Compliance, hysteresis, and collapsibility of human small airways

We tested the hypothesis that airway wall dimensions are important determinants for the mechanical properties of airways. Lung tissue was obtained from 31 smokers with different degrees of chronic obstructive pulmonary disease (COPD) who were operated on for a solitary lung lesion. Segments of small airways (n = 35) were mounted on cannulas in an organ bath and inflated and deflated cyclically between +15 and -15 cm H2O. For each airway this was done at baseline, after methacholine, and after isoprenaline. Specific compliance (sCdyn), specific hysteresis (s eta), and pressure at which the airways collapsed (Pcol) were calculated from each recording. Airway wall dimensions were measured morphometrically. Lung function parameters of airflow obstruction were correlated to sCdyn, s eta, and Pcol. At baseline, after methacholine, and after isoprenaline sCdyn was 0.059, 0.055 and 0.085 cm H2O-1, s eta was 13.5, 12.9, and 7.1%, and Pcol was -3.4 -3.5, and -1.9 cm H2O, respectively. Differences between sCdyn, s eta, and Pcol after methacholine and after Isoprenaline were highly significant (p < 0.001). Of all dimensions studied, smooth muscle area, but not total wall area, was the most important determinant for sCdyn and for s eta after methacholine. Specific hysteresis at baseline correlated to residual volume as a fraction of total lung capacity (RV/TLC) (r = 0.5, p = 0.05) and, in the presence of methacholine, to FEV1/FVC (r = -0.68, p = 0.02) and RV/TLC (r = 0.5, p = 0.05). We conclude that, in this study, smooth muscle area and smooth muscle tone, but not total wall area, are determinants for compliance, hysteresis, and collapsibility of isolated airways obtained from smokers.