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Red Blood Cells Products
The majority of RBC products are made from 450 to 500 mL of whole blood donated into an anticoagulant–preservative solution; about 14% of RBC products are collected via automated RBC apheresis. Whole blood is centrifuged to pack the RBCs, the platelet-rich plasma is expressed, and an additive solution (AS) is added. This product can undergo a number of modifications, including leukoreduction, freezing, rejuvenation, washing, irradiation, and/or volume reduction. Additionally, there has been recent interest in using whole blood, which contains viable platelets, in trauma patients with life-threatening hemorrhagic shock.
In the United States, approximately 10.7 million units of whole blood and 1.7 million apheresis units are manufactured into the ∼11 million RBC products that are transfused each year. Worldwide, it is estimated that ∼100 million units of RBC products are transfused. General characteristics of RBC products include ∼130–240 mL of RBCs, ∼50–80 g of hemoglobin (Hgb), and ∼150–250 mg of iron. The total volume, ∼250–350 mL, and hematocrit, 55%–80%, vary depending on the anticoagulant–preservative solution used; higher volume and lower hematocrit are found in AS-containing products. Small amounts of plasma, platelets, and leukocytes remain in RBC products, unless the leukocytes and platelets (for most filters) have been removed by leukoreduction, which is typical in about 80% of the RBC products used in the United States. Canada and many European countries have a universally leukoreduced blood supply.
RBC Storage Lesion
The RBC storage lesion is the term that collectively refers to the biochemical and physical changes occurring to the RBCs and supernatant during storage. In general, biochemical changes include progressive increases in free Hgb, lactate, and potassium concentrations paralleled by gradual decreases in RBC adenosine triphosphate (ATP) and pH during storage. In addition, proinflammatory cytokines accumulate during storage of nonleukoreduced units but not in leukoreduced units. Within the first week of storage, 2,3-diphosphoglycerate (2,3-DPG) declines rapidly; however, it regenerates within 24 hours after transfusion. The decreased 2,3-DPG results in a shift in the oxygen dissociation curve to the left, which leads to less oxygen release than normal RBCs at the same partial pressure of O 2 . Recent metabolomic studies reveal a number of other metabolites that change during storage. In addition to biochemical changes, RBCs change from a deformable biconcave disk, to reversibly deformed echinocytes, to irreversibly deformed spherechinocytes with increased membrane stiffness. These morphologic changes may also result in decreased oxygen transport owing to the inability of these RBC to flow through the microcirculation. On transfusion, storage-damaged RBCs are cleared from the circulation. Macrophage processing of these RBCs releases large amounts of iron into the circulation, consequences of which are currently being studied. However, large randomized trials comparing the standard of care with fresher RBC products have not observed differences in clinical outcome, leading to the recommendation of AABB current practice guidelines that patients, including neonates, receive RBC units selected at any point within their licensed dating period (standard issue), rather than limiting patients to transfusion of only fresh products. Controversy still exists regarding the safety of particularly old RBC products (i.e., those stored for 35–42 days), as randomized trials have not examined the safety of these products.
Transfusion Guidelines
Indications
RBCs are transfused to mitigate the signs and symptoms of anemia, reflecting a significant deficiency in oxygen-carrying capacity and/or tissue hypoxia due to an inadequate circulating RBC mass ( Table 33.1 ).
Table 33.1
Indications and Contraindications for Transfusion of Red Blood Cells
| Indications a |
| To mitigate tissue hypoxia due to decreased oxygen-carrying capacity associated with an inadequate RBC mass |
| As a source of replacement RBCs during RBC exchange (erythrocytapheresis) |
| Hgb < 7.0 g/dL in hemodynamically stable or burn patients |
| Hgb < 8.0 g/dL with orthopedic surgery or cardiac surgery and those with preexisting cardiovascular disease |
| Hgb < 9 g/dL in oncologic patients with septic shock b |
| Contraindications |
| To correct anemia due to iron deficiency |
| As a source of nutritional supplementation |
| For volume expansion or to increase oncotic pressure |
| To improve wound healing, recovery, or a sense of well-being |
a These recommendations do not apply to acute coronary syndrome, severe thrombocytopenia, and chronic transfusion–dependent anemia.
RBC Exchange
RBCs are used during RBC exchange (erythrocytapheresis) either therapeutically or prophylactically. RBC exchange can be required in hemolytic disease of the fetus and newborn for the prevention of kernicterus and in patients with sickle cell disease for the treatment of severe complications or prophylactically ( Chapter 52 ).
Transfusion Trigger
Because there are no precise indicators of tissue hypoxia, nor objective, measurable indicators of symptomatic anemia, it has become common practice to administer RBC products based on laboratory parameters including Hgb. These numbers constitute “transfusion triggers,” at which transfusion is generally considered appropriate and above which it is not. While the use of transfusion triggers is helpful in considering a patient’s general condition, there is no universal transfusion trigger, and therefore, the clinical assessment of each patient is imperative so that unnecessary transfusion can be avoided in patients who have adapted well to their current level of anemia and so that transfusion is not withheld when needed.
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