Is the chronotropic index applicable to children and adolescents?

Background The aim of the present study was to investigate whether the chronotropic index (CRI) is independent of age, sex, stage of exercise testing, functional capacity and resting heart rate in children, as it is in adult populations. Methods and Results A total of 129 normal children and adolescents underwent maximal treadmill exercise testing according to the Bruce protocol. Submaximal chronotropic response was assessed by the CRI at the end of stages 1, 2, 3, and 4 of Bruce treadmill exercise testing. The CRI assessed at each exercise stage was independent of sex, maximal oxygen uptake and resting heart rate. The CRI was independent of age when measured at stages 3 and 4 in boys and at stage 4 in girls, but was weakly correlated with age when measured at stages 1 and 2 in boys and at stages 1, 2 and 3 in girls. There was a significant effect of exercise intensity on the CRI: multiple comparisons revealed that, in girls, CRI assessed at stage 1 was greater than that assessed at stage 4. Also, lower exercise stages resulted in rather large variations in the measurement. Conclusion The results essentially confirm the potential applicability of the CRI for children and adolescents, although care should be taken in evaluating measurements of the CRI in the early stages of exercise testing to avoid misinterpretation of the results.