ObjectivesIntraparenchymal airways are involved in air flow regulation. Relaxation of intraparenchymal airways to volatile anesthetics varied by topographic location. This study was conducted to determine whether other bronchodilators (terbutaline, diltiazem, and aminophylline) relax bronchiolus to a greater degree than bronchus, as seen with volatile anesthetics.DesignIn vitro, controlled, randomized study.SettingAnimal research laboratory.SubjectsAdult dogs (n = 9).InterventionsProximal (outer diameter, 4-6 mm) and distal (outer diameter, 0.8-1.5 mm) airway rings of dogs were contracted in tissue baths with the effective concentration of acetylcholine that produces half the maximum response. Airway relaxant dose-response curves were constructed to measure isometric tension after administration of terbutaline (concentration range, 10-8to 10-4M), diltiazem (concentration range, 3 x 10-7to 1 x 10-4M), and aminophylline (concentration range, 10-7to 10-4M).Measurements and Main Results.05. At the concentrations tested, they were equally efficacious. No significant differences in relaxation between proximal and distal airways were noted with diltiazem or aminophylline in the entire dose range. However, terbutaline relaxed the distal airway more than the proximal airway in the entire dose range.ConclusionsThe results demonstrate that only terbutaline showed a differential airway relaxant effect between proximal and distal airways, as seen with volatile anesthetics. (Crit Care Med 1999; 27:1551-1556)