Micropuncture measurement of alveolar liquid pressure in excised dog lung lobes

We have investigated the mechanism of alveolar liquid filling in pulmonary edema. We excised, degassed, and intrabronchially filled 14 dog lung lobes from nine dogs with 75, 150, 225, or 350 ml of 5% albumin solution, and then air inflated the lobes to a constant airway pressure of 25 cmH2O. By use of micropipettes, we punctured subpleural alveoli to measure alveolar liquid pressure by the servo-null technique. Alveolar liquid pressure was constant in all lobes despite differences in lobe liquid volume and averaged 10.6 +/- 1.3 cmH2O. Thus, in all lobes a constant pressure drop of 14.4 cmH2O existed from airway to alveolar liquid across the air-liquid interface. We attribute this finding, on the basis of the Laplace equation, to an air-liquid interface of constant radius in all the lobes. In fact, we calculated from the Laplace equation an air-liquid interface radius which equalled morphological estimates of alveolar radius. We conclude that in the steady state, alveoli that contained liquid have a constant radius of curvature of the air-liquid interface possibly because they are always completely liquid filled.