Red Blood Cell Abnormalities
RBC abnormalities include either a decrease in RBC mass or the
presence of abnormally functioning RBCs. RBC mass is reduced by either decreased
production or increased loss. Decreased RBC production is associated with factor
deficiencies (iron, folate, vitamin B12
), bone marrow disease (aplastic
anemia, leukemia), chronic diseases (infection, neoplasm, renal disease), and rarely,
a congenital deficiency of RBC precursors (Diamond-Blackfan syndrome).[314]
RBC loss is either extracorporeal blood loss (hemorrhage) or cell destruction (hemolysis).
Hemolysis can be caused by intrinsic RBC abnormalities or by extrinsic factors.
Intrinsic anomalies include membrane defects (spherocytosis, elliptocytosis), enzyme
defects (glucose-6-phosphate dehydrogenase, pyruvate kinase deficiencies), and hemoglobinopathies
(hemoglobin S, C, D, E and thalassemia). Extrinsic abnormalities can be immune mediated
or infection and toxin mediated, or they may occur secondary to microangiopathic
destruction.[315]
Regardless of the cause of the decrease in RBC mass, the signs,
symptoms, and therapy depend on the acuity and severity of the blood loss. Acute
loss of massive amounts of blood (hemorrhage) can cause hypovolemic shock. Treatment
consists of oxygen, emergency
intravascular volume re-expansion, and ultimately, replacement of RBCs. In the absence
of hypovolemia, acute and chronic RBC loss is tolerated until the loss of oxygen-carrying
capacity is critical, which results in high-output cardiac failure. Treatment of
these severe anemias includes oxygen and replacement of the RBC mass, which should
be done cautiously and slowly in an attempt to avoid the complications of fluid overload.
Partial exchange transfusions are sometimes necessary to deliver an adequate RBC
load safely.
The main functional abnormality of the RBC is an inability to
carry oxygen; sulfhemoglobinemia and methemoglobinemia are the most common of these
abnormalities. Methemoglobinemia and sulfhemoglobinemia can be either congenital
or acquired disorders that produce hemoglobin that cannot bind oxygen. The congenital
forms are usually asymptomatic and require no therapy, but the acquired or toxic
forms may be fatal. Treatment includes oxygen, intravenous administration of methylene
blue, and occasionally, exchange transfusion.[316]
