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 degrees C, 28 degrees C and 35 degrees 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 (CVRESP), 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 (CVIHR) and CVRESP was used as a parameter to assess sympathetic nervous functions. The mean heart rate increased at high ambient temperature (35 degrees C) and also during mental task. Both the effects of ambient temperature and task conditions were significant on heart rate, and also on CVIHR. Moreover, the effects of ambient temperature and task conditions in CVIHR were divided into the effect of ambient temperature on CVRESP and the effect of task conditions on the difference between CVIHR and CVRESP. 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.