EFFECT OF INTRAVENOUS MAGNESIUM-SULFATE ON THE BIOPHYSICAL PROFILE OFTHE HEALTHY PRETERM FETUS

OBJECTIVE: The null hypothesis is that intravenous magnesium sulfate d oes not affect the biophysical profile of the healthy preterm fetus. S TUDY DESIGN: Thirty-one fetuses of 25 patients between the gestational ages of 24 and 35 weeks, median 31.4 and mean (+/- SD) 30.4 (+/- 2.9) , who required tocolysis for uterine contractions were prospectively s tudied. After normal fetal biophysical assessment was documented, intr avenous magnesium sulfate was started as a 4 or 6 gm loading dose and then infused at 2 to 3.5 gm/hr to achieve tocolysis. Blood was drawn f or measurement of maternal serum magnesium levels immediately before i ntravenous magnesium sulfate was administered and at 2 and 12 hours af ter the loading dose. Biophysical profiles, consisting of a possible 1 2 points, were performed at the same time as blood was drawn. Serum ma gnesium levels were compared with one-way analysis of variance for rep eated measures and biophysical profile scores with Friedman's test. St atistical significance was considered p < 0.05. RESULTS: Mean (+/- SD) serum magnesium levels were 1.7 (+/- 0.1) mg/dl before infusion, 4.3 (+/- 0.6) mg/dl at 2 hours, and 5.2 (+/- 0.7) mg/dl at 12 hours (p < 0 .001). Six fetuses did not have a 12-hour biophysical profile; three w ere delivered for severe variable decelerations, two progressed in lab or, and in one tocolysis was discontinued. The median biophysical prof ile score was 11 before intravenous magnesium sulfate, at 2 hours, and at 12 hours after the loading dose. The biophysical parameters presen t and the percentage of fetuses with each parameter were as follows: b reathing (>30 seconds), 88% (22/25) before magnesium sulfate, 84% (21/ 25) at 2 hours, and 92% (23/25) at 12 hours; nonstress test (reactive) , 84% (21/25) before magnesium sulfate, 68% (17/25) at 2 hours, and 80 % (20/25) at 12 hours; movement (normal), 100% (25/25) before magnesiu m sulfate, 100% (25/25), at 2 hours, and 96% (24/25) at 12 hours; tone (normal), 100% (25/25) before magnesium sulfate, 100% (25/25) at 2 ho urs, and 96% (24/25) at 12 hours. CONCLUSION: Intravenous magnesium su lfate did not significantly alter the biophysical profile in the 25 fe tuses evaluated by three biophysical profiles in spite of the signific ant increase in maternal serum magnesium levels.