Maternal and fetal effects of labetalol in pregnant ewes.

Labetalol has been advocated to rapidly decrease blood pressure in preeclamptic women and to blunt the hemodynamic response to tracheal intubation. To assess labetalol's actions in a setting of uterine vasoconstriction, saline or labetalol (1 mg/kg) was infused into maternal venous catheters in 12 pregnant ewes receiving continuous maternal intravenous infusions of norepinephrine. Norepinephrine increased maternal mean arterial pressure (MAP) to 120 +/- 4% of control and decreased uterine blood flow to 45 +/- 6% of control. Labetalol, but not saline, altered these parameters: it decreased MAP to 101 +/- 3% and increased uterine blood flow to 70 +/- 7% of prenorepinephrine values. In the second protocol, to assess the degree of labetalol-induced fetal adrenergic blockade, 7 pregnant ewes received, on separate days, saline or labetalol (0.3, 1.0, 3.0 mg/kg) via a maternal venous catheter. Maternally administered labetalol produced minor (less than 12%) decreases in maternal and fetal MAP, without significantly altering heart rate (HR). At each labetalol dose, the degree of alpha- and beta-adrenergic blockade, determined by phenylephrine and isoproterenol challenge, was greater in the ewe than in the fetus. In the near-term pregnant ewe, intravenous (iv) bolus administration of labetalol ameliorated the effects of increased circulating norepinephrine on maternal MAP, uterine blood flow, and fetal pH and arterial O2 tension (PaO2), and produced less adrenergic blockade in the fetus than in the mother.