Augmented baroreflex heart rate gain after moderate-intensity
dynamic exercise.
Halliwill, John R., J. Andrew Taylor, Timothy D. Hartwig, and Dwain L.
Eckberg.
Department of Anesthesiology, Mayo Clinic, Rochester, Minnesota
55905; Hebrew Rehabilitation Center for Aged, Beth Israel Hospital,
and Harvard Medical School, Division on Aging, Boston Massachusetts
02131-1097; Departments of Physiology and Medicine, Medical College
of Virginia and Hunter Holmes McGuire Department of Veterans Affairs
Medical Center, Richmond, Virginia 23298-0551
APStracts 2:0264R, 1995.
The occurrence of a sustained vasodilation and hypotension after
acute, dynamic exercise suggests that exercise may alter arterial
baroreflex mechanisms. Therefore, we assessed systemic hemodynamics,
baroreflex regulation of heart rate, and cardiac vagal tone following
60 min cycling at 60% peak oxygen consumption in 12 healthy untrained
men and women (ages 21-28 yr). We derived sigmoidal carotid-cardiac
baroreflex relations by measurement of R-R interval changes induced
by ramped, stepwise, R-wave triggered changes in external neck
pressure from 40 to -65 mmHg. We estimated tonic cardiac vagal
control with power spectral analysis of R-R interval variability in
the respiratory frequency band (0.2 - 0.3 Hz) during frequency- and
tidal volume-controlled breathing. Both mean arterial pressure and
total peripheral resistance were reduced post-exercise (pressure:
from 86+/-2 (mean+/-SE) to 81+/-2 mmHg; resistance: from 23+/-2 to
16+/-1 units; both P < 0.05). Cardiac output was increased post
-exercise (from 3.9+/-0.3 to 5.5+/-0.5 l min-1, P < 0.05). Both
slope and range of the carotid-cardiac baroreflex relation were
increased post-exercise (slope: from 4.7+/-0.7 to 6.1+/-0.9 ms mmHg
-1; range: from 186+/-23 to 238+/-30 ms, P < 0.05). Respiratory
R-R interval variability (cardiac vagal tone) was not changed at any
time after exercise, while heart rate and plasma norepinephrine
levels were elevated. Thus, moderate-intensity dynamic exercise
increases heart rate and cardiac output, reduces peripheral vascular
resistance, and augments baroreflex responsiveness. Our data suggest
that augmented baroreflex heart rate gain restrains rather than
contributes to post-exercise hypotension, which appears to be
mediated predominately by vasodilation.
Received 2 March 1995; accepted in final form 13 September 1995.
APS Manuscript Number R148-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 31 October 95