Cognitive performance in the heat: A prediction with heart rate variability (HRV) time-domain indices

Heat waves (HWs) appear more frequently and more intensively in recent years, producing a few high air temperature records and bringing more health issues. In order to explore the variation in human cognitive performance under hot-humid exposure, four air temperatures at 32°C, 36°C, 38°C and 41°C were selected and four corresponding 150-min hot exposure experiments were administrated. Subjects completed five kinds of cognitive tasks at these four environmental temperatures, during which electrocardiogram (ECG) data was continuously recorded. Time-domain analysis of the heart rate variability (HRV) was conducted based on the ECG data, and three time-domain indices were obtained, including the standard deviation of all normal to normal N-N intervals (STDNN), the percentage of intervals over 50 ms was different from the preceding interval (pNN50) and the square root of the mean of the squares of successive NN interval differences (RMSSD). Variations in these indices with exposure time and air temperature were analysed. Thereafter, a correlation analysis between cognitive performance and HRV time-domain indices was conducted. Results showed that these three HRV time-domain indices varied differently with the air temperature and exposure time. In addition, results of the correlation analysis showed that RMSSD was the most correlated index, based on which a cubic relation between the cognitive performance and RMSSD was acquired. This study provided a solution for the cognitive prediction of outdoor workers during HWs.