Plasma catecholamines during hypoxemia in fetal lamb

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Abstract
Changes in peripheral plasma catecholamines were studied during varying degrees of hypoxemia in fetal lambs of 95-140 days gestation. Epinephrine and norepinephrine concentrations were measured by radioenzymatic assay in plasma obtained simultaneously from the distal aorta and inferior vena cava in 15 chronically catheterized fetuses with the mother breathing 20, 10 or 8% O2 mixtures. Levels of both epinephrine and norepinephrine increased significantly after 5 min of hypoxemia and showed a strong inverse exponential correlation with PO2 [O2 partial pressure] (P < 0.001). Plasma norepinephrine always exceeded epinephrine concentrations (P < 0.01). Resting arterial and venous catecholamine levels were not different, but during hypoxemia arterial levels of both epinephrine and norepinephrine exceeded venous levels significantly, suggesting the adrenal medulla as a major source of these substances. Hypoxemia led to fetal bradycardia, the degree of which moderated somewhat with extremely low O2 tensions and associated very high catecholamine levels. Fetal blood pressure increased initially in proportion to norepinephrine levels but reached a plateau at concentrations > 10 ng/ml. Although concentrations of catecholamines were lower in the earlier gestation fetuses, similar qualitative relations between catecholamines and PO2 were evident at all gestational ages.