Seven days of hot water heat acclimation does not modulate the change in heart rate variability during passive heat...

We examined if the change in heart rate variability 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, before and after 7 days of HA (60 min at rectal temperature ≥38.6 °C). During passive heat exposure, heart rate, 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 heart rate (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 heart rate variability indices during passive heat exposure. Novelty: It remains unclear if HA 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.