vessels. The cessation of atrial contraction during vagal stimulation may act to increase the coronary flow and decrease the coronary vascular resistance. 4. In incomplete A-V block, a slight slowing of the heart rate during vagal stimulation was observed, and the coronary vascular resistance decreased by 16% below the control level. The mechanism of this response was discussed. Thus, the decrease in coronary vascular resistance seen during the vagal stimulation can be accounted for mainly on the basis of mechanical factors.