Pulmonary interstitial and capillary pressures estimated from intra-alveolar fluid pressures

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Abstract
The absorptive pressure for fluid in the alveolar and bronchiolar spaces of small occluded segments (PA) [arterial pulmonary pressure] and whole lung lobes was used to estimate the interstitial fluid pressure in the lungs of intact dogs. PA was found to be more negative in upper lung regions and less negative near the bottom of the lung. The mean PA value corrected to heart level was -6.0 Torr when the alveoli were filled with plasma and -10.6 Torr when filled with Tyrode solution. Osmotic reflection coefficients for the alveolobronchiolar epithelium and capillary endothelium was calculated from the PA response to changes in alveolar fluid colloid osmotic pressure, capillary hydrostatic pressure and plasma colloid osmotic pressure. From the mean PA values and from the simultaneously measured reflection coefficient for the alveolar membrane, it was possible to calculate interstitial fluid pressure (Pi). This was found to be between -7 and -8 Torr in normal lungs at heart level. A compliance curve of the interstitium based on calculated Pi values and the measured extravascular lung water (ELW) indicated a low compliance (0.042 g ELW/g dry wt per Torr) when Pi was negative, but an increased compliance (0.56 g ELW/g dry wt per Torr) when Pi exceeded atmospheric pressure. The PA values for Tyrode solution were also used to estimate pulmonary capillary pressures at different lung levels. A balance of fluid and osmotic pressures across the alveolar and capillary membranes which normally favors removal of fluid from the alveolar space is suggested. When this absorptive pressure was lost, the lungs rapidly gained extravascular fluid.