Effect of passive heat exposure on cardiac autonomic function in healthy children

Abstract

Purpose: The aim of this study was to examine the effect of passive heat stress on heart rate variability parameters in healthy children. Method: Fifteen children (9.3±1.6 years) of both sexes (8 male) participated in two randomized experimental conditions separated by 5 to 12 days. Children were seated for 2 h in an environmental chamber for 2 sessions: Neutral (22.4±0.1°C, 40.4±6.5%RH) and Hot (34.9±0.3°C, 36.6±6.2%RH) conditions. Electrocardiogram, mean skin temperature, tympanic temperature, and blood pressure were recorded. Five min epochs were averaged for analysis of cardiac autonomic function over the 2 h protocol. Result: Mean skin and tympanic temperatures and heart rate increased during the Hot condition (all p<0.01) while mean arterial pressure decreased (p<0.01). During the Hot condition, root-mean-square difference of successive normal RR intervals (45±9 to 38±7 ms), low- (LF, 1536±464 vs. 935±154 ms2) and high-frequency power (HF, 1544±693 vs. 866±355 ms2) decreased, whereas LF/HF ratio increased (1.64±0.24 vs. 2.40±0.23 au); all indices were different from Neutral (all p<0.05). These were all unchanged throughout the Neutral condition (all p>0.05), except for LF/HF ratio which decreased during the Neutral condition (p<0.05). Conclusion: Mild hyperthermia elicited marked changes in cardiac autonomic control in young children. These data suggest that, in healthy children, vagal withdrawal is responsible for the cardiac autonomic response to hyperthermia.