Plasma Catecholamine Responses to Hypoxemia in Fetal Lambs

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Abstract
The plasma catecholamine responses to hypoxemia were evaluated in 10 chronically catheterized fetal lambs in utero from 0.77 gestation to term using a highly sensitive radioenzymatic assay. Hypoxemia was produced by gradual constriction of the umbilical cord by an implanted cuff occluder. Fetal carotid arterial PO2 fell from 23 to 12 torr and was accompanied by an exponential increase in catecholamine concentration. Plasma norepinephrine (NE) increased from 256 ± 31 to 1,888 ± 384 pg/ml while epinephrine rose from 24 ± 5 to 1,159 ± 433 pg/ml. Fetal arterial PO2 returned to normal within 1 min of deflating the cuff occluder. Simultaneously, NE and epinephrine levels declined rapidly but, nevertheless, remained mildly elevated compared to controls. There was no maturational change in baseline NE levels during the last quarter of gestation, whereas epinephrine was significantly higher after 125 days than between 115 and 125 days (37 ± 7 vs. 16 ± 6 pg/ml, p < 0.05). However, maturation had no significant effect upon the magnitude of the epinephrine or NE response to hypoxemia. In conclusion, the late gestation fetal lamb is capable of manifesting a profound catecholamine response to hypoxemia which appears to play a major role in the compensatory cardiovascular adjustment to severe stress.