INFLUENCE OF ALVEOLAR OXYGEN ON PULMONARY VASOCONSTRICTION IN NEWBORN LAMBS VERSUS SHEEP

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Abstract
Fetal lung has vigorous hypoxic vasoconstriction and is exposed continuously to an O2 tension < 20 mm HG, which is well below that experienced by adult lungs. Whether the newborn lamb, with muscular arteries similar to the fetus, might not have a different O2 tension for developing alveolar hypoxic vasoconstriction in addition to having stronger vasoconstriction than the less muscular adult lung was investigated. Regional hypoxic pulmonary vasoconstriction in newborn lambs (mean of 5 days) and adult sheep (older than 2 yr) was compared at graded levels of alveolar O2 tension. Animals were anesthetized with pentobarbital, and each main bronchus was cannulated to allow ventilation of 1 lung with N2 or other O2/N2 mixtures as a hypoxic challenge while ventilation of the other lung with O2 maintained systemic oxgenation. Distribution of perfusion to the 2 lungs during bilateral O2 ventilation was first determined with 13N and a positron camera. The lambs significantly (P < 0.05) decreased the distribution of perfusion to the test lung compared with O2 control (PAO2 [arterial O2 tension] 620 .+-. 15) by 44 .+-. 1% SEM [standard error of the mean] with PAO2 of 22 .+-. 1 mm Hg, 22 .+-. 3% with PAO2 of 96 .+-. 1 mm Hg, and 12 .+-. 2% with PAO2 of 360 .+-. 3 mm Hg in contrast to the sheep that significantly decreased the distribution of perfusion to the test lung compared with O2 control distribution of perfusion by only 22% at a PAO2 of 19 .+-. 2 mm Hg and not at higher alveolar O2 tensions. The newborn lamb has not only stronger regional alveolar hypoxic pulmonary vasoconstriction than does the adult sheep but also, in contrast to sheep, can effectively express it at O2 tensions found in room air or higher.

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