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

The aim of this study was to examine the effect of passive heat stress on heart rate variability parameters in healthy children. Fifteen children (9.3 +/- 1.6 years) of both sexes (eight male) participated in two randomized experimental conditions separated by 5-12 days. Children were seated for 2 h in an environmental chamber for two sessions: neutral (22.4 +/- 0.1 A degrees C, 40.4 +/- 6.5% RH) and hot (34.9 +/- 0.3 A degrees 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. 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), and low- (LF, 1536 +/- 464 vs. 935 +/- 154 ms(2)) and high-frequency power (HF, 1544 +/- 693 vs. 866 +/- 355 ms(2)) 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). 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.