Decreased ovarian blood flow may confound the tocolytic effect of ritodrine

Pregnancies ending before 37 weeks gestation are the leading cause of infant morbidity and death. A recent study involving over 2,000 patients concluded that the beta-adrenergic agonist, ritodrine (Yutopar), the only tocolytic agent currently approved for clinical use, had no significant beneficial effect on perinatal mortality, the frequency of prolongation of pregnancy to term, or birth weight despite proven efficacy in suppressing uterine smooth muscle contraction for 24-48 h. Our previous studies in anesthetized, pregnant rats found that both isoproterenol and ritodrine decreased blood flow to the ovary, apparently via a 'vascular steal' phenomenon. Because decreased ovarian blood flow and blood flow-mediated decrease in progesterone secretion represents one potential mechanism to explain the long-term ineffectiveness of beta-agonist tocolysis, ovarian blood flow studies were conducted in conscious, pregnant rats. Our results indicate that, indeed, ritodrine infusion significantly decreases ovarian blood flow (40%) in the conscious, near-term pregnant rat.