The primary purpose of blood transfusion is to increase DO 2 , which is determined by cardiac output and arterial content of oxygen, the latter being dependent on the hemoglobin level. Hence, blood transfusions can, theoretically at least, limit tissue hypoxia [13, 25, 26] . But does this really happen in clinical practice? It is obvious that RBC transfusions can be lifesaving in situations of acute severe anemia or in bleeding patients in whom RBC administration can increase both oxygen arterial content and cardiac output. However, in the absence of bleeding, the increase in hemoglobin concentration could very well be offset by a decrease in cardiac output because of the increase in blood viscosity associated with a decreased sympathetic response [27, 28] . DO 2 has been shown to increase following RBC transfusion in numerous studies [26] , but not in all [29] .