Magnesium sulfate, maternal hypothermia, and fetal bradycardia with loss of heart rate variability

Background: Fetal bradycardia and decreased heart rate variability can indicate a nonreassuring fetal status. However, there can be iatrogenic, physiologic, or pathologic causes. Case: A patient in premature labor received toxic levels of magnesium sulfate for tocolysis. Elevated maternal serum magnesium levels correlated inversely with maternal temperature and both fetal heart rate and fetal heart rate variability. There was also a relative decrease of the maternal heart rate from baseline. When the magnesium levels returned to normal, these vital signs returned to normal. Conclusion: Magnesium sulfate therapy can result in maternal hypothermia and a decrease in fetal heart rate and heart rate variability. Maternal hypothermia might be the cause of fetal bradycardia. A direct action of magnesium on the fetal heart might be the cause of heart rate variability. (Obstet Gynecol 1998;92:691-3. (C) 1998 by The American College of Obstetricians and Gynecologists.).