Autonomic influences on atrioventricular conduction in conscious dogs

Autonomic innervation of the atrioventricular (AV) junction modulates conduction of the cardiac impulse, the sympathetic nerves facilitating and parasympathetic nerves impeding conduction. Experiments assessed the relative importance of parasympathetic vs. sympathetic control by pharmacologic blockade with atropine (0.2 mg/kg) or propranolol (1.0 mg/kg) while pacing the right atrium (100-400 beats/min) in normal, conscious resting dogs and in dogs that had undergone chronic, intrapericardial cardiac denervation. Maximum pacing rate with 1:1 atrioventricular conduction was termed Rmax. Control Rmax at rest was 125.69 +/- 9.49 beats/min and was slightly reduced after propranolol to 118.28 +/- 10.98 (P = 0.043). After atropine, Rmax was significantly increased to 344 beats/min. Propranolol and atropine together resulted in an Rmax of 308 beats/min, which was significantly less than after atropine alone. Rmax in cardiac-denervated dogs was 301 beats/min, which was not significantly different from that following total pharmacologic blockade. In conscious, resting, unsedated dogs, the upper limit of AV nodal conduction is associated with the level of parasympathetic rather than sympathetic tone; during exercise the sympathetics assume greater importance.