Transfusing allogenic red blood cells is an intervention that has both life-saving and life-threatening potential. Anesthesia providers are familiar with the deleterious effects of acute hemorrhagic anemia: Reduced cardiac output associated with severe blood loss can result in tissue hypoxia and eventually organ dysfunction and failure. Consequences of anemia depend on the patient’s ability to compensate for these physiologic changes. Because hemoglobin values are readily determinable in most intraoperative settings, it is tempting to transfuse packed red blood cells based on a number. However, the down sides to transfusion are more complex than just acute transfusion reaction alone. In order to fully assess the risks and benefits of transfusion, anesthesiologists must have a thorough understanding of their patient’s current medical condition and co-morbidities as well as the current literature on transfusion.

Laboratory experiments indicate that extreme hemodilution is well tolerated in healthy animals. Animals subjected to acute hemodilution tolerate decreasing hemoglobin concentrations down to around 5 g/dL, with ischemic electrocardiographic changes and depressed ventricular function noted at these levels. However, acute hemodilution is tolerated less well in animal models of coronary stenosis—ischemic electrocardiographic changes and depressed cardiac function are seen at hemoglobin concentrations between 7 and 10 g/dL. Human data regarding the limits of anemia tolerance are inadequate and often conflicting.

Guidelines regarding red cell transfusion have been presented by the American Society of Anesthesiologists Task Force on Blood Component Therapy. The authors strongly recommend reading the document in its entirety. The Task Force consensus states that transfusion is rarely indicated when hemoglobin concentration is greater than 10 g/dL and is almost always indicated when it is less than 6 g/dL, especially when the anemia is acute. Determining the need to transfuse for levels in between these numbers should take into account the individual’s risks of potential damage from possible inadequate oxygen delivery.

A patient’s likelihood of complications from anemia must be weighed against the risks of red cell transfusion. Some of the known risks of transfusion include threat of infection such as HIV, hepatitis B or C, or cytomegalovirus (CMV). Other, unquantified infectious risks almost certainly exist. There is also a danger of hemolytic or nonhemolytic reactions, allergic reactions, transfusion-related acute lung injury, and anaphylactic shock with any blood product transfusion. Despite numerous safeguards, transfusion mishaps due to human (often clerical) error remain the most common cause of serious transfusion reaction.

Numerous studies suggest that patients who are transfused more liberally have worse outcomes, in both the short and long term. Specific perioperative hazards associated with liberal transfusion include increased delirium and worsened pulmonary function. Some studies have shown increased postoperative infection, mortality, and length of stay. The seminal TRICC trial of 1998 concluded that intensive care unit (ICU) patients who received transfusions to maintain their hemoglobin greater than 9 g/dL (rather than greater than 7 g/dL) had increased mortality. These effects are thought to be secondary to immunomodulation from leukocytes in stored blood. There is speculation that routine transfusion of leukodepleted blood may reduce some of these adverse outcomes. Another important sequela of red blood cell transfusion is decreased long-term survival of some cancer patients, again possibly mediated by an alteration of immunologic status, although this is not borne out in large-scale studies.