Magnesium sulfate adversely affects fetal lamb survival and blocks fetal cerebral blood flow response during maternal hemorrhage.

Magnesium sulfate is commonly used in high-risk pregnancies, even though its actions in the fetus during maternal/fetal stress are not completely understood. The present study tested the hypothesis that magnesium sulfate alters the fetal cerebral blood flow response to hypoxemia produced during maternal hemorrhage. It was conducted in instrumented near-term fetal lambs at 123 days of gestation. Experimental treatment involved four periods of maternal hemorrhage over a 60-min period during fetal infusion of 0.25 g (n = 5) or 0.30 g (n = 6) magnesium sulfate, or normal saline (n = 11). The level of fetal cerx500l blood flow was determined using radiolabeled microspheres. For all three treatment groups, maternal hemorrhage produced fetal hypoxemia and some fetal demise. During fetal infusion of saline, 1 of 11 (9%) of the fetuses died; with the 0.25-g magnesium sulfate regimen, 1 of 5 (20%) died; and with the 0.30-g magnesium sulfate regimen, 3 of 6 (50%) of the fetuses died. Magnesium sulfate caused an increase in the proportion of fetal death produced by maternal hemorrhage (P < 0.05). Among surviving fetuses, hemorrhage-induced hypoxemia increased fetal cerebral blood flow during saline infusion. In contrast, infusion of magnesium sulfate had an inhibitory effect on this compensatory increase in fetal cerebral blood flow (P = 0.003). These data indicate that, in the sheep, magnesium sulfate increases fetal mortality and inhibits the compensatory increase in fetal cerebral blood flow during maternal hemorrhage-induced fetal hypoxemia.