Changes in heart rate variability during the induction and decay of heat acclimation.
Level 4 - case-series / case-control
Prospective single-arm pre-post experimental study without a control group.
PubMed 24957416 · doi:10.1007/s00421-014-2935-5
What was done
Ten healthy young males (mean age 23 ± 3 years, peak oxygen uptake 51.13 ± 4.61 mL/kg/min) completed a 14-day heat acclimation protocol of 90 minutes daily cycling at ~50% peak oxygen uptake in 40 °C and ~20% relative humidity. Core temperature (rectal and esophageal), heart rate, and 102 heart rate variability (HRV) measures were evaluated during standardized heat tolerance tests at baseline (day 0), post-induction (day 14), and post-decay (day 28, following 2 weeks without heat exposure).
What was found
Heat acclimation significantly reduced core temperature (rectal: χ² = 1298.14, p < 0.001; esophageal: χ² = 1069.88, p < 0.001) and heart rate (χ² = 1230.17, p < 0.001), while altering 75 of 102 HRV metrics (p < 0.001). Following the 14-day decay phase, 26% of rectal temperature, 40% of esophageal temperature, and 60% of heart rate reductions were lost, and 47 of 102 HRV measures remained significantly different from baseline.
Why it matters
This study demonstrates that cardiovascular and autonomic adaptations decay more rapidly than thermoregulatory core temperature adaptations during two weeks of heat non-exposure.
Limits
The sample size was very small (n = 10), restricted exclusively to young, fit males, and lacked a non-acclimated control group.
Cited by
- supports Heat acclimation from repeated heat exposure improves heart rate variability.