Analysis of heart rate variability during mental task with reference to ambient temperature

The purpose of this study was to evaluate the cardiac autonomic control over mental task under various ambient temperatures (21°C, 28°C and 35°C). Seven healthy male subjects engaged in the mental tasks, which consisted of distinctive reaction-time tasks. Respiratory coefficient of variation of instantaneous heart rate (CV_RESp), derived from the cross-correlation function between heart rate and respiratory curve, was used as a parameter to assess parasympathetic nervous functions. The difference between total coefficient of variation (CV_IHR) and CV_RESP was used as a parameter to assess sympathetic nervous functions. The mean heart rate increased at high ambient temperature (35°C) and also during mental task. Both the effects of ambient temperature and task conditions were significant on heart rate, and also on CV_IHR. Moreover, the effects of ambient temperature and task conditions in CV_IHR were divided into the effect of ambient temperature on CV_RESp and the effect of task conditions on the difference between C_IHR and CV_RESP. These results implied that respiratory modulated parasympathetic activity might control basal the effect of ambient temperature, and the other components including sympathetic activity contribute to the increase in heart rate due to mental task.