Anemia is a common medical problem, affecting approximately one quarter of the world’s population, especially children, pregnant and premenopausal women, the elderly, and the chronically ill.1-4 Anemia is not so much a disease as a sign or symptom. There are three broad causes of anemia: (1) blood loss, (2) decreased red blood cell production, and (3) increased red blood cell (RBC) destruction.
Anemia is a reduced concentration of RBCs. In healthy persons, normal erythropoiesis ensures that the concentration of RBCs present is adequate to meet the body’s demand for oxygen and that the destruction of RBCs balances the production. The average life of the circulating erythrocyte is approximately 110 to 120 days. Any process or condition that results in the loss of RBCs, that impairs RBC production, or that increases RBC destruction will result in anemia if the bone marrow cannot produce new cells to replace those lost (Table 231-1).
TABLE 231-1Classification of Anemia |Favorite Table|Download (.pdf) TABLE 231-1 Classification of Anemia
|Mechanism ||Example |
|Loss of red blood cells by hemorrhage ||Acute GI bleeding |
|Increased destruction ||Sickle cell disease |
| ||Drug-induced autoimmune hemolytic anemia |
|Impaired production ||Nutritional deficiency anemia (iron, folate) |
| ||Aplastic or myelodysplastic anemia |
|Dilutional ||Rapid IV crystalloid infusion |
Quantification of the erythrocyte concentration is reflected in (1) RBC count per microliter, (2) hemoglobin concentration, and (3) hematocrit (percentage of RBC mass to blood volume). Reference RBC values for adults vary between genders, with smaller variations for ethnicity and age (Table 231-2).
TABLE 231-2Reference Range Red Blood Cell Values for Adults |Favorite Table|Download (.pdf) TABLE 231-2 Reference Range Red Blood Cell Values for Adults
| ||Male ||Female |
|Red blood cell count (million/mm3) ||4.5–6.0 ||4.0–5.5 |
|Hemoglobin (grams/dL) || |
|Hematocrit or packed cell volume (%) ||42–52 ||36–48 |
|Mean corpuscular volume: MCV (fL) ||78–100 ||78–102 |
|Mean corpuscular hemoglobin: MCH (picograms/cell) ||25–35 ||25–35 |
|Mean corpuscular hemoglobin concentration: MCHC (grams/dL) ||32–36 (320-360 g/L) ||32–36 (320-360 g/L) |
|Red cell distribution width: RDW (%) ||11.5–14.5 ||11.5–14.5 |
|Reticulocytes (%) ||0.5–2.5 ||0.5–2.5 |
Anemia reduces the oxygen-carrying capacity of a given volume of blood. The body responds to anemia in ways to minimize the effect of this reduction in oxygen-carrying capacity. Mechanisms vary depending on the rapidity of onset, the degree of anemia, and the underlying condition of the patient. The cardiovascular response typically manifests with vasoconstriction in the peripheral vasculature and vasodilation in the central vasculature to help preserve blood flow to vital organs. As anemia worsens, systemic small-vessel vasodilation will occur, increasing blood flow to tissues. These processes result in decreased systemic vascular resistance, increased cardiac output, and ...