Do We Have Enough Evidence to Know when to Transfuse Erythrocytes?

This brief article reviews the evidence regarding the oxygen delivery / oxygen carrying capacity / hemoglobin concentration at which erythrocytes should be transfused in the perioperative period. The outdated threshold of a hemoglobin concentration (Hb) of 10 g/dl or an hematocrit of 30% has been replaced by recommendations of many practice guidelines, based on clinical experience and data. Generally, in humans, these data address surrogate endpoints, rather than the primary ones of intracellular oxygenation, because of the inability to assess the latter in clinical circumstances. In conscious, healthy, resting humans, oxygen delivery does not have a maximum at Hb 10 g/dl, but remains unchanged until Hb falls to 6–7 g/dl. Even at an Hb of 5 g/dl with an oxygen delivery of 10.7 ml O⊂2/kg/min, systemic markers of inadequate oxygen delivery do not indicate inadequate oxygenation. Anesthetized patients do not show altered oxygen consumption in response to acute reduction of Hb to 8 g/dl. However, those who are not able to increase their oxygen delivery in response to anemia should have a higher critical oxygen delivery than do normal humans. Patients aged 65 years or more and even those with coronary artery disease sufficiently severe to require revascularization surgery do not have decreased oxygen consumption when their Hb is reduced to 9 or 10 g/dl when they are anesthetized. Patients in an intensive care unit do not have a higher mortality when transfused to maintain an Hb of 8.5 g/dl rather than 10.7 g/dl. Assessment of critical organ oxygenation in humans has been difficult. Evaluation of transmyocardial lactate flux or maximal exercise capability has not demonstrated the critical value for the heart in elderly patients or in those with coronary artery disease. Maximal exercise capacity, however, would seem to be a poor surrogate for this purpose. Evaluation of mental function suggests that in healthy, conscious humans, inadequate cerebral oxygenation occurs at an Hb of 5–6 g/dl; however, this has not been evaluated in patients in the perioperative period. Until more specific methods and technology applicable to patients are developed to determine whether a specific patient requires augmentation of oxygen delivery, transfusion therapy in the perioperative period will most often be determined by clinical judgment.