Regional pulmonary blood flow during 96 hours of hypoxia in conscious sheep

The effects of acute hypoxia on regional pulmonary perfusion have been studied previously in anesthetized, artificially ventilated sheep (J. Appl. Physiol. 56: 338-342, 1984). That study indicated that a rise of pulmonary arterial pressure was associated with a shift of pulmonary blood flow toward dorsal (nondependent) areas of the lung. This study examined the relationship between the pulmonary arterial pressor response and regional pulmonary blood flow in five conscious, standing ewes during 96 h of normobaric hypoxia. The sheep were made hypoxic by N₂ dilution in an environmental chamber [arterial O₂ tension (Pa(O₂)) = 37-42 Torr, arterial CO₂ tension (Pa(CO₂)) = 25-30 Torr]. Regional pulmonary blood flow was calculated by injecting 15-μm radiolabeled microspheres into the superior vena cava during normoxia and at 24-h intervals of hypoxia. Pulmonary arterial pressure increased from 12 Torr during normoxia to 19-22 Torr throughout hypoxia (α<0.049). Pulmonary blood flow, expressed as %Q̇CO or ml·min^-1·g^-1, did not shift among dorsal and ventral regions during hypoxia (α>25); nor were there interlobar shifts of blood flow (α>0.10). These data suggest that conscious, standing sheep do not demonstrate a shift in pulmonary blood flow during 96 h of normobaric hypoxia even though pulmonary arterial pressure rises 7-10 Torr. We question whether global hypoxic pulmonary vasoconstriction is, by itself, beneficial to the sheep.