Chronic anemia in pregnant ewes: maternal and fetal effects.

To examine maternal and fetal responses to a sustained reduction in maternal hematocrit during days 110-138 of ovine gestation, 22 ewes and their singleton fetuses were instrumented. By repetitive exchange transfusions, the hematocrit in 12 ewes was decreased from 28.1 +/- 0.8 (baseline, day 110) to 14.0 +/- 0.7% (day 117) and maintained at that level. Anemic ewes demonstrated a transient increase in heart rate from 99 +/- 4 to 116 +/- 4 beats/min and a sustained increase in cardiac output from 117 +/- 8 to 153 +/- 11 ml.min-1.kg-1 compared with control animals. Uterine blood flow steadily increased in control animals from 868 +/- 100 (day 110) to 1,410 +/- 177 ml/min (day 138) but failed to increase in the anemic ewes. Uterine oxygen delivery fell from 83 +/- 10 to 39 +/- 4 ml/min with production of anemia and remained low in the anemic ewes. Arterial PO2 and oxygen content in the fetuses of anemic ewes fell transiently below control values (19.9 +/- 0.8 vs. 23.4 +/- 1.0 mmHg and 5.3 +/- 0.3 vs. 6.8 +/- 0.4 ml/dl, respectively) but did not persist. Fetal weights at 138 days gestation were lower in the anemic group (2,596 +/- 329 vs. 4,313 +/- 132 g). We conclude that chronic maternal anemia results in a decreased uterine blood flow as well as decreased uterine oxygen delivery. The fetus responds with decreased growth but does not develop sustained hypoxia or acidosis.