Breath-to-breath "noise" in the ventilatory and gas exchange responses of children to exercise.

The purposes of this investigation were to quantify the noise component of child breath-by-breath data, investigate the major determinants of the breath-to-breath noise, and to characterise the noise statistically. Twenty-four healthy children (12 males and 12 females) of mean (SD) age 13.1 (0.3) years completed 25 min of steady-state cycle ergometry at an exercise intensity of 50 W. Ventilatory and gas exchange variables were computed breath-by-breath. The mean (SD) oxygen consumption (VO2) ranged from 0.72 (0.16) to 0.92 (0.26) l x min(-1); mean (SD) carbon dioxide production (VCO2) ranged from 0.67 (0.20) l x min(-1) to 0.85 (0.16) l x min(-1); and mean (SD) minute ventilation ranged from 17.81 (3.54) l x min(-1) to 24.97 (5.63) l x min(-1). The majority of the breath-to-breath noise distributions differed significantly from Gaussian distributions with equivalent mean and SD parameters. The values of the normalised autocorrelation functions indicated a negligible breath-to-breath correlation. Tidal volume accounted for the majority of the VO2 (43%) and VCO2 (49%) variance. The breath-to-breath noise can be explained in terms of variations in the breathing pattern, although the large noise magnitude, together with the relatively small attainable response amplitudes in children reduces the certainty with which ventilatory and gas exchange kinetics can be measured.