A 55-year-old man comes to the office for an annual visit. Medical history includes hypertension and hypercholesterolemia, for which he takes amlodipine and atorvastatin. He is adherent to his medication regimen. The patient feels well in general, but over the past few months, he has had difficulty achieving and maintaining an erection, despite having a normal libido. He drinks 2 or 3 glasses of wine daily and does not smoke. He works for a lawn care company, frequently driving the grass cutter. Blood pressure is 160/100 mm Hg and pulse is 80/min. BMI is 45 kg/m². Cardiopulmonary examination is normal. The abdomen is soft and nontender. The liver and spleen are not palpable. Laboratory results are as follows: Which of the following is most likely responsible for this patient's increased hematocrit?

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11ec25c8_d2ff_0d56_bc81_899918b51a90_MD0003_00 This patient's presentation is most consistent with obstructive sleep apnea (OSA). Obesity (particularly BMI >35 kg/m²) is the strongest risk factor for OSA. Not all patients have obvious respiratory symptoms (eg, snoring, gasping for air); the sequelae of OSA (eg, erectile dysfunction, arterial hypertension) may be what prompts the initial evaluation. ² In patients with OSA, recurrent collapse of the pharynx during sleep results in transient airway obstruction. This causes short periods (20-40 seconds) of apnea or hypopnea, which reduce blood oxygen levels. The kidneys respond to the hypoxemia by increasing erythropoietin, which stimulates the bone marrow to differentiate more red blood cells (RBCs). Therefore, it is quite common for patients with OSA to have elevated hematocrit levels (polycythemia). The terms polycythemia and erythrocytosis are often used interchangeably. Polycythemia is a laboratory finding of elevated RBC count and hematocrit. Relative polycythemia is generally due to reduced plasma volume, whereas absolute polycythemia (ie, erythrocytosis) is due to increased RBC mass and can be primary (eg, polycythemia vera) or secondary (eg, due to chronic hypoxia or erythropoietin-producing tumors). (Choice A) Carbon monoxide (CO) poisoning increases carboxyhemoglobin levels, shifting the oxygen dissociation curve to the left and resulting in tissue hypoxia. The kidneys respond by increasing erythropoietin, which may cause erythrocytosis. However, patients with CO poisoning typically have dizziness, headaches, nausea, and confusion. This patient has obesity, hypertension, and erectile dysfunction, making OSA far more likely. (Choice B) The clonal proliferation of myeloid cells can cause erythrocytosis in polycythemia vera. Patients are often asymptomatic, but signs of increased blood viscosity (eg, transient neurologic symptoms, thrombosis), aquagenic pruritus, and gout are common. (Choice D) Significant plasma volume contraction causes relative polycythemia because RBCs are concentrated due to lower intravascular volume. This patient's normal blood urea nitrogen and creatinine levels make significant volume contraction very unlikely. (Choice E) Testosterone deficiency can result in erectile dysfunction. However, erythrocytosis can be caused by testosterone excess (eg, androgen use) rather than testosterone deficiency. OSA is an independent risk factor for erectile dysfunction, and treatment with continuous positive airway pressure may improve sexual function. Notably, testosterone supplementation (even to normal levels) can worsen OSA. Educational objective: Obstructive sleep apnea (OSA) can present without symptoms of snoring or witnessed apneic events. OSA causes transient periods of hypoxemia. The kidneys respond by increasing erythropoietin, which can result in erythrocytosis. References: The effect of CPAP and PDE5i on erectile function in men with obstructive sleep apnea and erectile dysfunction: a systematic review and meta-analysis. (http://www.ncbi.nlm.nih.gov/pubmed/31715462)

A 55-Year-Old Man Comes to the Office for an Annual