Anemia is a common medical problem worldwide, affecting approximately one quarter of the world's population, especially children, pregnant and premenopausal women, the elderly, and the chronically ill.1,2,3,4,5,6,7 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 destruction.
Anemia is a reduced concentration of red blood cells (RBCs) from the normal ranges based on age, gender, and race.8 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 body cannot produce enough new cells to replace those lost (Table 231-1). It is not uncommon for more than one mechanism to produce anemia in the same individual.
TABLE 231-1Classification of Anemia ||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). Normal RBC values for adults vary between genders, with small variations for ethnicity and age (Table 231-2).
TABLE 231-2Normal Red Blood Cell Values for Adults ||Download (.pdf) TABLE 231-2 Normal 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) ||14–17 ||12–15 |
|Hematocrit (%) ||42–52 ||36–48 |
|Mean corpuscular volume (fL) ||78–100 ||78–102 |
|Mean corpuscular hemoglobin (picograms/cell) ||25–35 ||25–35 |
|Mean corpuscular hemoglobin concentration (grams/dL) ||32–36 ||32–36 |
|Red cell distribution width (%) ||11.5–14.5 ||11.5–14.5 |
|Reticulocytes (%) ||0.5–2.5 ||0.5–2.5 |
The body responds to anemia in several ways in order to blunt the effect of a reduction in the oxygen-carrying capacity. The mechanisms vary, depending on the rapidity of onset, the degree of anemia, and the underlying condition of the patient. In acute forms of anemia that result from intravascular volume loss, the peripheral vasculature compensates by vasoconstriction, while the central vasculature vasodilates to help preserve blood flow to vital organs.9 As the condition worsens, systemic small-vessel vasodilation ...