Pharmacology and toxicology
Magnesium sulfate, although not actually an antihypertensive, has proven valuable in the treatment of pre-eclampsia. Magnesium sulfate is the drug of choice for treatment of seizures in eclampsia (Oettinger 1993). In a group of 31 fetuses where the mothers were administered the drug i.v. as an initial loading dose of 4-6g, followed by an infusion of 2-3.5 g/h. magnesium sulfate was found not to be harmful (Gray 1994). A significantly lower risk of repeated convulsions in eclampsia was seen in a study of 1700 women given parenteral magnesium sulfate compared with those given Phenytoin or diazepam (Duley 1995).
Magnesium sulfate also promotes improved uterine circulation and acts to inhibit contractions. In addition, cerebral paresis is reported to develop less often In extremely low birth weight newborns when magnesium is used prenatally to treat tocolysis or eclampsia (Nelson 1995).
In a comparative study of 400 pregnancies, neither magnesium sulfate nor phenytoin treatment for hypertension or prevention of eclampsia adversely affected the course of labor (Leveno 1998).
The effectiveness of parenteral magnesium in the treatment of premature labor has not yet been proven.
Magnesium, when used in higher doses, or when kidney function is limited, can cause marked muscle hypotonia in both mother and newborn. In extreme cases, especially when its effect is enhanced by a calcium antagonist such as nifedipine, a dangerous drop in maternal blood pressure can occur which may result in fetal hypoxia.
Recommendation. Magnesium sulfate can be used for appropriate indications such as pre-eclampsia and eclampsia. It is the drug of choice for treatment of seizures in eclampsia.
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