U01.16.015 Cyanide vs. Carbon Monoxide Poisoning

Cyanide and carbon monoxide are both potent toxins that inhibit aerobic metabolism by targeting complex IV (cytochrome c oxidase) in the electron transport chain. This leads to hypoxia that cannot be fully corrected with supplemental oxygen, forcing the body into anaerobic metabolism.

Clinical Presentation

Both toxins can cause similar symptoms, including seizures, coma, and pink or cherry-red skin, typically noted as postmortem findings.

Sources of Exposure

Toxin	Source
Cyanide	Odorless gas is produced by fires, car exhaust, or gas heaters.
Carbon Monoxide	Odorless gas produced by fires, car exhaust, or gas heaters.

Sources of Exposure

Treatment Approaches

Toxin	Treatment
Cyanide	– Hydroxocobalamin: Binds cyanide to form cyanocobalamin, which is excreted by the kidneys. – Nitrites: Oxidize hemoglobin to methemoglobin, which binds cyanide to form cyanomethemoglobin, reducing toxicity. – Sodium thiosulfate: Converts cyanide to thiocyanate, which is excreted renally.
Carbon Monoxide	– 100% Oxygen – Hyperbaric Oxygen: Increases the displacement of carbon monoxide from hemoglobin.

Treatment Approaches

Signs and Symptoms

Toxin	Signs and Symptoms
Cyanide	– Bitter almond odor on breath – Cardiovascular collapse
Carbon Monoxide	– Headache, dizziness – Multiple individuals may be affected simultaneously (e.g., families with similar symptoms in winter) – Bilateral globus pallidus lesions on MRI (classically, but rarely, seen with cyanide toxicity)

Signs and Symptoms

Effect on Oxygen-Hemoglobin Dissociation Curve

Toxin	Effect on Dissociation Curve
Cyanide	– The curve remains normal; oxygen saturation may initially appear normal.
Carbon Monoxide	– Leftward shift in the curve, increasing hemoglobin’s affinity for oxygen and reducing oxygen unloading in tissues. – Binds competitively to hemoglobin with 200 times greater affinity than oxygen, forming carboxyhemoglobin and reducing overall oxygen saturation.

Effect on Oxygen-Hemoglobin Dissociation Curve

Points to Remember

Both cyanide and carbon monoxide inhibit aerobic metabolism by interfering with complex IV, leading to hypoxia.
Cyanide poisoning may present with a bitter almond odor on the breath, while carbon monoxide poisoning often affects multiple people in enclosed spaces.
Treatment for cyanide poisoning includes hydroxocobalamin, nitrites, and sodium thiosulfate. Carbon monoxide poisoning is treated with 100% oxygen or hyperbaric oxygen.
Carbon monoxide causes a leftward shift in the oxygen-hemoglobin dissociation curve, whereas cyanide does not alter the curve significantly.

Bibliography

National Institutes of Health (NIH). Cyanide Poisoning. Retrieved from NIH.gov
Centers for Disease Control and Prevention (CDC). Carbon Monoxide Poisoning. Retrieved from CDC.gov
World Health Organization (WHO). Clinical Management of Chemical Exposure. Retrieved from WHO.int

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USMLE Step 1 Review Series

Curriculum

U01.16.015 Cyanide vs. Carbon Monoxide Poisoning

Clinical Presentation

Sources of Exposure

Treatment Approaches

Signs and Symptoms

Effect on Oxygen-Hemoglobin Dissociation Curve

Points to Remember

Bibliography

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