B Anemia
Definition
Anemia is a deficiency of erythrocytes caused by either too rapid a loss or too slow production of the cells. Therefore, numeric concentrations of hemoglobin are reduced, and the oxygen-carrying capacity of blood is decreased. This results in reduced oxygen delivery to peripheral tissues.
Pathophysiology
Anemia may result from acute blood loss; however, iron-deficiency anemia from persistent blood loss is the most frequent form of chronic anemia. Anemia is also associated with many chronic diseases, such as persistent infections, neoplastic processes, connective tissue disorders, and renal and hepatic disease. Other forms of anemia include aplastic anemia, which involves bone marrow depression, and megaloblastic anemias, which are related to deficiencies of vitamin B12 or folic acid. Finally, various anemias can result from intravascular hemolysis of erythrocytes.
Laboratory results
Erythrocyte production can be assessed from the reticulocyte count in peripheral blood. For instance, a low reticulocyte count in the presence of a low hematocrit suggests an erythrocyte production defect, rather than blood loss or hemolysis, as a cause of anemia. A decrease in hematocrit that exceeds 1% per day is most likely related to acute blood loss or intravascular hemolysis.
Clinical manifestations
A history of reduced exercise tolerance characterized as exertional dyspnea is a frequent clinical sign of chronic anemia. A functional heart murmur and evidence of cardiomegaly may be detected on physical examination. The decreased oxygen-carrying capacity of arterial blood is reflected in the arterial oxygen content equation (i.e., arterial oxygen content = [hemoglobin × 1.39] × oxygen saturation + [arterial oxygen tension × 0.003]). This decrease is compensated for by a rightward shift oxyhemoglobin dissociation curve and an increase in cardiac output. Decreased blood viscosity and vasodilation lower systemic vascular resistance and increase blood flow. Blood pressure and heart rate remain unchanged with increased cardiac output. The decreased exercise tolerance reflects the inability of cardiac output to increase to maintain tissue oxygenation when these patients become physically active.
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