Dynamic counterbalance between direct and indirect vagal controls of atrioventricular conduction in cats

The vagal system regulates the atrioventricular conduction time (T-AV) via two opposing mechanisms: a direct effect on the atrioventricular node and a n indirect effect through changes in heart period (T-AA). To evaluate how d ynamic vagal activation affects T-AV, we stimulated the Vagal nerve with fr equency-modulated Gaussian white noise and estimated the transfer function from vagal stimulation to the T-AV response under conditions of no pacing a nd constant pacing in anesthetized cats. The effect of changes in T-AA On T -AV was estimated by a random-pacing protocol. The transfer function from v agal stimulation to T-AV has low-pass filter characteristics. Constant paci ng increased the maximum step response in T-AV(2.4 +/- 1.2 vs. 6.3 +/- 2.2 ms/Hz, P < 0.01). The time constant did not differ between the vagal effect on T-AV and that on T-AV (2.9 +/- 1.2 vs. 2.3 +/- 0.5 s). Because changes in T-AA reciprocally affected T-AV without significant delay, the direct an d indirect effects were dynamically counterbalanced and exerted stable T-AV transient response during Vagal stimulation under normal sinus rhythm.