Effects of labetalol on uterine blood flow and cardiovascular hemodynamics in the hypertensive gravid baboon

Abstract

The purpose of this study was to investigate the effects of labetalol on uterine blood flow and cardiovascular parameters in acutely instrumented, hypertensive gravid baboons. During the latter half of pregnancy six gravid baboons were acutely instrumented, with ultrasonic flow probes placed on ipsilateral, external iliac, and uterine arteries and a flow-directed pulmonary artery catheter in the pulmonary artery. After a stable arterial pressure baseline was obtained, norepinephrine was infused to increase mean arterial pressure by at least 20%. A 20-minute hypertensive steady state was obtained. Labetalol at 0.5 and 1.0 mg/kg was randomly infused, followed by a 2.0 mg/kg dose, each over 1 minute. A 20-minute recovery period followed every labetalol infusion, allowing the hypertensive steady state to reestablish. External iliac and uterine blood flow measurements were continuously recorded during the baseline and experimental trials. Mean arterial blood pressure, heart rate, pulmonary artery and capillary wedge pressure, central venous pressure, and cardiac output were obtained at 5, 10, and 15 minutes during each steady state and after each labetalol infusion. Labetalol at all dosages significantly reduced the mean arterial pressure and the systemic vascular resistance at 1.0 and 2.0 mg/kg. External iliac blood flow was not consistently significantly reduced; however, uterine blood flow was significantly reduced after the 1.0 and 2.0 mg/kg labetalol dosages (p < 0.05). Although uterine vascular resistance tended to increase after the 1.0 and 2.0 mg/kg doses, statistical significance was not achieved. Low-dose labetalol (0.5 mg/kg) significantly reduces the pharmacologic hypertensive gravid baboon's mean arterial blood pressure without adversely affecting uterine blood flow.