Anesthesia & Analgesia, Vol 83, 493-499, Copyright © 1996 by International Anesthesia Research Society
Magnesium sulfate adversely affects fetal lamb survival and blocks fetal cerebral blood flow response during maternal hemorrhage
JD Reynolds, DH Chestnut, F Dexter, J McGrath and DH Penning
Department of Anesthesia, University of Iowa College of Medicine, Iowa City, USA.
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.