AJP - Heart and Circulatory Physiology, Vol 249, Issue 4 703-H709, Copyright © 1985 by American Physiological Society
Sustained increases in heart rate induced by timed repetition of vagal stimulation in dogs
T. Yang, M. D. Jacobstein and M. N. Levy
We determined the influence of the "free-running cycle length" (tau FR) on
chronotropic responses to one burst of right vagal stimuli per cardiac
cycle in anesthetized dogs (tau FR, cycle length that prevailed in absence
of right vagal stimulation). We varied tau FR by the following methods: 1)
tonic left vagal stimulation in pentobarbital-anesthetized animals; 2)
tonic left vagal stimulation plus sinus node cooling in
pentobarbital-anesthetized animals; and 3) anesthesia with fentanyl,
droperidol, and pentobarbital. When tau FR was less than a critical value
[1,019 +/- 60 (SE) ms], right vagal stimulus bursts always had the expected
negative chronotropic effect. However, when the tau FR was increased beyond
critical value, right vagal stimulus bursts delivered within a specific
portion of cardiac cycle actually had a positive chronotropic effect; i.e.,
cycle lengths diminished to values below tau FR. As tau FR was
progressively increased beyond critical value, positive chronotropic
response became greater and could be evoked by stimulus bursts delivered
within a greater fraction of cardiac cycle. The right vagal stimuli that
elicited the maximum positive chronotropic effect were those that were
given approximately 235 ms prior to beginning of next atrial
depolarization. This critical time probably occurs near the end of the
period of phase 4 depolarization of sinus node automatic cells.
