Cognitive performances under hot-humid exposure: An evaluation with heart rate variability

Attenuated cognitive performances caused by the hot-humid environment have been widely reported. However, the objective evaluation of the cognitive performance is still an open question. In pursuit of a possible biomarker of the cognitive performance under hot-humid exposures, 14 right-handed participants were recruited and exposed to 4 different hot-humid environments for 150 min, during which 5 kinds of cognitive tasks were administrated continuously till they gave up and quitted the experiment. The electrocardiogram (ECG) data were recorded during the whole experiment. Then the heart rate variability (HRV) analysis was conducted based on the ECG data, through which the time-domain indices, frequency-domain indices and non-linear indices were obtained. Thereafter, changes of these HRV indices with the exposure temperature and exposure time were investigated. Subsequently, the correlation analysis between the relative cognitive performance (RCP) calculated from the cognitive task results and these 3 kinds of HRV indices was conducted, producing the most correlated index, the mean LF/HF that represented the ratio of the low frequency and high frequency power from the HRV frequency analysis results. Based on that, the relationship between the mean LF/HF and RCP under different hothumid exposures was scrutinized. Results showed that the HRV indices changed with both the exposure temperatures and time, with certain patterns. Most importantly, a multi-segment negative relation was obtained between the mean LF/HF and the RCP. The mean RCP kept reducing when the mean LF/HF rose up to 3.5, or when the LF/HF was greater than 5, approximately. Between 3.5 and 5, however, slower reduction of the RCP was observed. This study produced a physiological approach to evaluate the cognitive performance alterations in hot-humid environments, which was of great significance to the productivity, safety and health of outdoor workers, especially under the circumstance of global warming.