The effects of RBC transfusion on the relationship between DO 2 and oxygen uptake (VO 2 ) are even more difficult to predict. Some studies reported that VO 2 increased following RBC transfusion, whereas others did not [26] , and variable effects have been reported on tissue perfusion as assessed by gastric mucosal pH or near-infrared spectroscopy (NIRS) [30] . The reasons for these contradictory results lie primarily in the degree of severity of hypoxia preceding the RBC transfusion [31] , which influences the dependency of VO 2 on DO 2 . Methodological problems (imprecision in determination of VO 2 , assessment of global VO 2 instead of regional VO 2 , poor correlation between systemic oxygenation parameters, and oxygenation in the microcirculation [13] ) also may contribute to these discrepancies [31] .