CARDIAC-FUNCTION IN FETAL SHEEP DURING 2 WEEKS OF HYPOXEMIA

Although several studies have examined fetal cardiac responses to acute hypoxemia, relatively little is known of the response to prolonged hypoxemia. To determine the effects of long-term hypoxemia on ovine fetal cardiac function, we measured right (Q(RV)) and left ventricular outputs (Q(LV)) and determined the effects of increasing preload (ventricular function curve) and afterload (arterial pressure sensitivity curve) on the left ventricle. Six days after fetal surgical instrumentation with catheters and electromagnetic flow probes (similar to 123 days gestation), we administered N-2 into the maternal trachea for 14 days to reduce maternal PO2 to similar to 55 Torr (hypoxemic group, Hyp, n = 6). Normoxic animals were used as controls (Cont, n = 6). With the onset of hypoxemia, fetal arterial PO2 was reduced from similar to 27 to similar to 18 Torr. Fetal heart rate in Hyp fetuses decreased similar to 22% on day 14 compared with Cont (P < 0.05). Mean arterial pressure in the Hyp group was higher than that of Cont but not significantly so. Right and left atrial pressures were not affected by hypoxemia. Q(RV) in Hyp fetuses was maintained on day 1 but decreased significantly by day 3 (similar to 19%) and further decreased on days 7 (similar to 28%) and 14 (similar to 34%). Q(LV) was not depressed until day 7 (similar to 20%), with a further decrease on day 14 (similar to 38%). In association with the decreased Q(LV) the plateau of the ventricular function curve in Hyp fetuses was depressed significantly on days 7 and 14. In contrast, the slope of the arterial pressure sensitivity curve in the Hyp group did not differ from Cont. Fetal growth rate in Hyp fetuses was significantly depressed compared with Cont. In conclusion, 2 wk of hypoxemia during the last third of gestation significantly depressed LV function but did not affect LV sensitivity to afterload. Q(RV) was decreased significantly by day 3, whereas Q(LV) was not decreased until day 7. The mechanisms for this difference in response remain to be explored.