Evaluation of the effect of heat exposure on the autonomic nervous system by heart rate variability and urinary catecholamines

The aim of this study was to investigate the usefulness of heart rate variability (HRV) and urinary catecholamines (CA) as objective indices of heat stress effect. We examined physiological responses, subjective symptoms, HRV and urinary CA to evaluate the effect of heat exposure on the autonomic nervous system. Six healthy male students volunteered for this study. They were exposed on different days to either a thermoneutral condition at wet bulb globe temperature (WBGT) 21 degrees C, or a heated condition at WBGT 35 degrees C for 30 min, while seated on a chair. In the thermoneutral condition, differences of all parameters between the values before and after 30 min exposure were not statistically significant. In the heated condition, heart rate, body temperature and scores for subjective symptoms (feverishness, sweating, mood, and face flushing) significantly increased after 30 min exposure (p<0.05). Also, the high frequency component (HF%) of HRV significantly decreased and the low frequency/high frequency (LF/HF) ratio of HRV significantly increased after 30 min exposure to the heated condition (p<0.05). There were no significant differences between the amounts of urinary CA before and after the 30 min exposures; however, the norepinephrine amount after 30 min exposure to the heated condition was significantly greater than that of the thermoneutral condition (p<0.05). The heat exposure (WBGT 35 degrees C) induced activation of the sympathetic nervous system and a withdrawal of the parasympathetic nervous system. These findings coincide with observed changes of heart rate, body temperature and subjective symptoms. It is suggested that HRV (HF% and LF/HF ratio) and urinary norepinephrine may be useful objective indices of heat stress; HRV seems to be more sensitive to heat stress than urinary CA.