[Perioperative blood transfusions. Value, risks, and guidelines]

Surgical blood loss and trauma are the major causes of allogeneic blood transfusions, which still bear considerable risks. After the correction of hypovolemia, the anesthesiologist often has to deal with normovolemic anemia. The clinical relevance of this isolated decrease in hemoglobin concentration consists in an eventually compromised global or regional oxygen supply with the development of tissue hypoxia below a critical threshold. This is an individual threshold for each patient and depends on his or her capacity to compensate the decrease in blood oxygen content. Therefore, physiologic transfusion triggers should primarily be applied and not rigid numeric transfusion triggers, such as hemoglobin concentration, which do not take into account each patient's individual reserve.