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 AA (2.9 ± 1.2 vs. 2.3 ± 0.5 s). Because changes in T AA reciprocally affected T AVwithout significant delay, the direct and indirect effects were dynamically counterbalanced and exerted stable T AV transient response during vagal stimulation under normal sinus rhythm.