Serum magnesium levels during magnesium sulfate tocolysis in high-order multiple gestations

The objective of this study was to determine the serum levels of magnesium associated with various intravenous infusion rates of magnesium sulfate (MgSO4) in high-order multiple gestations. A retrospective review of 12 triplet and 4 quadruplet pregnancies cared for in Phoenix from August 1988 to January 1992 was performed. The mothers received MgSO4 for tocolysis of preterm labor. Serum magnesium levels were drawn to help manage patients with symptoms of an excessive magnesium level or an inadequate tocolytic effect. The results are presented for triplets and quadruplets with and without pregnancy-induced hypertension (PIH). The serum levels of Mg++ in triplets and quadruplets were similar to those in singleton pregnancies described in the literature. Higher serum levels of Mg++(7.0-7.5 mg/dL) seemed to be necessary for successful tocolysis. Serum levels of Mg++ in patients with underlying PIH were significantly higher at the same intravenous infusion rate. MgSO4 may need to be administered at infusion rates of 4-5 g/h to achieve therapeutic levels in women with triplets and quadruplets. PIH should require lower infusion rates to achieve therapeutic serum levels of Mg++.