Alveolar pressure and lung volume as determinants of net transvascular fluid filtration

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Abstract
[The effect of positive pressure ventilation on lung edema was studied via transvascular fluid filtration.] The influence of changes in alveolar pressure (PAlv) and in lung volume on the net transvascular fluid filtration rate (FFR) was investigated. The preparation was isolated, perfused zone III rabbit lungs. In observation periods the outflow pressure was kept constant at a level generally causing net filtration. All pressures were measured relative to atmospheric. FFR was measured by continuous monitoring of preparation weight. Elevation of PAlv at constant lung volume caused reversible reductions in FFR, also at constant capillary hydrostatic pressure (Pa-v < 2 Torr). Increases in lung volume at constant PAlv caused reversible increases in FFR. When both PAlv and Ptp [transpulmonary pressure] were increased, a reduction in FFR was seen in the majority of cases. At constant pulmonary arterial pressure, the size and the direction of the influence of positive airway pressure on FFR seems to depend on the relative change in lung volume and in alveolar pressure per se. Under the present experimental conditions, a rise in PAlv will be transmitted to interstitial fluid pressure and affect the transvascular fluid balance.