Effect of intravenous magnesium sulfate on the biophysical profile of the healthy preterm fetus.
OBJECTIVE
The null hypothesis is that intravenous magnesium sulfate does not affect the biophysical profile of the healthy preterm fetus.


METHODS
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 studied. After normal fetal biophysical assessment was documented, intravenous 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 for measurement of maternal serum magnesium levels immediately before intravenous magnesium sulfate was administered and at 2 and 12 hours after the loading dose. Biophysical profiles, consisting of a possible 12 points, were performed at the same time as blood was drawn. Serum magnesium levels were compared with one-way analysis of variance for repeated measures and biophysical profile scores with Friedman's test. Statistical 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 were delivered for severe variable decelerations, two progressed in labor, and in one tocolysis was discontinued. The median biophysical profile score was 11 before intravenous magnesium sulfate, at 2 hours, and at 12 hours after the loading dose. The biophysical parameters present and the percentage of fetuses with each parameter were as follows: breathing (> 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 magnesium sulfate, 100% (25/25) at 2 hours, and 96% (24/25) at 12 hours.


CONCLUSIONS
Intravenous magnesium sulfate did not significantly alter the biophysical profile in the 25 fetuses evaluated by three biophysical profiles in spite of the significant increase in maternal serum magnesium levels.
Gray
S E
SE
Department of Obstetrics and Gynecology, University of Connecticut Health Center, Farmington.
Rodis
J F
JF
Lettieri
L
L
Egan
J F
JF
Vintzileos
A
A
D000328:Adult; D005260:Female; D005323:Fetal Monitoring; D005324:Fetal Movement / Q000187:drug effects; D005333:Fetus / Q000187:drug effects*; D005865:Gestational Age; D006801:Humans; D008278:Magnesium Sulfate / Q000494:pharmacology*; D007752:Obstetric Labor, Premature; D011247:Pregnancy; D011446:Prospective Studies; D012119:Respiration / Q000187:drug effects; D015145:Tocolysis
D008278:Magnesium Sulfate