Effects of autonomic activity and changes in heart rate on atrioventricular conduction

The effects of autonomic neural stimulation and changes in atrial pacing frequency on atrioventricular (AV) conduction were determined in anesthetized open-chest dogs. Increases in vagal stimulation frequency and in pacing rate significantly increased the AV interval, whereas increases in sympathetic stimulation frequency reduced the AV interval. Vagal stimulation (1.4 Hz) prolonged the AV interval by 17 ms when the atrial pacing rate was 2 Hz. On the other hand, the same vagal stimulation increased the AV interval by 29 ms when the pacing rate was 2.73 Hz. In addition, sympathetic stimulation (1.2 Hz) reduced the AV interval by 29 ms when the pacing rate was 2 Hz. In contrast, the same sympathetic stimulation reduced the AV interval 54 ms when the pacing rate was 2.73 Hz. However, the increase in vagal stimulation did not significantly alter the positive dromotropic response of the AV node to sympathetic stimulation. Therefore, the response of AV conduction to combined sympathetic and vagal stimulation was essentially the algebraic sum of the responses to the individual stimulations. Furthermore, the level of activity in one autonomic division did not alter appreciably the interaction between the pacing rate and the activity in the other autonomic division; i.e., the interaction between pacing rate, sympathetic stimulation, and vagal stimulation was not significant.