Lesser tachycardia during isometric exercise in older humans is
associated with impaired cardiac vagal withdrawal.
Taylor, J. Andrew, Junichiro Hayano, and Douglas R. Seals.
From the Departments of Medicine, Harvard Medical School and Beth
Isreal Hospital, Boston, Massachusetts; the Third Department of
Internal Medicine, Nagoya City University Medical School, Nagoya,
Japan; and the Department of Kinesiology, University of Colorado at
Boulder, Boulder, Colorado
APStracts 2:0188A, 1995.
The tachycardia which accompanies isometric exercise decreases with
age in humans, however the mechanism of this decline is unknown. To
determine if less cardiac vagal withdrawal is associated with the
age-related decrease in this response, we assessed high frequency R-R
interval variability (0.15-0.40 Hz), an index of cardiac vagal tone,
before (control) and during isometric exercise to exhaustion in
healthy young (21-29 yrs, N=12) and older (61-72 yrs, N=11) men.
During control, the two groups did not differ in respiratory rate,
mean arterial pressure, or heart rate, although the older subjects
had a lower high frequency amplitude (12.2+/-1.1 vs. young: 29.4+/
-4.6 ms/Hz, P<0.05). During isometric exercise, the respiratory rate
and arterial pressure responses did not differ; however, the older
men had a lower absolute heart rate and a smaller increase in heart
rate, and only the young men demonstrated a significant decline in
high frequency amplitude. The heart rate responses to exercise were
correlated to the declines in high frequency amplitude (initial
responses: r= -0.808, midpoint responses: r= -0.714, peak responses:
r= -0.632; all P<0.005), which were, in turn, correlated to control
high frequency amplitude (initial: r= -0.727, midpoint: r= -0.643,
peak: r= -0.610; all P<0.01). Thus, smaller declines in high
frequency amplitude, related to lower baseline amplitude,
corresponded to smaller increases in heart rate throughout isometric
exercise. Therefore, we conclude that the smaller tachycardic
response to isometric exercise in older humans is associated with an
inability to decrease cardiac vagal tone below an already reduced
baseline level.
Received 29 August 1994; accepted in final form 11 April 1995.
APS Manuscript Number A912-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 9 May 1995.