title: Changes in heart rate variability during the induction and decay of heat acclimation.
abstract: OBJECTIVE
  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.
authors: FAME Laboratory, Department of Exercise Science, University of Thessaly, Karies, 42100, Trikala, Greece, andreasflouris@gmail.com.
authors: Andreas D
authors: AD
authors: Flouris
authors: Martin P
authors: MP
authors: Poirier
authors: Andrea
authors: A
authors: Bravi
authors: Heather E
authors: HE
authors: Wright-Beatty
authors: Christophe
authors: C
authors: Herry
authors: Andrew J
authors: AJ
authors: Seely
authors: Glen P
authors: GP
authors: Kenny
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D000328:Adult; D001831:Body Temperature; D006339:Heart Rate; D006358:Hot Temperature; D006801:Humans; D008297:Male