A 34-year-old man was admitted to our emergency center because of coma. He had been exposed to vehicle exhaust. His limbs were cyanotic and oxyhemoglobin saturation measured by pulseoximeter (SpO₂) was 82%, although he was suspected of CO poisoning. Arterial blood gas analysis under 100% oxygen showed PaO₂ 348mmHg, PaCO₂ 32.2mmHg, pH 7.41, SaO₂ 98.2%, O₂Hb 42.4%, COHb 32.6% and MetHb 24.2%. In a case of CO poisoning with cyanosis by vehicle exhaust, we would expect methemoglobinemia to exist simultaneously with CO hemoglobinemia.

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