Relationship between the site of airflow limitation and localization of the bronchodilator response in asthma.

In an effort to determine the relationship between the major sites of airflow limitation and bronchodilator action, physiologic studies were performed in 65 asthmatics before and after the administration of isoproterenol, metaproterenol, or terbutaline. Before the administration of a bronchodilator, the results of routine pulmonary function tests were abnormal; the mean increment in maximal expiratory flow at 50% of forced vital capacity (delta VEmax50) after breathing 80% helium-20% oxygen (He-O2) was reduced (p < 0.001). Prebronchodilator values of delta VEmax50 correlated linearly with VEmax50 breathing air (r = + 0.68, p < 0.001). These data suggested that airflow limitation becomes more peripheral as asthma becomes more severe. After the bronchodilator administration, the results of routine pulmonary function studies improved; delta VEmax50 either increased or decreased, depending upon the prebronchodilator values. Density-dependence tended to (1) increase in subjects with low prebronchodilator values of delta VEmax50, and (2) decrease in subjects with high prebronchodilator values of delta VEmax50. Hence, these results suggested that predominant sites of bronchodilatation in asthmatics are related to the prebronchodilator sites of airflow limitation.