Lung fluid balance in hypoxic, awake newborn lambs and mature sheep

To study the pulmonary vascular response to hypoxia and associated transvascular flow of fluid into the lungs of newborn lambs and mature sheep, we collected lung lymph and measured pulmonary vascular pressures and blood flow in 14 unanesthetized lambs and 9 sheep breathing 10-12% oxygen for 3-6 h after a 2-hour control period in air. In both groups of animals, pulmonary arterial pressure, blood flow, and lung vascular resistance increased with hypoxia, but only in lambs did the flow of lymph increase and th concentration of protein in lymph decrease with hypoxia. These results suggest that hypoxia increased transvascular filtration of fluid into the lungs of the lambs by increasing hydraulic pressure in the pulmonary microcirculation. In sheep, the vascular response to hypoxia did not increase lung fluid filtration. Hypoxia had no effect on microvascular permeability to protein in the lungs of either group of animals.