Magnesium sulfate is the drug of choice for seizure prophylaxis. Although magnesium has no substantial effect on blood pressure after a bolus dose and the mechanism for the anticonvulsant activity of magnesium is unknown, it attenuates the vascular responses to endogenous and exogenous pressor substances. It also dilates the vascular beds. Other anticonvulsants (e.g., phenytoin, diazepam) have been evaluated, but magnesium sulfate therapy is more effective in preventing recurrent seizures than either phenytoin or diazepam. The benefit of magnesium sulfate administration in women with mild disease is unclear. However, women with severe disease should received magnesium sulfate prophylaxis during labor and after delivery. Acute severe hypertension should be treated to prevent cerebrovascular and cardiovascular complications. Blood pressures should not be normalized. However, blood pressures of at least 160/105 mm Hg should be treated with antihypertensive medications. Because uteroplacental perfusion is proportionate to blood pressure, precipitous decreases in blood pressure can cause fetal compromise.
Antihypertensive agents should be titrated. Although hydralazine is the most commonly administered agent, it was recently compared to intravenous labetalol or oral nifedipine in a metaanalysis of trials. In the systematic review labetalol or nifedipine therapies were as effective as hydralazine in controlling blood pressure with fewer side effects. Labetalol is representative of a class of drugs that act as competitive antagonists at both a- and p-adrenergic receptors. Although systemic vascular resistance is reduced, cardiac output is maintained. Labetalol has also been shown to preserve uteroplacental perfusion. Hydralazine, an arteriolar vasodilator, remains a popular drug for patients with preeclampsia because it increases renal and uterine artery blood flow. The onset and duration are often unpredictable. Side effects include reflex tachycardia and ventricular dysrhythmias. Nifedipine is a calcium channel blocker that prevents the release of calcium, resulting in smooth muscle relaxation. Sodium nitroprusside is another alternative antihypertensive most often administered to patients with refractory hypertension and severe hypertension. Sodium nitroprusside administration requires placement of an arterial catheter to facilitate continuous assessment of maternal blood pressure. Advantages include a fast onset and short duration. It also preserves uteroplacental perfusion. Side effects include reflex tachycardia, cerebral vasodilation, and potential cyanide toxicity.
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