The measurement of oxygen uptake kinetics in children
Traditional approaches to exercise testing in children may not provide the most appropriate measures of a child's physiological responses to exercise, partly because they do not reflect children's normal intermittent activity patterns. The measurement of the rate and magnitude of change of oxygen uptake to dynamic exercise, oxygen uptake kinetics (V02 KINETICS provides an alternative approach to exercise testing. A submaximal, intermittent, pseudo-random binary sequence (PRBS) exercise test to measure V02 KINETICS may provide a useful method of measuring the metabolic responses of children to exercise. Traditional methods used in the analysis of V02 KINETICS require the fitting of explicit models in order to characterise the data. These models have not however been validated for use in children. As the responses to the PRBS protocol are analysed in the frequency domain, explicit models and their physiological correlates are not required to characterise the data. Another potential problem in the measurement of V02 KINETICS in children are the small work rate changes that can be employed to stimulate the exercise response whilst constraining the test to the aerobic range. In respiratory gas measurement, breath-by-breath variability (noise) can be large in comparison to the magnitude of the metabolic response and this signal noise can obscure some characteristics of the response. The aim of the study was to develop appropriate measurement techniques to reduce the effects of breath-by-breath variability and to apply the techniques to the measurement of V02 KINETICS in children. The main experimental study compared the V02 KINETICS of children with those of adults. Ten children (3 females) in the age range 8 to 13 and twenty adults (10 females) in the age range 20 to 28 years completed a PRBS test to measure V02 KINETICS and an incremental ramp protocol on a cycle ergometer (Bosch 550 ERG) to establish V02 MAX, T VENT and delta efficiency. Breath-by-breath respiratory gas analysis was undertaken using a respiratory mass spectrometer (MGA1100). Estimates of alveolar gas exchange were made using the algorithm of Beaver et al. (1981) and a post hoc value of an effective lung volume was calculated to minimise the breath-by-breath variability. A cross-correlation technique (CC) was used to filter out the effects of anomalous (nonphysiologic) V02 responses recorded during the PRBS protocol. Subsequent Fourier analysis of the auto-correlation and CC functions provided a description of V02 KINETICS in the frequency domain in terms of amplitude ratio and phase delay over the frequency range of 2.2-8.9mHz. At each of the frequencies assessed amplitude ratio was higher in children (P<0.001) than in either of the adult groups. Phase delay was also significantly shorter in children compared to adults males (P<0.01) and adult females (P<0.001) but this effect was not identifiable at any specific frequency. Maximal oxygen uptake was not significantly different in adult males (42.5 ml"kg "min) and children (44.7 ml-kg'-min') but was lower in adult females (36.9 ml"kg "min) than adult males (P<0.01) and children (P<0.001). Ventilatory threshold (% V02 MAX) was not different between groups. Delta efficiency was significantly lower in children than adult males (P<0.05) and adult females (P<0.01). These results support the contention that there are maturational differences between adults and children in the metabolic processes involved in the utilisation of oxygen during physical activity. It has been argued, theoretically, that in adults the control of V02 KINETICS is driven by ATP demand in the skeletal muscle. As the mitochondria] capacity and the concentration of oxidative enzymes is higher in children than in adults it is likely that the controlling factor(s) for V02 KINETICS in children also relates to some aspect of peripheral metabolism. It is suggested that the PRBS protocol, with appropriate noise reduction techniques, is considered a suitable method for investigating the metabolic responses of children to dynamic exercise.