Changes in distribution of ventilation with lung growth.

We systematically studied the effects of varying preinspiratory lung volumes and expiratory flow rates on the alveolar plateau of the single-breath oxygen test in children and adults. With inhalations of oxygen beginning at functional residual capacity (FRC) compared with residual volume (RV), the slope of phase III increased in 52 of 54 children and 6 of 6 adults (mean increase 29.2%, P less than 0.001) but then decreased at preinspiratory volumes greater than FRC. With maximal expiratory flows, the slopes were smaller than slopes from conventional maneuvers in 14 of 15 children by a mean 24.1%, P less than 0.001. These data suggest that apex-to-base differences in regional lung volume are a major determinant of the slope. Slopes (FRC maneuver) decreased as a function of the height of the children (r = -0.73, P less than 0.001), but differences in nitrogen concentration over the alveolar plateau increased with height (r = 0.70, P less than 0.001). This indicates that the apex-to-base differences in regional lung volume increase with lung size. An estimate of mixing efficiency between resident and inspired gas derived from the alveolar plateau increased with height for both RV (r = 0.40, P less than 0.005) and FRC maneuvers (r = 0.45, p less than 0.005) and was greater for FRC than RV (P less than 0.001). These increases in mixing efficiency are consistent with previously demonstrated decreases in closing volume with growth.