Anemia is common in
critically ill patients. Although the goal of transfusion of red blood cells is to increase
oxygen-carrying capacity, there are contradictory results about whether
red blood cell transfusion to treat moderate
anemia (e.g.,
hemoglobin 7-10 g/dL) improves tissue oxygenation or changes outcomes. Whereas increasing levels of
anemia eventually lead to a level of critical
oxygen delivery, increased cardiac output and
oxygen extraction are homeostatic mechanisms the body uses to prevent a state of dysoxia in the setting of diminished
oxygen delivery due to
anemia. In order for cardiac output to increase in the face of
anemia, normovolemia must be maintained. Transfusion of red blood cells increases blood viscosity, which may actually decrease cardiac output (barring a state of
hypovolemia prior to transfusion). Studies have generally shown that transfusion of red blood cells fails to increase
oxygen uptake unless
oxygen uptake/
oxygen delivery dependency exists (e.g., severe
anemia or strenuous exercise). Recently, near-infrared spectroscopy, which approximates the
hemoglobin saturation of venous blood, has been used to investigate whether transfusion of red blood cells increases tissue oxygenation in regional tissue beds (e.g., brain, peripheral skeletal muscle). These studies have generally shown increases in near-infrared spectroscopy derived measurements of tissue oxygenation following transfusion. Studies evaluating the effect of transfusion on the microcirculation have shown that transfusion increases the functional capillary density. This article will review fundamental aspects of
oxygen delivery and extraction, and the effects of
red blood cell transfusion on tissue oxygenation as well as the microcirculation.