Twelve newborn lambs less than 48 hr old had their high oxygen affinity blood exchanged for low-affinity fresh adult blood. Tissue oxygenation, hemodynamic status, blood gases, and myocardial function were compared before and after the exchange transfusion. The P50was increased from 18 to 29 mm Hg after exchange transfusion; pH and hemoglobin levels remained constant; and there was no change in myocardial function, arteriovenous O2content difference, O2consumption, or cardiac output. However, mixed venous Po2was significantly increased with low affinity red cells (33versus23 mm Hg).SpeculationThis study suggests that in the immediate neonatal period adequate a lowering of red cell oxygen affinity under normoxemic conditions can result in a increase in tissue Po2.