Effect of prolonged infusion of ritodrine on uteroplacental blood flow and cardiac output distribution in unanesthetized pregnant guinea pigs

Abstract The effects of ritodrine, 15 μg · min −1 · kg −1 administered intravenously, on the maternal circulation and uteroplacental blood flow were investigated in eight chronically catheterized pregnant guinea pigs near term. Cardiac output and its distribution were measured by means of the radionuclide-labelled microsphere technique before, and after 2 and 6 h of infusion of ritodrine. Ritodrine produced a sustained elevation of cardiac output to 135 and 137% of the control value after 2 and 6 h, respectively. The carcass and gastrointestinal tract together accounted for the greatest portion of the extra blood flow, 77 and 84% at the times of the two flow measurements during treatment. After 2 h of ritodrine, mean placental blood flow was reduced to 85% of the control level (n.s.), uteroplacental vascular resistance was 127% of its initial value (n.s.), and the placental fraction of cardiac output had decreased from 18 to 12% ( P P