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Fresh, warm whole blood most effectively restores red cell mass, plasma volume, clotting factors, and platelets. However, given shortages of blood products, the use of whole blood transfusions is not realistic. The use of component product transfusion is the mainstay of blood banking and transfusion practice, effectively utilizing a scare resource while matching the components transfused to the specific needs of the patient. Whole blood is usually separated into packed red blood cells (PRBC), fresh frozen plasma (FFP), and platelet concentrate soon after donation. The plasma can be further separated into cryoprecipitate and cryopoor plasma, or undergo further fractionation to individual plasma proteins.
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Indications for blood component therapy can be divided into two main categories: (1) enhancement of oxygen-carrying capacity by increasing red blood cell (RBC) mass and (2) replacement of coagulation components due to loss, dysfunction, or consumption.
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Anemia is one of the most common abnormal laboratory findings among critically ill patients. The effect of anemia on outcome and the determination of transfusion triggers has been the subject of much debate in recent literature.
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Historically, the decision to transfuse has been guided by the hemoglobin (Hb) concentration, usually 10 mg/dL. However, given the risks associated with PRBC transfusion and recent literature supporting better or similar outcomes with lower transfusion triggers, the optimal Hb level at which to transfuse patients remains unclear.
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Benefits of RBC Transfusion
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The main function of RBCs is to transport oxygen from the lungs to the peripheral tissues. Oxygen delivery (DO2) is calculated by multiplying the cardiac output (CO) times the arterial oxygen content (CaO2):
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where DO2 is in milliliters per minute, CO in deciliters per minute, and CaO2 in milliliters per deciliter.
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And CaO2 is calculated by the following equation:
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where SaO2 is the arterial oxygen saturation (%), 1.34 the oxygen-carrying capacity of Hb (mL/g), [Hb] the Hb concentration (g/dL), 0.0031 the solubility of oxygen in plasma at 37°C, and Pao2 is measured in millimeters of mercury.
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Under normal conditions, DO2 exceeds oxygen consumption (VO2) by three to five times. However, in situations where the VO2 of the peripheral tissues is greatly increased, or DO2 is decreased by anemia or decreased CO, VO2 can exceed DO2 and result in tissue hypoxia. Increasing the [Hb] is one of the ways to increase the blood's oxygen-carrying capacity, and therefore increase DO2. Additionally, transfusion can increase blood volume for patients following acute blood loss or hemorrhage and alleviate symptoms of anemia such as dyspnea, weakness, and fatigue.
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Drawbacks of RBC Transfusion
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Despite the theoretical benefits of transfusion described above, it is associated with multiple risks. There is risk of human error resulting in a transfusion ...