Regional Vascular Responses to Chronic Anemia in Pregnant Sheep

Abstract

The increased circulatory and metabolic demands of pregnancy lead to a variety of adaptive responses in many regional vascular beds. The concomitant development of anemia could modify these adaptive processes. The purpose of our studies was to quantify the regional vascular responses to chronic anemia in pregnant sheep and to compare those responses to those occurring with acute anemia. We measured regional blood flows and O₂ deliveries and calculated vascular resistances in 10 near-term pregnant sheep, five of which were made anemic (hematocrit = 14%) for 6 days by isovolemic hemodilution with fresh plasma. The remaining five sheep served as controls (hematocrit = 28%). As a result of 6 days of anemia, almost all regional beds (except for the small intestines, colon, and spleen) had increases in blood flow, which helped maintain tissue O₂ deliveries at or near normal levels. Tissues with increased flow accounted for 88% of cardiac output, in contrast to what has been reported for acute anemia of comparable severity, where tissues receiving increased flow accounted for only 9% of cardiac output. Our data indicate that regional vascular responses to chronic anemia differ from those to acute anemia during pregnancy. The global increase in perfusion with chronic anemia plays an important role in counteracting the potential adverse effects of the anemic state on both mother and fetus.

Key Words:

• Cardiac output distribution
• Systemic and regional O₂ delivery and vascular resistance
• Uteroplacental adaptation to anemia