EFFECTS OF ACUTE PASSIVE HEAT EXPOSURE
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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.