We examined if the change in heart rate variability (HRV) during passive heat exposure is modified by hot water heat acclimation (HA). Sixteen healthy adults (28 ± 5 years, 5 females/11 males) underwent heat exposure in a water-perfused suit, pre and post 7 days of HA (60 minutes at rectal temperature ≥38.6°C). During passive heat exposure, heart rate (HR), the standard deviation of NN intervals (SDNN), the square root of the mean squared differences of successive NN intervals (RMSSD) and the power in the high frequency range (HF) were measured. No difference in HR (P=0.22), SDNN (P=0.87), RMSSD (P=0.79) and HF (P=0.23) was observed at baseline. The increase in HR (pre-HA: 43 ± 10, post-HA: 42 ± 9 bpm, P=0.57) and the decrease of SDNN (pre-HA: -54.1 ± 41.0, post-HA: -52.2 ± 36.8 ms, P=0.85), RMSSD (pre-HA: -70.8 ± 49.5, post-HA: -72.7 ± 50.4 ms, P=0.91) and HF (pre-HA: -28.0 ± 14.5, post-HA: -23.2 ± 17.1%, P=0.27) were not different between experimental visits at fixed increases in esophageal temperature. These results suggest that 7 consecutive days of hot water HA does not modify the change in HRV indices during passive heat exposure.
Novelty bullets:
- It remains unclear if heat acclimation alters the change in heart rate variability that occurs during passive heat exposure.
- At matched levels of thermal strain, 7 consecutive days of hot water immersion did not modulate the change in indices of heart rate variability during passive heat exposure.
Effect of sauna-based heat acclimation on plasma volume and heart rate variability
