Quantifying the effects of mixed venous PO 2 on regional pulmonary perfusion with positron emission tomography during lobar hypoxic ventilation in dogs

Abstract

The mixed venous PO 2 (P vO 2) can affect vascular tone during hypoxic pulmonary vasoconstriction. Therefore, a decrease in intrapulmonary shunt would be expected if P vO 2 decreases during regional alveolar hypoxia. We tested this possibility by first ventilating the left lower lobe (LLL) with 0% oxygen and the rest of the lung (RU with 100% oxygen. As a result, regional pulmonary blood flow (rPBF) to the LLL, measured by positron emission tomography, decreased to 49% of its baseline value. When the FiO 2 of the RL was then lowered to room air (but the LLL was still ventilated with 0% oxygen), P vO 2 fell from 51 ± 8 to 38 ± 5 mm Hg ( P < .05). However, rPBF to the LLL actually increased by 33%, mean pulmonary perfusion pressure increased by 63%, and cardiac output did not change. In contrast, after ventilating the LLL with 10% oxygen, no change in rPBF occurred after altering the FiO 2 in the RL in the same fashion, despite a comparable decrease in P vO 2 , as mean pulmonary perfusion pressure only increased by 29%. Thus, the effect of physiologic changes in P vO 2 (about 13 mm Hg) on vascular tone is modest (resulting in an approximate 30% change). However, the effect on intrapulmonary shunt will depend on the balance of effects in parallel compartments caused by the maneuver which changes the P vO 2 .