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 an indirect effect through changes in heart period (T(AA)). To evaluate how dynamic vagal activation affects T(AV), we stimulated the Vagal nerve with frequency-modulated Gaussian white noise and estimated the transfer function from vagal stimulation to the T(AV) response under conditions of no pacing and 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 vagal stimulation to T(AV) has low-pass filter characteristics. Constant pacing 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 and indirect effects were dynamically counterbalanced and exerted stable T(AV) transient response during Vagal stimulation under normal sinus rhythm.