MAGNESIUM-SULFATE LOADING - PREECLAMPSIA VS PRETERM LABOR - (A CLINICAL PEARL)

Objective: To measure the apparent volume of distribution (AVOD) for m agnesium (Mg) in preeclampsia and preterm labor and determine if a sta ndard 4 gm loading dose of magnesium sulfate (MgSO4) is sufficient to attain therapeutic levels. Methods: Twenty-five patients with preeclam psia and 25 with preterm labor received 4 g of MgSO4 intravenously ove r 15 minutes. Serum Mg levels were determined before and one minute af ter loading and the AVOD for Mg was calculated. Stepwise linear regres sion with AVOD as the dependent variable was performed and comparisons between the groups were made. Results: Preeclamptics were heavier, ha d greater surface areas, and presented at a later stage of pregnancy t han did patients with preterm labor. Despite these differences AVOD di d not differ between the groups. Predose magnesium levels were slightl y higher in the preeclamptic group (p = .04). Postloading levels were nearly identical due to similar AVOD's and, because of the lower level s required for seizure prevention as opposed to tocolysis, were therap eutic 88% of the time in preeclampsia but only 12% of the time in pret erm labor (p < .001). Multivariate analysis revealed that only ideal b ody weight, degree of underweight, and current therapy with betamimeti cs were significantly related to AVOD. Conclusion: AVOD was found to b e similar in preeclamptic and preterm labor patients. A 4 g loading do se of MgSO4 is usually adequate to achieve therapeutic levels in preec lampsia but not in preterm labor.