Adaptive responses during anemia and its correction in lambs

There is limited information available on which to base decisions regarding red blood cell (RBC) transfusion treatment in anemic newborn infants. Usin g a conscious newborn lamb model of progressive anemia, we sought to identi fy accessible metabolic and cardiovascular measures of hypoxia that might p rovide guidance in the management of anemic infants. We hypothesized that s evere phlebotomy-induced isovolemic anemia and its reversal after RBC trans fusion result in a defined pattern of adaptive responses. Anemia was produc ed over 2 days by serial phlebotomy (with plasma replacement) to Hb levels of 30-40 g/l. During the ensuing 2 days, Hb was restored to pretransfusion baseline levels by repeated RBC transfusion. Area-under-the-curve methodolo gy was utilized for defining the Hb level at which individual study variabl es demonstrated significant change. Significant reciprocal changes (P < 0.0 5) of equivalent magnitude were observed during the phlebotomy and transfus ion phases for cardiac output, plasma erythropoietin (Epo) concentration, o xygen extraction ratio, oxygen delivery, venous oxygen saturation, and bloo d lactate concentration. No significant change was observed in resting oxyg en consumption. Cardiac output and plasma Epo concentration increased at Hb levels <75 g/l, oxygen delivery and oxygen extraction ratio decreased at H b levels <60 g/l, and venous oxygen saturation decreased and blood lactate concentration increased at Hb levels <55 g/l. We speculate that plasma Epo and blood lactate concentrations may be useful measures of clinically signi ficant anemia in infants and may indicate when an infant might benefit from a RBC transfusion.