Estimates of isogravimetric capillary pressures during alveolar hypoxia

Isogravimetric capillary pressures (Pci) and pre-(Ra) and postcapillary (Rv) pulmonary vascular resistances were estimated in isolated dog lungs when ventilated with 95% O2-5% CO2 and during alveolar hypoxia, which was simulated by ventilation with a 95% N2-5% CO2 gas mixture. In addition, the filtration coefficient (KF,C) was measured during alveolar hypoxia and compared with prehypoxic measurements. When both arterial and venous pressures were maintained constant during the alveolar hypoxia, Pci decreased from 8 to 6.9 mmHg, and total pulmonary vascular resistance increased from 0.040 to 0.083 (cmH2O . ml-1 . min . 100 g lung). Ra increased significantly from 0.027 to 0.067, and Rv remained unchanged, averaging 0.013 and 0.015 for control and hypoxic conditions, respectively. The changes in vascular resistance observed in alveolar hypoxia appear to be confined to blood vessels that are probably located in larger preseptal blood vessels. KF,C increased significantly from 0.092 to 0.179 ml . min-1 . cmH2O-1 . 100 g lung-1 during hypoxia. Either the surface area available for fluid exchange or the vascular permeability increases with alveolar hypoxia in the isolated dog lung. Inasmuch as vascular pressures were maintained constant in the present study, then the increase in KF,C with alveolar hypoxia is probably associated with changes in vascular permeability rather than an increased surface area of the fluid exchange vessels.