EFFECTS OF ISOPROTERENOL ON THE CARDIOVASCULAR-SYSTEM OF FETAL SHEEP EXPOSED TO LONG-TERM HIGH-ALTITUDE HYPOXEMIA

We exposed fetuses to high-altitude (3,820 m) hypoxemia from 30 to 130 days gestation, when we measured fetal heart rate, right and left ven tricular outputs with electromagnetic flow probes, and arterial blood pressure during an isoproterenol dose-response infusion. We also measu red the distribution of cardiac output with radiolabeled microspheres during the maximal isoproterenol dose. Baseline fetal arterial blood p ressure was higher in long-term hypoxemic fetuses (50.1 +/- 1.3 vs. 43 .4 +/- 1.0 mmHg) but fell during the isoproterenol infusion to 41.3 +/ - 1.4 and 37.5 +/- 1.4 mmHg, respectively, at the highest dose. Heart rate was the same in both groups and did not differ during isoproteren ol infusion. Baseline fetal cardiac output was lower in the hypoxemic group (339 +/- 18 vs. 436 +/- 19 ml . min(-1). kg(-1)) due mainly to a reduction in right ventricular output. During the isoproterenol infus ion, right ventricular output increased to the same extent in both hyp oxemic and normoxic fetuses (similar to 35%); however, left ventricula r output increased only similar to 15% in the hypoxemic group compared with similar to 40% in the normoxic group. The percent change in indi vidual organ blood flows during isoproterenol infusion in the hypoxemi c groups was not significantly different from the normoxic group. All of the mechanisms that might be responsible for the differential respo nse of the fetal left and right ventricles to long-term hypoxia are no t understood and need further exploration.