Sites of airway dilatation in asthma following inhaled versus subcutaneous terbutaline: Comparison of physiologic tests with radionuclide lung images

In 12 asthmatic subjects with mild to severe airways obstruction, we compared the relative magnitude and sites of airway dilatation of a beta-adrenergic stimulant administered by different routes. Specific airway conductance (SG_aw), peak expiratory flow, the sum of the absolute volume of isoflow and residual volume as a percentage of total lung capacity (capacity of isoflow) and the ratio of maximal expiratory flow at 50 per cent of vital capacity breathing 80 per cent helium-20 per cent oxygen to that breathing air (ratio V?max₅) were determined before and after the administration of aerosolized terbutaline (0.5 mg), subcutaneous terbutaline (0.5 mg) or placebo. Increases in SG_aw and peak expiratory flow of ≥ 25 per cent were considered to indicate significant dilatation of central airways; increases in ratio Vmax₅₀ of ≥ 0.10 and decreases in capacity of isoflow of ≥10 per cent were assumed to reflect dilatation of peripheral airways. In addition, radioaerosol and radioxenon lung imaging was performed to determine the relationship between changes in lung imaging patterns and changes in physiologic indices in response to bronchodilator therapy. Placebo caused little change in lung function or lung imaging. After inhaled terbutaline, SG_aw and peak expiratory flow increased ≥25 per cent in seven subjects, ratio Vmax₅₀ increased ≥0.10 in only three subjects, capacity of isoflow decreased ≥10 per cent in only one subject, radioaerosol images showed less central deposition in nine subjects and radioxenon images showed improved distribution and/or washout of xenon in five subjects. After the administration of subcutaneous terbutaline, SG_aw and peak expiratory flow increased ≥25 per cent in 10 subjects, ratio Vmax₅₀ increased ≥0.10 in 10 subjects, capacity of isoflow decreased ≥10 per cent in 11 subjects, and radioaerosol and xenon images showed improvement in 11 and eight subjects, respectively. These findings are consistent with the action of inhaled terbutaline mainly on large airways and of subcutaneous terbutaline on both large and small airways. Although reduced central radioaerosol deposition correlated well with physiologic evidence of large airway dilatation, improvement in xenon distribution and washout could be attributed to dilatation of either large and/or small airways.