Maximal static pressures in healthy children.

Maximal static pressures (Pmax) were measured at the mouth in 119 Caucasian school children aged 7-13 years. Lung volumes were determined by body plethysmography. Pressures were generated near RV, at FRC and near TLC. As in adults, inspiratory Pmax decreased with increasing lung volume, whereas expiratory Pmax increased. At all lung volumes expiratory and inspiratory Pmax increased with age and were greater in males than females. Net respiratory muscle force was derived from the product of pressure and surface area (SA) over which pressure was applied. SA was estimated at different lung volumes on the basis of chest wall measurements. The dimensions obtained were utilized to calculate the SA of a truncated cone. This model was shown to be appropriate at TLC and FRC, but not RV. Since both SA and Pmax increased with age, the derived forces, as compared with pressures, proportionally increased more with age. Thus changes in Pmax underestimated changes in force during growth.