Autonomic Nervous System Response to Heat Stress Exposure by Means of Heart Rate Variability

In this study, a heart rate variability (HRV) analysis guided by respiration is conducted to assess changes in autonomic nervous system (ANS) regulation during pronounced heat stress induced by repetitive exposures to dry sauna in 13 young healthy subjects. The spectral power at the low frequency (LF) band [0.04, 0.15] Hz and at the high frequency band centered around the mean respiratory rate, estimated from ECG-derived respiration signals, are studied. The mean heart rate (HR), the total power of HRV, and the normalized LF power are also analyzed.Results show that mean HR and normalized LF power increase significantly after basal stage, not only during the sauna sessions, but also at the intermediate short rest stages and recovery phase after 30 min. A significant reduction in the total power of HRV during all sauna sessions is observed, while an increased power in LF band is shown only during recovery which might be related to improved cardiac function (increased cardiac output and reduced peripheral vascular resistance) after heat exposure. Respiratory rate does not change significantly during the protocol but a negative trend at stress stages may indicate a reverse reaction against an over-activation of the sympathetic branch. In conclusion, exposure to heat stress shifts sympathovagal balance of ANS toward sympathetic dominance that increases HR and diminishes HRV.