Effect of endogenous opioid blockade on fetal cardiovascular and sympathoadrenal responses to hypoxemia induced by umbilical cord constriction.

The cardiovascular and plasma catecholamine responses to hypoxemia following endogenous opioid blockade were evaluated in 18 late gestation fetal lambs (0.75-0.83 gestation) in utero. Hypoxemia was produced by progressive constriction of the umbilical cord with an inflatable silicone rubber cuff while fetal heart rate, blood pressure, and arterial blood gases were monitored. Fetal arterial blood samples were obtained for plasma catecholamine analysis using a radioenzymatic method. The animals were divided into 4 experimental groups. Control animals during normoxemia and following hypoxemia are represented by groups I and III, respectively. During umbilical constriction, arterial oxygen tension declined from 21.4 +/- 0.3 to 13.4 +/- 0.3 torr (p less than 0.001, mean +/- SE) and was accompanied by an increase in blood pressure (44 +/- 1 to 53 +/- 1 torr), decrease in heart rate (176 +/- 4 to 114 +/- 4 beats/min) and rise in plasma norepinephrine (437 +/- 90 to 3,410 +/- 617 pg/ml) and epinephrine (53 +/- 10 to 1,074 +/- 325 pg/ml). Naloxone infusion had no significant effect on the fetus during either normoxemic (group II) or hypoxemic conditions (group IV). Plasma catecholamines rose less in group IV than in hypoxemic control fetuses (norepinephrine: 2,407 +/- 406 vs. 3,410 +/- 617 pg/ml; epinephrine: 966 +/- 641 vs. 1,074 +/- 305 pg/ml), but the differences were not significant (p greater than 0.2). It is concluded that endogenous opioids do not play a major role in the modulation of cardiovascular and sympathoadrenal adaptations to moderate hypoxemia in the fetus.