MATERNAL AND FETAL CARDIOVASCULAR-RESPONSES OF THE NORMOTENSIVE AND HYPERTENSIVE PREGNANT SHEEP TO PARENTERAL LABETALOL

Chronic and pregnancy-induced hypertension complicate 13% of all human pregnancies. Although several antihypertensive agents have been investigated for use during pregnancy, limited experimental information is available on the effects of labetalol (an alpha-1- and beta-1- adrenergic blocking agent) on uterine blood flow. This study was designed to test the hypothesis that labetalol safely lowers blood pressure in the normotensive and hypertensive pregnant ewe without adversely affecting uterine blood flow or fetal blood pressure and heart rate. Twelve normotensive and three renal hypertensive pregnant ewes were chronically catheterized to study the effects of bolus intravenous injections of labetalol (12.5 mg to 200 mg) on maternal and fetal blood pressure, heart rate, and uterine blood flow. All doses of labetalol produced significant reductions in both maternal mean arterial pressure and uterine blood flow (p < 0.05). Cardiac output responses were measured only at the 12.5 and 25 mg dose of labetalol, but showed consistent increases in cardiac output and decreases in systemic vascular resistance. Renal hypertensive sheep had both elevated maternal arterial pressure and elevated uterine vascular resistance. Administration of labetalol (50 or 100 mg) to hypertensive sheep showed changes in maternal blood pressure and uterine blood flow which were equivalent to normotensive sheep. We conclude that labetalol effectively lowers maternal arterial blood pressure but that it also decreases uterine blood flow in both the pregnant normotensive and renal hypertensive sheep.