The effects of magnesium sulfate infusion on blood pressure and vascular responsiveness during pregnancy.

Magnesium sulfate is primarily used for its anticonvulsive effects in treating hypertensive disorders of pregnancy. However, we have frequently observed significant falls in blood pressure in patients suffering from severe pregnancy-induced hypertension in whom little or no additional hypotensive medication was necessary for maintenance of diastolic blood pressure at 90 to 100 mm Hg while using recommended doses of this drug. We compared resting mean arterial pressure and vascular response to infusions of 50 ng/kg/min of angiotensin II and 2500 ng/kg/min of Levophed (norepinephrine) before and during infusion of a loading dose of 200 mg of magnesium sulfate intravenously in 15 minutes, followed by continuous infusions of 50 and 100 mg/hr intravenously in chronically prepared virgin (n = 14) and primigravid (n = 12) rabbits at 24 to 28 days' gestation (term = 30 +/- 1 day). Resting mean arterial pressure and vascular response to angiotensin II and norepinephrine were significantly decreased in the primigravid and virgin animals. Magnesium sulfate may attenuate blood pressure by decreasing the vascular response to pressor substances.