A 47-year-old man comes to the office due to occasional daytime headaches, dizziness, and nausea. He has not had any chest pain, shortness of breath, or cough. He gets restful sleep and does not snore. The patient has no significant past medical history and takes no medication. He works as a traffic controller in an underground parking garage. He has smoked 2 packs of cigarettes daily for 25 years. Blood pressure is 140/90 mm Hg and pulse is 84/min. Oxygen saturation is 97% on room air. BMI is 24 kg/m². There is no jugular venous distension or lymphadenopathy. Pulmonary examination is unremarkable. A grade 2/6 apical holosystolic murmur is present. The abdomen is soft and nontender. The liver span is 8 cm, and the spleen is not palpable. The extremities have no cyanosis, clubbing, or edema. Neurologic examination reveals no abnormalities. Laboratory results are as follows: Which of the following is most likely responsible for this patient's current condition?

Question

Quizplus · Accepted Answer

11ec25c8_d428_e754_bc81_092da819ab95_MD0003_00 This patient, who is exposed to automobile exhaust in an enclosed space, has intermittent headaches, dizziness, nausea, and polycythemia (elevated hematocrit), most likely due to chronic carbon monoxide (CO) poisoning. CO is a byproduct of combusting organic matter (eg, oil, gas, wood). Exposure to toxic levels is more likely in enclosed or poorly ventilated areas. CO tightly binds hemoglobin - forming carboxyhemoglobin - with an affinity much greater than that of oxygen. Nonsmokers have low levels (<3%) of carboxyhemoglobin (due to normal enzymatic reactions). Cigarette smokers may have carboxyhemoglobin levels as high as 10%. Even though patients at this level are generally asymptomatic, small additional amounts of CO exposure may cause toxicity manifesting as headache, malaise, and nausea. Carboxyhemoglobin shifts the oxygen dissociation curve to the left, impairing the ability of heme to unload oxygen at the tissue level. This results in tissue hypoxia. The kidney responds to tissue hypoxia by producing more erythropoietin (EPO). EPO stimulates the bone marrow to differentiate more red blood cells. Chronic CO toxicity is a cause of secondary polycythemia. Pulse oximetry does not differentiate between carboxyhemoglobin and oxyhemoglobin; it cannot be used in the diagnosis of CO poisoning. Diagnosis is made by arterial blood gas with cooximetry. (Choice B) Lung cancer often presents with chest pain, cough, dyspnea, or hemoptysis. Erythrocytosis (increased red blood cell mass)/polycythemia may develop due to EPO-producing tumors (eg, renal cell carcinoma) or as part of a paraneoplastic syndrome, but this patient's intermittent symptoms and employment history make CO poisoning much more likely. (Choice C) Nocturnal hypoventilation may cause erythrocytosis from tissue hypoxia. However, this patient with a normal BMI has restful sleep and does not snore, making many causes of nocturnal hypoventilation (eg, obstructive sleep apnea, obesity hypoventilation) unlikely. (Choice D) Polycythemia vera is a clonal myeloproliferative disorder that causes erythrocytosis. Patients are often asymptomatic, but signs of increased blood viscosity (eg, transient neurologic symptoms, thrombosis), aquagenic pruritus, gout, and increases in all 3 cell lines are common. (Choice E) If severe, right-to-left shunting can cause chronic systemic hypoxemia that may result in an EPO-mediated erythrocytosis. This patient's normal pulse oximetry reading makes a right-to-left shunt unlikely. His cardiac auscultation findings suggest mitral valve regurgitation; most patients with this condition are asymptomatic. Educational objective: Carbon monoxide poisoning should be considered in patients with environmental risk factors who present with headache, nausea, and dizziness.

A 47-Year-Old Man Comes to the Office Due to Occasional