Labetalol, a combined α– and β–blocker, in hypertension of pregnancy

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Abstract
Labetalol was given to women with hypertension of pregnancy in their last trimester to study its acute effect on circulation and metabolism. Seven women were given 50 mg labetalol i v. There was a significant decrease of blood pressure from a mean of 143/101± 4/2 (SEM) to 127/88 ± 5/2 mm Hg. Maternal heart rate fell significantly from 77 ± 5 to 68 ± 3 beats per min. These changes persisted during a three–hour observation period. The hypotensive response was accompanied by a significant increase in plasma noradrenaline from 1.54 ± 0.16 to 2.37 ± 0.41 nmol/l, suggesting sympathetic activation. Plasma cyclic AMP, which is increased by β2–adrenoceptor stimulation, was significantly elevated after labetalol. This supports the hypothesis of partial β–agonist activity of labetalol. Lipid metabolism, as judged from measurements of plasma FFA, glycerol and 3–hydroxybuturic acid, showed little change. The acute effect of labetalol on uteroplacental blood flow was determined in eight women with pregnancy hypertension using a gammacamera on line with a computer. 0.5 mCi indium–113m was given i v before and 30 min after labetalol was administered i v in a dose of 1 mg per kg body weight. After the injections of indium–113m, serial scintigrams were recorded during 10 s periods for 240 s. By computerized summation of the scintigrams, an image was obtained in which the placenta could be outlined for time–activity analysis of the isotope accumulation curve. From this curve a uteroplacental blood flow index could be calculated. Labetalol induced a significant drop of mean arterial blood pressure from114 ± 2 mm Hg to 100 ± 3 mm Hg after 30 min in this group of women. However, the uteroplacental blood flow index did not change. As we have earlier shown with this technique that uteroplacental blood flow can be severely impaired in hypertension of pregnancy, the Finding of substained uteroplacental blood flow simultaneously with a decrease in blood pressure should be of clinical importance. Taken together with other studies of clinical effects, these results indicate that labetalol is useful in the treatment of hypertension of pregnancy