Micropuncture measurement of microvascular pressures in isolated lamb lungs during hypoxia.

To study the effect of alveolar hypoxia on the pulmonary circulation of the newborn, we determined the longitudinal distribution of vascular pressures and the distribution of blood flow in isolated blood perfused lungs of newborn lambs during normoxia and hypoxia. We also examined the effect of hypoxia on the slope and intercept of the vascular pressure-flow (P/Q) relation. In 14 lungs, we used the micropipette servonulling technique to measure pressures in 20-80 micron subpleural arterioles and venules during normoxia and hypoxia at constant blood flow. In 5 other lungs, we used radiolabelled microspheres to determine the effect of hypoxia on regional distribution of blood flow. Under baseline normoxic conditions, segmental pressure drops in arteries, microvessels, and veins were approximately equal. During hypoxia pulmonary artery pressure always increased, resulting in a 60% increase in total vascular resistance. All segmental pressure drops increased with hypoxia. Arterial and venous pressure drops increased by 80 and 70% respectively, contributing almost equally to the increase in total pulmonary vascular resistance. Regional distribution of blood flow was uniform during normoxia and was unchanged during hypoxia. Hypoxia increased both the slope and intercept of the P/Q relation significantly, with an increase in fluid filtration rate. We conclude that in the newborn lamb alveolar hypoxia leads to pulmonary arterial and venous constriction, resulting in increased microvascular pressure and fluid filtration.