Chronic anemia in the newborn lamb: cardiovascular adaptations and comparison to chronic hypoxemia.

The cardiovascular adaptations to chronic anemia were studied in the newborn lamb and then compared with the adaptations to chronic hypoxemia. Eight chronically instrumented newborn lambs underwent repeat isovolemic exchange transfusions to maintain their Hb concentrations at 60% of normal for age. Hemodynamic studies were performed twice weekly for 2 wk after which time regional blood flows were measured using radionuclide-labeled microspheres. The major compensatory responses after 2 wk of anemia were moderate increases in heart rate (229 +/- 20 versus 187 +/- 15 beats/min) and cardiac output (226 +/- 36 versus 165 +/- 38 ml/kg/min), an increase in fractional extraction of oxygen (65 versus 40%), and a redistribution of regional blood flow. Blood flows to the heart and brain increased whereas blood flows to the viscera and carcass did not change. These compensatory responses were different from those that occur during chronic hypoxemia: specifically, cardiac output did increase, growth was not suppressed, and the pattern of redistribution of regional blood flows was different. The dissimilar effects of anemia (decreasing systemic oxygen content) versus hypoxemia (decreasing systemic oxygen tension) on local tissue receptors and peripheral chemoreceptors may account for these differences.