AJP - Heart and Circulatory Physiology, Vol 244, Issue 1 102-H108, Copyright © 1983 by American Physiological Society
Autonomic influences on atrioventricular conduction in conscious dogs
J. X. Thomas Jr and W. C. Randall
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.
