4.5 Article

Repeated warm water baths decrease sympathetic activity in humans

Journal

JOURNAL OF APPLIED PHYSIOLOGY
Volume 133, Issue 1, Pages 234-245

Publisher

AMER PHYSIOLOGICAL SOC
DOI: 10.1152/japplphysiol.00684.2021

Keywords

autonomic; heart rate variability; heat therapy; stress; sympathetic

Funding

  1. National Institutes of Health [MPI R01 HL141198, UL1 TR002014]

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This study found that repeated moderate heat exposure (RMHE) decreases resting sympathetic activity and heart rate, which can be considered beneficial effects. However, RMHE did not alter the sympathetic responses to stress.
Acute whole body heat stress evokes sympathetic activation. However, the chronic effects of repeated moderate heat exposure (RMHE) on muscle sympathetic nerve activity (MSNA) in healthy individuals remain unclear. We performed RMHE with 4 wk (5 days/wk) of warm baths (similar to 40 degrees C, for 30 min) in nine healthy older (59 +/- 2 yr) volunteers. Hemodynamic variables and MSNA were examined before, 1 day after, and 1 wk following 4 wk of RMHE in a laboratory at similar to 23 degrees C. Cold pressor test (CPT) and hand-grip (HG) exercise were performed during the tests. Under normothermic condition, the resting MSNA burst rate (prior, post, post 1-wk: 31.6 +/- 2.0, 25.2 +/- 2.0, and 27.7 +/- 1.7 bursts/min; P < 0.001) and burst incidence (P < 0.001) significantly decreased after RMHE. Moreover, the resting heart rate significantly decreased after RMHE (62 +/- 2, 60 +/- 2, and 58 +/- 2 beats/min, P = 0.031). The sensitivity of baroreflex control of MSNA and heart rate were not altered by RMHE, although the operating points were reset. The MSNA and hemodynamic responses (i.e., changes) to handgrip exercise or cold pressor test were not significantly altered. These data suggest that the RMHE evoked by warm baths decreases resting sympathetic activity and heart rate, which can be considered beneficial effects. The mechanism(s) should be examined in future studies. NEW & NOTEWORTHY To our knowledge, this is the first study to observe the effects of repeated warm baths on sympathetic nerve activity during rest and stress in healthy middle age and older individuals. The data suggest that the repeated warm baths decreased resting sympathetic activity and heart rate, which can be considered beneficial effects. This study also provides the first evidence that the repeated warm baths did not alter the baroreflex sensitivity and the sympathetic responses to stress.

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