Hypoxic constriction of alveolar and extra-alveolar vessels in isolated pig lungs

To examine whether hypoxia causes constriction of alveolar or extra-alveolar vessels, we determined the relationships among transpulmonary pressure, pulmonary arterial pressure (Ppa), blood flow (Q̇), and transvascular fluid filtration rate (Ẇ) during normoxia (Po₂ = 200 Torr) and hypoxia (PO₂ = 50 Torr) in isolated pig lungs perfused with autologous blood. Left atrial pressures were always subatmospheric. The effects of lung inflation and hypoxic vasoconstriction on the Ppa-Q̇ relationship were similar; when transpulmonary pressure was >5 Torr, both shifted the curve to higher pressures in a parallel fashion. When transpulmonary pressure was 0-5 Torr, however, inflation had no effect on the Ppa-Q̇ relationship during either normoxia or hypoxia. During normoxia at a transpulmonary pressure of 3.5 Torr, the relationship between fluid filtration rate and flow was characterized by a Ẇ of zero at Q̇ < 1.5 l/min and a rapid increase in Ẇ with Q̇ above this value. Both hypoxia and inflation shifted this relationship to higher filtration rates in a parallel fashion. Furthermore, the combined effects of hypoxia and inflation on filtration rate were additive. These results suggest that hypoxia caused constriction of both alveolar and extra-alveolar vessels, resulting in increased intraluminal pressure and filtration of fluid in vessels upstream from the sites of constriction.