Obese women may benefit from a higher dose of magnesium sulfate to protect against preeclampsia, based on data from a randomized trial.
Pharmacokinetic models have shown that, “in women who received a 4-g intravenous loading dose followed by a 2-g/h IV maintenance dose, obese women took approximately twice as long as women of mean body weight in the sample to achieve these previously accepted therapeutic serum magnesium concentrations,” which suggests the need for alternate dosing based on body mass index, wrote Kathleen F. Brookfield, MD, of Oregon Health & Science University, Portland, and colleagues.
In a study published in Obstetrics & Gynecology, the researchers randomized 37 women aged 15-45 years with a BMI of 35 kg/m2 or higher who were at least 32 weeks’ gestation to receive the standard Zuspan regimen of magnesium sulfate (4 g intravenous loading dose, followed by a 1-g/hour infusion) or to higher dosing (6 g IV loading dose, followed by a 2-g/hour infusion).
Higher dose increases effectiveness
Serum magnesium concentrations were measured at baseline, and after administration of magnesium sulfate at 1 hour, 4 hours, and delivery; the primary outcome was the proportion of women with subtherapeutic serum magnesium concentrations (less than 4.8 mg/dL) 4 hours after administration.
After 4 hours, the average magnesium sulfate concentrations were significantly higher for women in the high-dose group vs. the standard group (4.41 mg/dL vs. 3.53 mg/dL). In addition, 100% of women in the standard group had subtherapeutic serum magnesium concentrations compared with 63% of the high-dose group.
No significant differences in maternal side effects or neonatal outcomes occurred between the groups. However, rates of nausea and flushing were higher in the higher dose group, compared with the standard group (10.5% vs. 5.5% and 5.2% vs. 0%, respectively).
The study findings were limited by several factors including the lack of statistical power to evaluate clinical outcomes and lack of generalizability to extremely obese patients, as well as to settings in which the higher-dose regimen is already the standard treatment, the researchers noted. However, the results were strengthened by the use of prospective pharmacokinetic data to determine dosing.
The researchers also noted that the study was not powered to examine preeclampsia as an outcome “and there is no evidence to date to suggest women in the United States with higher BMIs are more likely to experience eclampsia,” they said. “Therefore, we caution against universally applying the study findings to obese women without also considering the potential for increased toxicity with higher dosing regimens,” they added.
Current results may not affect practice
The study objectives are unclear, as they do not change the dosing for magnesium sulfate already in use, said Baha M. Sibai, MD, of the University of Texas Health Science Center at Houston, in an interview.
Dr. Sibai said he was not surprised by the findings. “This information has been known for almost 30 years as to serum levels with different dosing irrespective of BMI,” he said. Based on current evidence, Dr. Sibai advised clinicians “not to change your practice, since there are no therapeutic levels for preventing seizures.” In fact, “the largest trial that included 10,000 women showed no difference in the rate of eclampsia between 4 grams loading with 1 g/hour [magnesium sulfate] and 6 g loading and 2 g/hour,” he explained.
Future research should focus on different outcomes, said Dr. Sibai. “The outcome should be eclampsia and not serum levels. This requires studying over 6,000 women,” he emphasized.
The study was supported by the National Institutes of Health Loan Repayment Program and a Mission Support Award from Oregon Health & Science University to Dr. Brookfield and by the Oregon Clinical & Translational Research Institute grant. Dr. Brookfield also disclosed funding from the World Health Organization. Dr. Sibai had no financial conflicts to disclose.
SOURCE: Brookfield KF et al. Obstet Gynecol. 2020 Dec. doi: 10.1097/AOG.0000000000004137.