Changes in heart rate variability during the induction and decay of heat acclimation

• Published in: European journal of applied physiology Vol. 114; no. 10; pp. 2119 - 2128
• Main Authors: Flouris, Andreas D., Poirier, Martin P., Bravi, Andrea, Wright-Beatty, Heather E., Herry, Christophe, Seely, Andrew J., Kenny, Glen P.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.10.2014; Springer Nature B.V
• Subjects: Acclimatization - physiology; Adult; Biomedical and Life Sciences; Biomedicine; Body Temperature; Cardiac arrhythmia; Esophagus; Health sciences; Heart Rate; Heat; Hot Temperature; Human Physiology; Humans; Male; Maximum oxygen consumption; Occupational Medicine/Industrial Medicine; Original Article; Physiology; Sports Medicine; Temperature; Thoracic surgery; Canada; Acclimatization; Hot exposure; Exercise; Core temperature; Heart rate variability; HRV
• ISSN: 1439-6319; 1439-6327
• DOI: 10.1007/s00421-014-2935-5

Purpose We evaluated the changes in core temperature, heart rate, and heart rate variability (HRV) during the induction and decay of heat acclimation. Methods Ten males (23 ± 3 years; 79.5 ± 3.5 kg; 15.2 ± 4.5 percent body fat; 51.13 ± 4.61 mLO 2 ∙kg −1 ∙min −1 peak oxygen uptake) underwent a 14-day heat acclimation protocol comprising of 90-min cycling at ~50 % peak oxygen uptake at 40 °C and ~20 % relative humidity. Core temperature, heart rate, and 102 HRV measures were recorded during a heat tolerance test conducted at baseline (day 0) and at the end of the induction (day 14) and decay (day 28) phases. Results Heat acclimation resulted in significantly reduced core temperature [rectal ( χ 2  = 1298.14, p  < 0.001); esophageal ( χ 2  = 1069.88, p  < 0.001)] and heart rate ( χ 2  = 1230.17, p  < 0.001). Following the decay phase, 26, 40, and 60 % of the heat acclimation-induced reductions in rectal temperature, esophageal temperature, and heart rate, respectively, were lost. Heat acclimation was accompanied by profound and broad changes in HRV: at the end of the induction phase, 75 of the 102 variability measures computed were significantly different ( p  < 0.001), compared to only 47 of the 102 at the end of the decay phase. Conclusions Heat acclimation is accompanied by reduced core temperature, significant bradycardia, and marked alterations in HRV, which we interpret as being related to vagal dominance. The observed changes in core temperature persist for at least 2 weeks of non-exposure to heat, while the changes in heart rate and HRV decay faster and are only partly evident after 2 weeks of non-exposure to heat.