A 63-year-old woman is hospitalized because of dyspnea on exertion and generalized fatigue. She has been hospitalized 3 times during the past year for heart failure exacerbations. Her other medical problems include essential hypertension for 20 years, myocardial infarction at the age of 58, and hypercholesterolemia. She has a 36-pack-year smoking history in addition to a 10-year history of alcohol abuse. Physical examination reveals tachycardia, distended neck veins, bilateral crackles on lung auscultation, a third heart sound on cardiac auscultation, and pedal edema. The appropriate therapy is initiated. On the third day of hospitalization, her hematocrit level increases to 50%, up from 44% on admission. An arterial blood gas analysis shows an arterial O₂ partial pressure (PaO₂) of 70 mm Hg. A ⁵¹Cr-tagged red blood cell infusion indicates normal red blood cell mass. Which of the following is the most likely cause of this patient's polycythemia?

Question

Quizplus · Accepted Answer

Polycythemia (erythrocytosis) is defined as a hematocrit level > 52% in men and > 48% in women. It may be the result of a true increase in the red blood cell (RBC) mass (absolute erythrocytosis) or decrease in the plasma volume (relative erythrocytosis). 11ec160b_3ccb_e805_9249_114aa4af09d5_MD0001_00 In a patient with erythrocytosis, the following tests can help determine the cause: 1. Absolute versus relative erythrocytosis: Hemoglobin and hematocrit levels cannot accurately differentiate these conditions. Direct measurement of the RBC mass is necessary. An increased total RBC mass indicates an absolute erythrocytosis; a normal RBC mass indicates a relative erythrocytosis. This patient does not have an increased RBC mass and is likely experiencing relative erythrocytosis due to aggressive diuretic therapy for her heart failure exacerbation. 2. Primary versus secondary erythrocytosis: Serum erythropoietin levels can be used to differentiate primary from secondary erythrocytosis. Primary erythrocytosis is associated with low erythropoietin levels and is caused by myeloproliferative disorders such as polycythemia vera. Secondary erythrocytosis is characterized by increased erythropoietin levels due to chronic hypoxia from high altitudes, smoking, or chronic obstructive pulmonary disease; or abnormal secretion by neoplastic or otherwise diseased tissues. 3. Hypoxic versus other causes of secondary erythrocytosis: Measurement of the arterial oxygen saturation (SaO₂) is important to exclude hypoxemia as a cause of the erythrocytosis. SaO₂ < 92% (PaO₂ < 65 mm Hg) can cause secondary polycythemia. SaO₂ (measured as a percentage) should not be confused with PaO₂ (measured as a partial pressure in mm Hg). (Choice A) Polycythemia vera is a myeloproliferative disorder associated with marked erythrocytosis and more modest increases in granulocytes and platelets. A low erythropoietin level is characteristic, along with increased RBC mass. (Choice B) Hypoxia is a common cause of secondary erythrocytosis. SaO₂ < 92% (PaO₂ < 65 mm Hg) appears to be the threshold for the development of secondary erythrocytosis. This patient has hypoxemia above this threshold, an acute (< 3 day) increase in hematocrit, and normal RBC mass; none of these features are consistent with hypoxemia-induced erythrocytosis. (Choices C and D) Occasionally, renal disease or occult neoplasia can cause secondary erythrocytosis due to abnormal erythropoietin production by the diseased tissue. Work-up will reveal an increased RBC mass and an increased erythropoietin level. However, chronic renal disease typically leads to decreased erythropoietin production. Educational objective: Erythrocytosis is defined as a hematocrit level > 52% in men and > 48% in women. Measurement of red blood cell mass is necessary to distinguish absolute from relative erythrocytosis. A normal red blood cell mass indicates plasma volume contraction as the cause of polycythemia. __________ References: Investigation of patients with polycythaemia. (http://www.ncbi.nlm.nih.gov/pubmed/8949586) Oxygen desaturation in apparent and relative polycythaemia. (http://www.ncbi.nlm.nih.gov/pubmed/1998762)

A 63-Year-Old Woman Is Hospitalized Because of Dyspnea on Exertion