The effect of a low-dose infusion of ritodrine on cardiac output distribution and uteroplacental blood flow in unanaesthetized pregnant guinea pigs.

Cardiac output and its distribution were measured using the microsphere technique before and at 2 and 6 h during the infusion of ritodrine, 1.5 microgram X min-1 X kg-1 i.v., into near-term pregnant guinea pigs. Average cardiac output increased 16 and 25%, and total peripheral resistance decreased 16 and 19% after 2 and 6 h of ritodrine infusion. The carcass and gastrointestinal tract together received two-thirds of the extra cardiac output. Uteroplacental blood flow and uteroplacental vascular resistance were not significantly different from the preinfusion values after 2 h of ritodrine treatment, but were increased 35% and decreased 31%, respectively (p less than 0.05 and less than 0.01), after 6 h of infusion. The increase in uteroplacental blood flow between the 2- and 6-h measurements was positively, and the decrease in uteroplacental vascular resistance negatively correlated with the increase in cardiac output during the same period.