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

The effects of ritodrine, 15 micrograms X min-1 X 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 less than 0.02). Continuation of the infusion to 6 h resulted in an increase in mean placental blood flow to 107% of the control value and return of uteroplacental vascular resistance to its initial level. The placental fraction of cardiac output increased to 14%, significantly (P less than 0.05) different from both the initial and the 2-h values. Blood flow to the skin, lungs, spleen and mammary glands decreased between the 2- and 6-h flow measurements. These observations are compatible with the hypothesis that the changes in uteroplacental blood flow during administration of ritodrine are passive, resulting from the balance between the increase in cardiac output and vasodilatation in extrauterine vascular beds.