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

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Abstract

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 mLO2∙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.

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Abbreviations

ANS:

Autonomic nervous system

HRV:

Heart rate variability

NN:

Normal-to-normal R–R intervals

R–R interval:

Time between two consecutive R waves in the electrocardiogram

Tes :

Esophageal temperature

Tre :

Rectal temperature

\({\dot{\text{V}}\text{O}}_{{2{\text{peak}}}}\) :

Peak oxygen uptake

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Acknowledgments

This research was supported by the Natural Sciences and Engineering Research Council (RGPIN-298159-2009), the Canadian Mining Industry Research Organization (CAMIRO) and the Canada Foundation for Innovation—Leaders Opportunity Fund (22529); (Grants held by Dr. Glen P. Kenny). Dr. Heather Wright was supported by a MITACS-Accelerate Fellowship (sponsored by Vale Ltd.). Mr. Martin Poirier was supported by Queen Elizabeth II Graduate Scholarship in Science and Technologies (sponsored by CAMIRO). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Conflicts of interest

Andrew J.E. Seely is the founder and Chief Science Officer, and Geoffrey Green is the Product Manager of Therapeutic Monitoring Systems (TMS). TMS aims to commercialize patented Continuous Individualized Multi-organ Variability Analysis (CIMVA) technology, with the objective of delivering variability-directed clinical decision support to improve quality and efficiency of care. All the other authors have no conflicts of interest to disclose.

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Authors and Affiliations

  1. FAME Laboratory, Department of Exercise Science, University of Thessaly, Karies, 42100, Trikala, Greece

    Andreas D. Flouris

  2. Human and Environmental Physiology Research Unit, Faculty of Health Sciences, School of Human Kinetics, University of Ottawa, Ottawa, K1N 6N5, Canada

    Martin P. Poirier, Heather E. Wright-Beatty & Glen P. Kenny

  3. Divisions of Thoracic Surgery and Critical Care Medicine, Ottawa Hospital-General Campus, Ottawa Health Research Institute, Ottawa, ON, K1H 8L6, Canada

    Andrea Bravi, Christophe Herry & Andrew J. Seely

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  1. Andreas D. Flouris
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  2. Martin P. Poirier
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Correspondence to Andreas D. Flouris.

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Communicated by George Havenith.

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Flouris, A.D., Poirier, M.P., Bravi, A. et al. Changes in heart rate variability during the induction and decay of heat acclimation. Eur J Appl Physiol 114, 2119–2128 (2014). https://doi.org/10.1007/s00421-014-2935-5

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