Ventilation distribution and density dependence of expiratory flow in asthmatic children

Of 114 asthmatic children, 21% had abnormally steep phase III slopes from a modified single-breath oxygen (SBO2) procedure. We hypothesized that the steep slopes reflect inequality of time constants caused by obstruction of peripheral airways and tested this by using a bronchodilator to reduce overall time constants in subgroups of 10 children with steep slopes (SS) and 20 children with normal slopes (NS). Maximum expiratory flow increased by equivalent degrees (0.65-0.70 l/s) in both groups, but slope decreased significantly only in the SS group. Moreover, the single-breath mixing efficiency of inspired oxygen with resident nitrogen was normal in the NS group but significantly low in the SS group. Density dependence of maximum expiratory flow (DD) was abnormally small in the SS group [15 +/- 6% (SD) increase compared with 57 +/- 13% increase in a separate group of normal children] and was independent of the anatomical dead space. In contrast, DD was normal and varied inversely with anatomical dead space (r = 0.62, P less than 0.01) in the NS group. These results indicate that 1) steep SBO2 slopes found in asthmatic children between acute episodes reflect unequal time constants caused by obstruction of peripheral airways and 2) part of the variation in DD among asthmatic children is caused by variation in convective accelerative pressure losses in major airways.