AIRWAY SURFACE LIQUID THICKNESS AS A FUNCTION OF LUNG-VOLUME IN SMALLAIRWAYS OF THE GUINEA-PIG

The average thickness and distribution of airway surface liquid (ASL) on the luminal surface of peripheral airways were measured in normal g uinea pig lungs frozen at functional residual capacity (FRC) and total lung capacity (TLC). Tissue blocks containing cross sections of airwa ys of internal perimeter 0.188-3.342 mm were cut from frozen lungs and imaged by low-temperature scanning electron microscopy (LTSEM). Measu rements made from LTSEM images were found to be independent of freezin g rate by comparision of measurements at rapid and slow freezing rates . At both lung volumes, the ASL was not uniformly distributed in eithe r the circumferential or longitudinal direction; there were regions of ASL where its thickness was < 0.1 mu m, whereas in other regions ASL collected in pools. Discernible liquid on the surfaces of airways froz en at FRC followed the contours of epithelial cells and collected in p ockets formed by neighboring cells, a geometry consistent with a low v alue of surface tension at the air-liquid interface. At TLC airway liq uid collected to cover epithelial cells and to form a liquid meniscus, a geometry consistent with a higher value of surface tension. The ave rage ASL thickness (h) was approximately proportional to the square ro ot of airway internal perimeter, regardless of lung volume. For airway s of internal perimeter 250 and 1,800 mu m, h was 0.9 and 1.8 mu m at FRC and 1.7 and 3.7 mu m at TLC, respectively. For a given airway inte rnal perimeter, h was 1.99 times thicker at TLC than at FRC; the diffe rence was statistically significant (P < 0.01; 95% confidence interval 1.29-3.08). These measurements suggest that the ASL layer is highly d ynamic and that movement of liquid across the airway wall and along th e airway tree are both significant factors in establishing ASL thickne ss under normal conditions. Transepithelial pressure gradients, ion tr ansport, and intraluminal surface tension gradients are potential mech anisms driving ASL flow, and accumulation or depletion of the ASL via these mechanisms can occur on time scales of < 1 min.