Brief vagal stimuli and AV conduction in dogs: effects on retrograde exceed antegrade

We opened the chests of 20 mongrel dogs (16-28 kg) that were anesthetized with pentobarbital sodium (22 mg/kg), given propranolol (1 mg/kg), and bipolar catheters placed into the right atrium and ventricle. We crushed the sinoatrial (SA) node and paced the heart to produce either an antegrade (A-V) or a retrograde (V-A) conduction (where A is atrial activation and V is ventricular activation). Both cervical vagosympathetic trunks were ligated and connected to electrodes that were driven by a computer-controlled, isolated stimulators. Three brief vagal stimulus bursts (at the same intensity but at different times in the cardiac cycle) were given at least 1 min apart, the pacing site was changed, and the identical three-stimulus bursts were again applied. This procedure was continued until the phase of the stimulus had scanned the entire cardiac cycle. Forty-four sets of vagal effect curves were generated. The composite data indicate that brief vagal stimuli have about twice the effect on the peak amplitude of the change in retrograde than in antegrade AV nodal conduction in the dog. Unexpectedly, the duration of the parasympathetic effect is also considerably greater (approximately 40%) and the decay is slower (approximately 48%) for retrograde than for antegrade conduction, even when the vagal effect curves have been normalized to the same amplitude. This latter result suggests that some factor other than diffusion and inactivation of ACh determines the duration of the A-V conduction response to a brief vagal stimulus.