Cardiac and uterine hemodynamic responses to ritodrine hydrochloride administration in pregnant sheep.

The changes in the maternal circulation following administration of ritodrine hydrochloride were investigated in chronically prepared pregnant sheep. Low infusion rates of ritodrine (see text) elevated the maternal heart rate and cardiac output and decreased peripheral vascular resistance. Stroke work fell while minute work increased. The distribution of uterine blood flow did not change, as measured with microspheres. Simultaneously measured fetal cardiac output and umbilical blood flow were not altered. When ritodrine infusion rates (see text) were increased there was a slight but significant decrease in uterine perfusion pressure, and an increase in uterine vascular resistance with uterine blood flow decreasing. These changes were observed when the ewes were not in labor, and similar changes were again recovered with ewes in labor despite the simultaneous inhibition of uterine contractions. Selective beta blockade with practolol during ritodrine administration decreased the maternal tachycardia without affecting cardiac output, peripheral vascular resistance, or uterine vascular resistance.