Effects of Acute Passive Heat Exposure
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Date
2016-06
Authors
Harris, Sarianne
Journal Title
Journal ISSN
Volume Title
Publisher
University of Oregon
Abstract
BACKGROUND: Passive heat therapy (regular hot tub or sauna use) has gained
attention for its potential to improve cardiovascular health, and recent evidence suggests
that it produces beneficial vascular adaptations. However, the cardiovascular responses
to a single bout of hot water immersion have not yet been characterized; therefore the
mechanisms that produce long-tenn adaptations are not yet fully known. PURPOSE:
To examine the acute physiological and perceptual effects as well as the safety and
acceptability ofa 60 minute bout of hot water immersion. METHODS: Thirteen
healthy, young (23 ± 1 years) subjects were immersed to heart level (both anns out) in
40.5°C water for 60 min (average peak rectal temperature: 38. 7 ± 0.1 °C). All
measurements were taken at baseline prior to and during immersion. Subjective
experiences were measured with scales of perceived exertion, skin wettedness, and
thermal sensation. Heart rate was measured by commercially available chest strap,
blood pressure by an automated sphygmomanometer, and blood glucose with a
commercially available glucose monitor. Cardiac output was measured with ah opencircuit
acetylene wash-in method. Carotid and brachia} artery diameter and blood
velocity were measured using Doppler ultrasonography. Values were used to calculate blood flow and shear rate, a frictional force exerted on the endothelium by blood which
is associated with beneficial vascular adaptation. Skin blood flow was measured using
laser Doppler flowmetry and is presented as a percentage of maximal cutaneous
vascular conductance (%CVCmax; CVC = laser Doppler flux/mean arterial pressure),
determined by local heating to 43.5˚C at the end of hot water immersion. Temperatures
were measured with thermocouples on the skin, a rectal thermistor, tympanic, and
sublingual probes. Peak changes were compared to baseline using Student’s paired ttest,
and significance was set to P<0.05. Data are presented as mean ± S.E. RESULTS:
Heart rate increased from 60 ± 2 at baseline to a peak of 119 ± 4 bpm (p<0.01) during
immersion. Cardiac output increased from 6.3 ± 0.4 to 10.2 ± 0.6 L/min (p<0.01).
Blood flow increased in the carotid (618.7 ± 26.1 to 1057.3 ± 56.8 ml/min; p<0.01) and
brachial (56.4 ± 9.4 to 470.6 ± 38.0 ml/min; p<0.01) arteries. Total shear rate (4 x
velocity / diameter) also increased in the carotid artery (205.7 ± 13.5 to 278.6 ± 17.8 s-
1; p<0.01) and even more so in the brachial artery (93.2 ± 10.5 to 508.9 ± 50.2 s-1
p<0.01). Skin blood flow reached a plateau of 50.5 ± 3.4 %CVCmax (p<0.01) during
immersion. CONCLUSION: Hot water immersion caused substantial increases in core
temperature, cardiac output, arterial blood flow, skin blood flow, and shear rates. These
changes are similar to those seen during a single bout of exercise, which suggests that
repeated hot water immersion may cause beneficial vascular adaptations through similar
mechanisms.
Description
42 pages. A thesis presented to the Department of Human Physiology and the Clark Honors College of the University of Oregon in partial fulfillment of the requirements for degree of Bachelor of Science, Spring 2016.
Keywords
Human physiology, Cardiovascular, Passive heating, Hot water immersion, Cardiac output, Shear stress, Atheroclerosis