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Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas 75231; and Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas 75235
The
effects of whole body heating on human baroreflex function are
relatively unknown. The purpose of this project was to identify whether
whole body heating reduces the maximal slope of the carotid baroreflex.
In 12 subjects, carotid-vasomotor and carotid-cardiac baroreflex
responsiveness were assessed in normothermia and during whole body
heating. Whole body heating increased sublingual temperature (from
36.4 ± 0.1 to 37.4 ± 0.1°C, P < 0.01)
and increased heart rate (from 59 ± 3 to 83 ± 3 beats/min,
P < 0.01), whereas mean arterial blood pressure (MAP)
was slightly decreased (from 88 ± 2 to 83 ± 2 mmHg,
P < 0.01). Carotid-vasomotor and
carotid-cardiac responsiveness were assessed by identifying the maximal
gain of MAP and heart rate to R wave-triggered changes in carotid sinus transmural pressure. Whole body heating significantly decreased the
responsiveness of the carotid-vasomotor baroreflex (from 
0.20 ± 0.02 to 0.13 ± 0.02 mmHg/mmHg, P < 0.01)
without altering the responsiveness of the carotid-cardiac baroreflex
(from 0.40 ± 0.05 to 0.36 ± 0.02 beats · min
1 · mmHg1,
P = 0.21). Carotid-vasomotor and carotid-cardiac
baroreflex curves were shifted downward and upward, respectively, to
accommodate the decrease in blood pressure and increase in heart rate
that accompanied the heat stress. Moreover, the operating point of the
carotid-cardiac baroreflex was shifted closer to threshold (P = 0.02) by the heat stress. Reduced
carotid-vasomotor baroreflex responsiveness, coupled with a reduction
in the functional reserve for the carotid baroreflex to increase heart
rate during a hypotensive challenge, may contribute to increased
susceptibility to orthostatic intolerance during a heat stress.
hyperthermia; baroreceptor; orthostatic intolerance
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