S.M. Harris, B.R. Ely, V.E. Brunt, E.S. Wright, C.T. Minson, FACSM

University of Oregon, Eugene, OR

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-term adaptations are not yet fully known. PURPOSE: To examine the acute cardiovascular hemodynamic effects of a 60 min bout of hot water immersion by observing changes in cardiac output, arterial blood flow, shear rate, and skin blood flow. METHODS: Four healthy, young (26 ± 4 yrs) subjects were immersed to heart level (both arms 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. Heart rate was measured by commercially available chest strap. Cardiac output was measured with open-circuit acetylene wash-in method. Carotid and brachial 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. Peak changes were compared to baseline using Student’s paired t-test, and significance was set to P<0.05. Data are presented as mean ± S.E. RESULTS: Heart rate increased from 61 ± 4 at baseline to a peak of 122 ± 6 bpm (p<0.01) during immersion. Cardiac output increased from 5.6 ± 0.6 to 10.0 ± 1.5 L/min (p=0.02). Blood flow increased in the carotid (611.9 ± 88.3 to 1200.9 ± 127.7 ml/min; p<0.01) and brachial (47.5 ± 7.6 to 560.2 ± 50.6 ml/min; p<0.01) arteries. Total shear rate (4 x velocity / diameter) also increased in the carotid artery (193.5 ± 8.7 to 311.5 ± 31.2 s-1; p=0.03) and even more so in the brachial artery (87.5 ± 20.8 to 646.9 ± 88.3 s-1 p<0.01). Skin blood flow reached a plateau of 52.3 ± 7.6 %CVCmax (p<0.01) during immersion. CONCLUSION: Hot water immersion caused substantial increases in 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.

Supported by the University of Oregon Undergraduate Research Opportunity Program and Robert D. Clark Honor’s College.

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