ADJUSTMENT OF MAGNESIUM-SULFATE INFUSION RATE IN PATIENTS WITH PRETERM LABOR

OBJECTIVE: Our purpose was to investigate factors that might influence serum magnesium levels during intravenous magnesium sulfate tocolytic therapy. STUDY DESIGN: Thirty-three women receiving magnesium sulfate for preterm labor participated in this prospective, observational stu dy. Gestational ages were 24 to 34 weeks. Four groups of women were id entified according to the maintenance magnesium infusion rate required for arresting preterm labor after 5 g of therapy induction: 1.5, 2, 2 .5, and 3 g/h. Serum magnesium samples were drawn after a predefined p eriod of at least 18 hours of arrested preterm labor, at a minimum of every 6 hours. Variables examined included serum albumin; serum protei n; serum ionized calcium; serum creatinine; creatinine clearance; 24-h our urine output; maternal height, weight, body surface area; and body mass index.RESULTS: By use of a multivariate stepwise regression mode l we identified four variables that independently and significantly co ntributed to the model: magnesium infusion rate (P<.001); total serum protein level (P <.001); serum creatinine level (P=.009); and maternal weight squared (P =.026). Seventy-two percent of the variance was acc ounted for by use of these parameters. A predictive linear model, deve loped to relate these factors, produced the following formula: Suggest ed magnesium infusion rate = 0.89 x Serum magnesium concentration (mg/ dl) - 3.16 x Serum creatinine (mg/dL) - 0.66 x Serum total proteins (g /dL) + 0.0001 x (materna[ weight)2 (kg) + 2.30. CONCLUSIONS: Serum cre atinine, serum protein, and maternal weight can be used to adjust the dose of magnesium sulfate in patients with premature labor to achieve therapeutic serum levels of magnesium more rapidly and safely.