This study aimed to see if gas trapping or ventilation inhomogeneity during tidal breathing increases in young asthmatic subjects when recumbent over a period of 1 hr. Ten asthmatic children and teenagers and 12 control subjects performed tidal breathing multiple-breath N(2)-washout in the sitting position and immediately, 30 min, and 60 min after assuming the supine position. The asthma group also performed the same lung function tests on another day, but then remained in the sitting position throughout. When assuming the supine position, both groups showed significant falls in vital capacity (4-8%) and in functional residual capacity (FRC) (>20%). Gas trapping increased significantly after 30 and 60 min in the supine position only in the asthma group, but other indices of ventilation inhomogeneity did not change with body position. When in the supine position, the trapped gas volume was on average 10% of FRC in the asthma group, which was twice as much as when sitting and three times higher than in supine control subjects. It is concluded that assuming the supine position leads to increased gas trapping in young asthmatics but not in healthy subjects; this could be one of several mechanisms that contribute to nocturnal worsening of asthma.
Copyright 2003 Wiley-Liss, Inc.