Magnesium sulfate therapy during asphyxia in near-term fetal lambs does not compromise the fetus but does not reduce cerebral injury

OBJECTIVE: Our purpose was to investigate (1) the safety of fetal magnesium sulfate treatment and (2) possible beneficial effects on the brain during perinatal asphyxia. STUDY DESIGN: In 20 chronically instrumented fetal lambs (gestational age 125.8 +/- 3.5 days) four total umbilical cord occlusions for 5 minutes were repeated at 30-minute intervals. Fetuses received either saline solution (n = 11) or magnesium sulfate (n = 9) as a bolus of 300 mg intravenously 2 hours before occlusions, followed by an infusion of 100 mg/hr until 1 hour after occlusions. RESULTS: In the treated fetuses plasma magnesium levels rose from 0.85 +/- 0.20 to 2.23 +/- 0.40 mmol/L. Occlusions induced asphyxia, associated with mortality; 4 of 11 fetuses in the control group versus 1 of 9 in the magnesium-treated group died (not significant). Fetal electroencephalographic activity decreased and cerebral impedance increased during occlusions. Maximum spike and seizure activity occurred 5 to 10 hours after asphyxia. Neuronal loss was primarily localized in the corpus striatum. Magnesium caused no alterations in blood pressure, heart rate, or cerebral and peripheral blood flow, nor did it influence electrophysiologic responses or neuronal loss. CONCLUSIONS: Administration of magnesium sulfate was safe but did not offer significant cerebral protection from asphyxia in the near-term fetal lamb.