Perivascular interstitial fluid pressure measured by micropipettes in isolated dog lung.

Micropipettes in conjunction with a servo-nulling system were used to measure fluid pressure (Pf) in the interstitium around the partially exposed vein near the hilus of the right upper lung lobes of the dog. Lobes were studied at constant transpulmonary pressure (Ptp). In the absence of extravascular water accumulation, Pf was -1.5 cmH2O relative to pleural pressure at Ptp of 6 cmH2O and vascular pressure (Pv) of 0 cmH2O and was more negative in lobes tested at higher Ptp values. In five lobes made edematous with plasma at Ptp of 6 cmH2O and Pv of 15 cmH2O, mean Pf increased from -1 to 4.4 cmH2O as lung weight increased up to 400% of the initial excised weight. In four other lobes, at Ptp of 15 cmH2O and Pv of 20 cmH2O, Pf increased from -2.4 to 8.8 for a similar increase in weight. In lobes degassed and filled with saline or plasma, Pf always equilibrated to alveolar pressure (PA). Results suggest that alveolar surface tension (tau) in air-filled lobes with gross edema prevented Pf from reaching PA. Reduction in Pf below PA was larger at higher Ptp, consistent with increased tau with lung inflation.