ABSTRACT
Medisoft) and muscle deoxygenation characterized by HHb signal (Oxymon, Artinis Medical system BV) were continuously monitored. 2 phase 2 and HHb kinetics were modelled and characterized by time to achieve 63% of the phase amplitude (() and 63% of the overall signal (mean response time – MRT) respectively. 23.2.2 Pulmonary oxygen uptake During all the tests, adults and children breathed through an adult or pediatric mask of low dead space adapted to their face. Respiratory gas-exchanges were measured breath-by-breath using an automated gas analysis system to determine ventilation, oxygen uptake ( 2), and carbon dioxide output. 23.2.3 Near infrared spectroscopy During exercise, the NIRS probe was firmly attached to the skin overlying the lower third of the vastus lateralis muscle, parallel to the major axis of the thigh. A 4cm distance was kept between the optodes which were housed in a plastic holder, thus ensuring that the position of the optodes, relative to each other, was fixed and invariant. NIRS is based on the relative tissue transparency for light in the near-infrared region and on the oxygen-dependent absorption changes of hemoglobin and myoglobin. By using a continuous wave near-infrared spectrophotometer that generates light at 905, 850 and 770 nm, it is possible to differenciate between oxyand deoxy-hemoglobin/myoglobin (O2Hb/O2Mb and HHb/HMb, respectively). Data were sampled at 10Hz from the beginning to the end of the exercise session, displayed real-time and stored for offline analysis. 23.2.4 Modelling procedures
2 and HHb data were interpolated to give second-by-second values. The four transitions were time aligned to the start of exercise and averaged to yield a single response for each subject. Nonlinear regression techniques were used to fit data with a monoexponential model and an iterative process ensured the sum of squared error was minimized. The 2 phase 2 was modelled after a time delay (TD) of 15s (Springer et al., 1991), as follows:
2 (t) = 2 (b) + Ax(1 – e [- (t-TD/$)]) (DeLorey et al., 2004) The HHb data (ie. muscular deoxygenation) were modelled as follows:
HHb(t) = HHb(b) + Ax(1 – e [- (t-TD/$)]) (DeLorey et al., 2004) Where HHb(t) represents HHb at any time (t), HHb(b) is the baseline value of HHb before the onset of exercise, A is the amplitude of the increase in HHb above the baseline value, TD is the time delay before HHb increase above preexercise values,
and $ is the time constant defined as the duration of time through which HHb increases to a value equivalent to 63% of A. Experimental data are presented as means ± standard deviations (mean ±SD). Mean values comparisons between children and adults were made with the Student’s t test after verification for distribution normality. In all analysis, the level of significance was set at p<0.05. 23.3 RESULTS Anthropometric characteristics and maximal graded test parameters are presented on Table 23.1. No significant difference in2max related to body mass between children and adults. The2 kinetics and NIRS derived parameters at the onset of exercise are presented on Table 23.2. For2 as for HHb, the kinetics responses were significantly faster in children when compared with adults. Table 23.1. Anthropometric characteristics and maximal graded test parameters.
