Cardiac autonomic function during hypothermia and its measurement repeatability.

This study examined the effect of mild hypothermia (a 0.5°C decrease in rectal temperature) on heart rate variability (HRV), with the identical hypothermia protocol performed twice and compared using intraclass correlation coefficient (r) analysis to study the repeatability. Twelve healthy males each completed a Neutral (23°C) and two Cold (0°C) trials. In the Neutral trial participants sat quietly for 30 min. In the Cold trials, baseline data were obtained from a 5-min sample following 30 min of quiet sitting at 23°C, followed by passive exposure to 0°C; hypothermic measures were taken from a 5-min period immediately prior to rectal temperature decreasing by 0.5°C. HRV was obtained from a three-lead ECG. There were no differences (all p>0.05) in baseline measures between the Neutral and the two Cold trials, suggesting no pre-cooling anxiety related to the Cold trials. Heart rate along with HRV measures RMSSD, TINN, LF, and HF increased (all p<0.05) with mild hypothermia and showed excellent reliability between the two Cold trials (all r≥0.81). In contrast, HF/LF ratio decreased (p<0.05) and had only fair reliability between the two Cold trials (r=0.551). In general, hypothermia led to increases in heart rate along with most measures of HRV. While counter-intuitive that both sympathetic and vagal influence would increase simultaneously, these changes likely reflect increased stress from whole body cooling along with marked cardiovascular strain and sympathetic nervous system activity from shivering to defend core body temperature. An important methodological consideration for future studies is the consistent and repeatable HRV responses to hypothermia.