Circulatory effects of magnesium sulfate in normotensive and renal hypertensive pregnant sheep.

We have studied the effects of magnesium sulfate (MgSO4) administration in chronically instrumented normotensive and hypertensive pregnant sheep. Animals of 90 to 144 days' gestation were divided in two groups; those with normotensive arterial pressure and those in which renovascular hypertension had been produced. Our results show that bolus injection of 2 or 4 Gm. of MgSO4 caused a transient decrease in systolic and diastolic pressure which returned to control values within 5 to 10 minutes. The continuous infusion of MgSO4 in doses of 2 or 4 Gm. per hour produced a slight increase in uteroplacental blood flow but did not alter the blood pressure. Heart rate showed a slight increase. Because MgSO4 produced no significant hemodynamic changes, we feel that: (1) its use in obstetrics must be based on its anticonvulsant action and (2) MgSO4 in either normotensive or hypertensive patients is not associated with impairment of uteroplacental blood flow.