Rigidity of tracheae and bronchi during muscular constriction.

Abstract

To determine how muscular constriction affects ability of cartilaginous airways to withstand compressive narrowing, we isolated airways of cats in an in vitro system. Viscoelastic properties of the relaxed and constricted tracheae or intralobar bronchi were studied under both "semistatic" and dynamic conditions. Specific compliances of bronchial segments exceeded those of tracheae when their muscles were relaxed. Muscular constriction with acetylcholine reduced volumes to 44 and 38% of control for tracheae and bronchi, respectively, and reduced specific compliances from 0.077 [plus or minus] 0.023 SD/cm H2O to 0.029 [plus or minus] 0.013/cm H2O (tracheae) and from 0.124 [plus or minus] 0.033 SD to 0.36 [plus or minus] 0.023/cm H2O (bronchi). Contraction of tracheal muscle increased circumferential tension, whereas contraction of bronchial muscle increased both circumferential and longitudinal tensions. Muscular constriction caused the airways to become not only less distensible but also less compressible. For the tracheae, with compressive pressures of 25 cm H2O or more, the volumes were greater when constricted than when relaxed. Constricted airways withstood transient forces of compression better than sustained forces of compression.