Effect of maternal body mass index on serum magnesium levels given for seizure prophylaxis

Objective: To estimate the effect of body mass index (BMI) on magnesium levels for eclampsia prophylaxis. Methods: This is a retrospective study from 2004 to 2011, examining magnesium levels in women receiving seizure prophylaxis. Women received 6 g and then 2 g/h. Women had 4-hour and 12-hour levels drawn. Levels were considered subtherapeutic at less than 4.9 mg/dL, therapeutic from 4.9 to 8.4 mg/dL, and supratherapeutic at 8.5 mg/dL or more. If the 4-hour value was not therapeutic, the dose was adjusted and a 12-hour level was drawn. Levels at 4 and 12 hours were compared among the women with different BMI classifications and clinical characteristics. Results: During the study period,106,265 women delivered, and 7,799 (7.4%) had preeclampsia diagnosed and received magnesium sulfate for seizure prophylaxis. A total of 5,304 (68%) of these women had a recorded BMI. At 4 hours, 2,698 (51%) were subtherapeutic. These women were more likely to be older, parous, undergo cesarean delivery, have a higher systolic blood pressure, and have central nervous system manifestations. At 12 hours, 2,342 (90%) of therapeutic women remained therapeutic, and 5% became subtherapeutic (n=118) or supratherapeutic (n=140). Using logistic regression, we were able to predict being subtherapeutic in women with greater BMI and to predict being supratherapeutic if women had labor longer than 12 hours and worsening severity of preeclampsia. Conclusion: Women receiving seizure prophylaxis with a BMI of more than 30 may benefit from routine serum magnesium evaluation 4 hours after the loading dose.