A 68-year-old widow is seen by her physician because of complaints of fatigue and mild memory loss. The patient does not abuse alcohol and has not had a history of surgery in the last 5 years. Blood gases (Sao2, Po2, Pco2, and pH) are normal. Blood analysis shows a white cell count of 5,200 cells/mm3; Hb, 9.0 gm/dL; and a hematocrit of 27%. Her serum vitamin B12 is low, but her serum folate, thyroxin-stimulating hormone (TSH), and liver enzymes are normal. Her peripheral blood smear is unremarkable.
1. Why are SaO2 and arterial Po2 normal in anemic patients who have hypoxemia?
2. How does anemia affect the oxygen diffusing capacity of the lungs?
3. Why might this patient be deficient in vitamin B12?
1. Hemoglobin increases the oxygen carrying capacity of the blood, but has no effect on arterial Po2. By way of illustration, if 100 mL of blood are exposed to room air, the Po2 in the blood will equal atmospheric Po2 after equilibration. Removing the red cells, leaving only plasma, will not affect Po2. An otherwise healthy patient with anemia will have a normal SaO2 because both O2 content and capacity are reduced proportionately. Hypoxemia in anemic patients is a result of low oxygen content, not a low Po2.
2. DLCO decreases with anemia because there is less hemoglobin available to bind CO.
3. There are several causes of vitamin B12 deficiency. In older individuals, especially those who live alone, insufficient dietary intake of animal protein may be the cause; other causes include loss of gastric mucosa or regional enteritis.
Wintrobe MM. Clinical Hematology. 9th Ed. Philadelphia: Lea &
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