A 35-year-old man is brought to the emergency department after collapsing in his garage. He recently bought a classic 1960s convertible and was repairing it when he collapsed. The patient was inside the garage but had the door half-open. He lost consciousness after working on the car for 2 hours with the engine running. The patient has no known medical problems and takes no medications. He does not use tobacco, alcohol, or illicit drugs. Which of the following best reflects the combination of findings that would have been expected in an arterial blood sample taken when the patient lost consciousness?

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Carbon monoxide (CO) is a colorless, odorless gas product of the combustion of carbon-containing compounds. CO poisoning can occur after exposure to automobile exhaust (especially in older cars without catalytic converters), fire smoke, or improperly vented natural gas appliances. CO has 2 major effects on oxygen delivery to tissues: CO's effects on arterial blood oxygen content are seen in the graphs below, which show the lower oxygen content with CO poisoning (similar to anemia) and the leftward shift of the dissociation curve that becomes increasingly more pronounced at increasingly higher HBCO concentrations (this does not occur with anemia). 11ec160b_3cc9_4ff1_9249_7d3154be24a5_MD0001_00 (Choices A, B, C, and E) CO poisoning does not affect PaO₂ and it does not cause methemoglobinemia. Methemoglobin is formed when the Fe²⁺ (ferrous iron) in heme is oxidized to Fe³⁺ (ferric iron). Methemoglobinemia results from drug exposures (ie, dapsone, nitrites) as well as enzyme deficiencies and hemoglobinopathies. Educational objective: Carbon monoxide (CO) binds to hemoglobin with much higher affinity than oxygen, thereby preventing oxygen binding to hemoglobin. It also reduces oxygen unloading from hemoglobin in the tissues. CO poisoning increases carboxyhemoglobin concentrations but does not affect the partial pressure of oxygen and does not precipitate methemoglobinemia.

A 35-Year-Old Man Is Brought to the Emergency Department After