Compliance, hysteresis, and collapsibility of human small airways

We tested the hypothesis that airway wall dimensions are important determin ants for the mechanical properties of airways. Lung tissue was obtained fro m 31 smokers with different degrees of chronic obstructive pulmonary diseas e (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 an d deflated cyclically between +15 and -15 cm H2O. For each airway this was done at baseline, after methacholine, and after isoprenaline. Specific comp liance (sCdyn), specific hysteresis (s eta), and pressure at which the airw ays collapsed (Pcol) were calculated from each recording. Airway wall dimen sions were measured morphometrically. Lung function parameters of airflow o bstruction were correlated to sCdyn, s eta, and Pcol. At baseline, after me thacholine, 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 methacholin e and after Isoprenaline were highly significant (p < 0.001). Of all dimens ions studied, smooth muscle area, but not total wall area, was the most imp ortant determinant for sCdyn and for s eta after methacholine. Specific hys teresis at baseline correlated to residual volume as a fraction of total lu ng capacity (RV/TLC) (r = 0.5, p = 0.05) and, in the presence of methacholi ne, to FEV1/FVC (r = -0.68, p = 0.02) and RV/TLC (r = 0.5, p = 0.05). We co nclude that, in this study, smooth muscle area and smooth muscle tone, but not total wall area, are determinants for compliance, hysteresis, and colla psibility of isolated airways obtained from smokers.