Acute anemic hypoxemia produces a transient depression in fetal respiratory activity

Isovolemic anemia was produced in 11 unanesthetized fetal sheep by withdrawal of blood and replacement with saline-dextran. Fetal hematocrit fell from 36 ± 1 to 19 ± 1% (SE). Fetal breathing movements, which were present during 34.4 ± 5.5% of 3 h before the anemia, occurred 10.1 ± 5.3, 14.8 ± 4.4, and 27.1 ± 6.7% in the 3 h following. The anemia caused a fall in arterial O₂ concentration from 8.4 ± 0.3 to 3.6 ± 0.1 vol% and sagittal vein PO₂ fell from 15.4 ± 0.5 to 12.4 ± 0.3 Torr. Cerebral metabolic rate during the period of anemia was 2.9 ± 0.1 ml·100 g^-1·min^-1, which was unchanged from the control value of 3.0 ± 0.2 ml·100 g^-1·min^-1. Sagittal vein PCO₂ (54.2 ± 1.4 Torr) remained constant after the fetus was made anemic. We conclude that respiratory activity in the sheep fetus is depressed by anemic hypoxemia but that the effect is transient.