Anemia is characterized by an insufficient number of red blood cells (RBCs). RBCs carry oxygen from the lungs to tissues throughout the body. All cells require oxygen to function.
Red blood cells originate in bone marrow as erythroblasts (a "blast" is a primitive cell that develops into a mature cell). Hemoglobin (Hb), a protein that binds to oxygen, is the main component of red blood cells. Once RBCs become filled with hemoglobin they enter the bloodstream as erythrocytes. Healthy hemoglobin holds the oxygen molecules with a precise degree of force. If it binds oxygen molecules in the lungs too loosely, it cannot hold onto them and carry them away. If it binds them too tightly, it cannot release them to tissues.
Red blood cell production is stimulated by the hormone erythropoietin (EPO), which is produced in the kidneys. If the kidneys fail to produce adequate EPO, anemia develops.
Hospitals use blood supplied by blood banks (companies that collect, prepare, and store blood for medical and emergency uses). Blood banks type blood and test the compatibility of donor and recipient blood before transfusion (called cross-matching). Blood types are A, B, AB, and O. Whether the type is positive or negative depends on whether the Rh factor is present on the person's red blood cells.
All types can receive O negative blood, but may not be compatible with other types:
- Recipients with A+ blood type can receive A+, A-, O+ and O- blood types
- Recipients with B+ blood type can receive B+, B-, O+ and O- blood types
- Recipients with AB+ blood type can receive AB+, AB-, O+ and O- blood types.
- Recipients with O+ blood type can receive O+ and O- blood types
- Recipients with A- blood type can receive A- and O- blood types
- Recipients with B- blood type can receive B- and O- blood types
- Recipients with AB- blood type can receive AB- and O- blood types.
- Recipients with O- blood type can receive O- blood type.
Blood products commonly transfused in intensive care units (ICUs) include
- red blood cells (RBCs) – contain hemoglobin, which carries oxygen to all tissues;
- plasma – straw-colored fluid that carries the blood cells, enzymes, and hormones throughout the body; and
- platelets – cell-like bodies that control bleeding.
Blood banks also test blood for anemia and pathogens (disease-causing bacteria and viruses), including hepatitis viruses B and C, human immunodeficiency virus (HIV), and Treponema pallidum (bacterium that causes syphilis).
Despite the many regulations in place to assure the safety of blood supplies, transfusions are not risk free. Possible complications of blood transfusions include
- allergic reaction (caused by an allergen in the donor blood) and
- hemolytic transfusion reaction (caused by incompatible blood).
Managing patients in ICU requires strategies to minimize blood loss and increase production of blood in bone marrow. Limiting laboratory testing and phlebotomy (drawing blood) are important components of blood management.
Other Treatment for Anemia
Injectable EPO (e.g., PROCRIT®, EPOGEN®) is an alternative to blood transfusion to treat critically ill patients with anemia. Exogenous EPO is identical to the natural hormone in its role of stimulating the bone marrow to produce red blood cells. EPO has been used safely in many clinical settings, including chronic renal failure, oncology, and surgery. In the ICU, use of EPO has been shown to reduce the amount of blood transfused by almost 50%, at the same time significantly increasing hemoglobin levels.