After studying this chapter you should:
Understand how anemia impacts oxygen transport and the ways in which anemic patients compensate for decreased oxygen-carrying capacity.
Develop a coherent approach to the clinical and laboratory evaluation of patients with anemia.
Understand and use classification of anemia based on production versus destruction and on red cell size (mean cell volume).
Be able to explain the phenomenon of ineffective erythropoiesis—its diagnosis and role in pathogenesis.
Understand the process by which red cells are broken down, the catabolism of heme in macrophages, and the transport of bilirubin in the plasma and its conjugation in the liver.
As stated in Chapter 1, the primary function of the red blood cell is to transport oxygen to respiring cells and tissues. Anemia is defined as a significant deficit in the mass of circulating red blood cells. As a result, the capacity of the blood to deliver oxygen is compromised. Clinicians document the presence of anemia by measurement of either the concentration of hemoglobin in the blood or the hematocrit, which is the ratio of the volume of red cells to the total volume of a blood sample. A patient is anemic if the hemoglobin or hematocrit value is more than two standard deviations below normal. As shown in Figure 3-1, the lower limits of normal vary with the age of the individual and, in adults, with gender. Occasionally, the documentation of anemia is confounded by a concurrent alteration in the plasma volume. For example, if a patient with a low mass of circulating red blood cells is also hypovolemic, owing to a concurrent loss of plasma volume from dehydration, the blood hemoglobin and hematocrit levels will be falsely elevated and may even be in the normal range. Another important example, discussed in more detail later in this chapter, is acute hemorrhage, in which there is concomitant loss of both red blood cells and plasma.
Lower limits of normal levels of hemoglobin and hematocrit at different ages. Mo, month; Yr, year; F, female; M, male.
OXYGEN TRANSPORT IN ANEMIA
Oxygen transport to a given region in the body, as expressed in the Fick equation, is a product of three independent variables: blood flow, hemoglobin concentration, and the fraction of hemoglobin that has unloaded oxygen during transit from artery to vein. In anemic patients, the oxygen-carrying capacity of the blood is, by definition, low. As shown in Figure 3-2 and explained below, the two other components in the Fick equation undergo compensatory changes.
The Fick equation applied to anemia. Hb, hemoglobin; Asat, arterial O2 saturation; Vsat, venous O2 saturation; DPG, 2,3-diphosphoglycerate.
*Response to erythropoietin is blunted in underproduction anemias