The use of standard dose of magnesium sulphate in prophylaxis of eclamptic seizures: do body mass index alterations have any effect on success?

OBJECTIVE: We anticipated that the universal use of a standard magnesium sulfate infusion to prevent eclamptic convulsions in preeclamptic patients would result in alterations in circulating magnesium levels that were negatively correlated with the patient's body mass index. We postulated that the highest failure rate with seizure prophylaxis would occur in patients with the highest body mass index.
MATERIALS AND METHODS: After discarding 6 patients, this study was performed in 194 of 200 preeclamptic patients admitted to our high risk pregnancy unit between February 2000 and August 2000, who were divided into four groups determined by body mass indices. A standard magnesium sulfate infusion protocol (loading dose 4.5 g/15 minutes followed by 1.8 g/hour) was administered to 194 preeclamptic patients. One hundred and thirty-eight severe preeclamptic patients received magnesium sulfate during both antepartum and postpartum periods. The remaining 56 patients only received the therapy during the postpartum period. Serial serum magnesium levels of each groups were recorded and compared.
RESULTS: The 1.8 g infusion rate produced acceptable magnesium levels in the majority of patients but most were in the lower 50% of the therapeutic range. Levels were lowest in patients with high body mass indices (this group recorded most of the subtherapeutic levels, particularly when patient were infused antepartum). Apart from 13 referred patients who had convulsed prior to admission no eclampsia occurred during the antepartum period while seizures occurred in nine women during the postpartum period. Two hours after the initiation of the therapy, magnesium levels were inversely related to the body mass index (BMI) both during the ante- and postpartum periods (Prepartum; group I: 5.97 mg/dl, group II: 4.90 mg/dl, group III: 4.35 mg/dl, group IV: 3.88 mg/dl; Postpartum; group I: 5.89 mg/dl, group II: 5.71 mg/dl, group III: 4.82 mg/dl and group IV: 4.61 mg/dl, Table 4). Although the lowest levels were detected in patients with high body mass indices, in contrast to our hypothesis, eclamptic seizures occurred in four patients with low body mass indices. Furthermore therapeutic serum magnesium levels were detected in three of these patients. There was no association between treatment failures and body mass or with magnesium levels.
CONCLUSION: The infusion regimen described herein resulted in therapeutic levels in the majority of patients that correlated inversely with body mass index. However most levels fell within the lower range of what many studies consider "therapeutic" suggesting that maintenance infusion rates of at least 2-2.5 g/hour would be more appropriate. This would be particularly true in patients with body mass indices exceeding 30, where subtherapeutic levels occurred most frequently. The study's limited power prevents conclusions on outcomes but what is of interest is that eclamptic convulsions did not correlate with either body mass index or circulating plasma magnesium levels.