Emergency Medicine · Environmental Emergencies
The facts most likely to be tested
Carbon monoxide binds to hemoglobin with an affinity 200-250 times greater than oxygen, causing a leftward shift of the oxyhemoglobin dissociation curve.
Pulse oximetry is falsely normal because standard devices cannot distinguish between oxyhemoglobin and carboxyhemoglobin.
The gold standard for diagnosis is an arterial blood gas (ABG) with co-oximetry to measure the carboxyhemoglobin level directly.
Patients classically present with headache, nausea, dizziness, and altered mental status in the setting of faulty heating equipment or enclosed space fires.
The classic physical exam finding of cherry-red skin is rarely seen and is considered a post-mortem finding.
Initial management requires immediate removal from the source and administration of 100% high-flow oxygen via a non-rebreather mask.
Hyperbaric oxygen therapy is indicated for patients with syncope, seizures, coma, myocardial ischemia, or pregnancy.
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A 34-year-old male is brought to the emergency department by his spouse after being found unconscious in their garage. The patient was working on his car with the engine running for several hours. On arrival, he is confused and complaining of a throbbing headache and nausea. His physical exam reveals tachycardia and tachypnea, but his pulse oximetry reads 99% on room air. His skin is warm and dry with no signs of trauma.
What is the most appropriate next step in management?
Administration of 100% high-flow oxygen via a non-rebreather mask.
The patient's presentation of altered mental status and headache in an enclosed space with a running engine is classic for CO poisoning; the falsely normal pulse oximetry is a hallmark, necessitating immediate high-flow oxygen therapy.
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Etiology / Epidemiology
Caused by incomplete combustion of carbon-based fuels; suspect in patients with faulty furnaces or enclosed space fires.
Clinical Manifestations
Classic cherry-red skin is rare; suspect in patients with unexplained headache, nausea, and altered mental status.
Diagnosis
Standard pulse oximetry is unreliable; order arterial blood gas with co-oximetry to measure carboxyhemoglobin (COHb) levels.
Treatment
Administer 100% high-flow oxygen via non-rebreather; consider hyperbaric oxygen therapy for severe cases.
Prognosis
Delayed neuropsychiatric sequelae can occur weeks later; monitor for cognitive impairment and personality changes.
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Epidemiology & Etiology
Common in winter months due to faulty heating systems and indoor use of generators. Often presents as multiple family members with similar flu-like symptoms. Always consider in cases of suicide attempts via motor vehicle exhaust.
Pertinent Anatomy
Carbon monoxide binds to hemoglobin with 200-250 times the affinity of oxygen. This creates a left-shift in the oxyhemoglobin dissociation curve, preventing oxygen release to peripheral tissues.
Pathophysiology
CO binds to hemoglobin forming carboxyhemoglobin, reducing oxygen-carrying capacity. It also binds to cytochrome c oxidase, disrupting cellular respiration and causing mitochondrial toxicity. The resulting tissue hypoxia disproportionately affects high-metabolic organs like the brain and heart.
Clinical Manifestations
Early symptoms include headache, dizziness, and malaise, often misdiagnosed as a viral syndrome. Altered mental status, seizures, and myocardial ischemia indicate severe poisoning. The cherry-red skin is a rare, late-stage finding; do not wait for it to diagnose.
Diagnosis
Standard pulse oximetry cannot distinguish between oxyhemoglobin and carboxyhemoglobin, leading to a falsely normal SpO2. The gold standard is co-oximetry via ABG. COHb levels >3% in nonsmokers or >10-15% in smokers are diagnostic.
Treatment
Immediate removal from the source is mandatory. 100% high-flow oxygen reduces the half-life of COHb from 320 minutes to ~80 minutes. Hyperbaric oxygen therapy is indicated for COHb >25%, pregnancy, or end-organ damage (e.g., coma, ischemia).
Prognosis
Patients may develop delayed neuropsychiatric sequelae (DNS) including memory loss and parkinsonism weeks after recovery. Close follow-up is required to monitor for cognitive decline.
Differential Diagnosis
Methemoglobinemia: presents with chocolate-colored blood and cyanosis
Cyanide poisoning: associated with bitter almond odor and lactic acidosis
Diabetic ketoacidosis: presents with Kussmaul breathing and hyperglycemia
Meningitis: presents with nuchal rigidity and fever
Food poisoning: usually involves gastrointestinal symptoms without neurological deficits