The apparent response of airway function to deep inspiration depends on the method of assessment

The response to deep inspiration (DI) has been reported in terms of specific airway conductance (SGaw) pre- and post-DI, and by comparing forced expiratory flows following both maximal and partial inspiration (M/P ratio). The lung volume at which M/P has been obtained has varied between studies. We have investigated how the method of assessment affects the apparent response to DI. In 16 asthmatic and 16 control subjects both SGaw and M/P were measured, the latter at various lung volumes from 40 to 15% vital capacity (VC) (remaining). In each group M/P increased as lung volume decreased (r=-0.98, P<0.001 for each group). In contradistinction to M/P ratio, the SGaw ratio (post-DI: pre-DI) suggested a bronchoconstrictor asthmatic response (mean 0.938, SEM 0.029) and a bronchodilator control response (mean 1.063, SEM 0.029). SGaw ratio and M/P were correlated strongly in normals but not in asthmatics. These results confirm that the observed response to DI depends on the method used to measure it. The results in normals are explicable by DI-induced smooth muscle stretch increasing unstressed airway calibre, but also increasing compressibility on forced expiration. The results in asthma suggest the existence of an additional DI-related bronchoconstricting mechanism.