Effects of maternal anemia on cardiac output, systemic oxygen consumption, and regional blood flow in pregnant sheep.

We measured regional blood flows and cardiac output and calculated regional oxygen deliveries (blood flow X arterial blood oxygen concentration) and systemic oxygen delivery, oxygen extraction, and oxygen consumption as maternal hematocrit was varied randomly between 31% and 8% by exchange transfusion with plasma or packed sheep red blood cells. We observed three patterns of response: In the heart, brain, and adrenal glands, oxygen delivery was constant or nearly constant over the range of hematocrits studied; in the kidneys, splanchnic organs, and uteroplacental tissues, oxygen delivery was a linear function of hematocrit; in the carcass tissues, oxygen delivery decreased slowly as hematocrits were reduced from 31% to 15% and dropped rapidly at hematocrits less than or equal to 15%. Systemic oxygen consumption was maintained at normal levels, despite reductions in hematocrit to about 15%, because oxygen extraction increased as compensation. At lower hematocrits, systemic oxygen consumption fell in association with a metabolic acidemia, suggesting that, with this severity of acute anemia, oxygen supply to some tissues in the pregnant sheep was inadequate for tissue metabolic demands.