Anemia in Critically Ill Patients: How Much Can They Tolerate?

SUMMARY Anemia is a common occurrence in intensive care unit (ICU) patients and frequently results in allogeneic blood transfusion. In critical illness, patient tolerance to a reduction in the blood's oxygen‐carrying capacity may be reduced because of a blunting of the physiologic compensatory response mechanisms to – anemia and/or to an increase in tissue metabolic needs. Anemia has indeed been associated with an increased risk of mortality in critically ill patients, particularly in those with pre‐existing cardiovascular disease or in those with significant blood loss. Clinical trials comparing a liberal versus restrictive transfusion strategy defined by a specified hemoglobin (or hematocrit) concentration showed that mortality, morbidity and length of hospital stay seem unaffected by the transfusion strategy, even in hemodynamically stable cardiovascular disease patients. The absence of outcome improvement associated with a more liberal transfusion strategy remains largely to be elucidated. There is, however, some experimental evidence that blood transfusion may have adverse effects on tissue oxygen delivery because of erythrocyte storage‐related changes. The decision to transfuse a given patient must also take into account the risks associated with allogeneic blood, in particular those related to its potential immunosuppressive effects. There is no one “optimal” hemoglobin value for ICU patients, and the decision to transfuse should be made on a case‐by‐case basis, with each patient being assessed according to his or her clinical status, the underlying cause of anemia and the risk of potential complications. In any case, preventive measures should be taken whenever possible to limit the development of anemia in the ICU.