Toxicology & Substance Use · Topic 10 of 17

Carbon Monoxide & Cyanide Poisoning

Toxicology · asphyxiants and smoke inhalation

Carbon Monoxide & Cyanide Poisoning

Give 100% oxygen and measure carboxyhemoglobin by CO-oximetry. A fire victim with lactate ≥ 10 mmol/L gets hydroxocobalamin 5 g IV for cyanide.

Key numbers

100% oxygen by nonrebreather nowt½ 4.5 h → 1.5 h on O₂

Consider hyperbaric oxygen if COHb > 25%COHb > 25%

Hydroxocobalamin 5 g IV over 15 minutes5 g IV over 15 min

Fire victim with lactate ≥ 10: treat cyanidelactate ≥ 10 mmol/L

01 · Do100% oxygen by nonrebreather now

Remove the patient from the source and give 100% oxygen by nonrebreather: COHb half-life falls from about 4.5 hours on room air to 1.5 hours, and to about 20 minutes with hyperbaric oxygen.

02 · Defined byConsider hyperbaric oxygen if COHb > 25%

Consider hyperbaric oxygen, with poison center or hyperbaric input, for any loss of consciousness, altered mental status, ongoing chest pain or cardiopulmonary complications, COHb > 25%, or pregnancy at lower levels.

03 · GiveHydroxocobalamin 5 g IV over 15 minutes

Treat known or suspected cyanide poisoning with hydroxocobalamin 5 g IV over 15 minutes; a second 5 g (total 10 g) may follow, depending on severity and response.

Presentation

What you'll see

Fire victim with lactate ≥ 10: treat cyanide must knowSuspect cyanide in smoke inhalation from an enclosed-space fire with soot in the airway and altered mental status; hypotension or lactate ≥ 10 mmol/L strongly suggests it. Treat empirically without waiting for a cyanide level.

CO symptoms track carboxyhemoglobin levelHeadache and nausea start at COHb 10–20%; > 20% causes dizziness, weakness and poor judgment; > 30% exertional dyspnea, chest pain and confusion; then syncope, seizures, coma and death, usually > 60%.

Flu-like illness in one household: think COCO poisoning is easily missed: suspect it when several people from the same heated dwelling have flu-like symptoms, or with unexplained metabolic acidosis.

Cyanide: sudden coma, seizures and collapseCyanide causes headache, confusion, tachypnea, early hypertension then hypotension, seizures, coma and cardiovascular collapse with severe lactic acidosis — it can kill within minutes.

Diagnosis & monitoring

Work-up

ECG and troponin in moderate–severe COIn moderate to severe CO poisoning obtain an ECG and cardiac biomarkers: acute myocardial injury predicts poor outcome.

Draw labs before hydroxocobalaminHydroxocobalamin's deep red color interferes with colorimetric labs and CO-oximetry and can trip a hemodialysis blood-leak alarm; draw COHb and a pretreatment blood sample first, without delaying the antidote.

Do right now

In this order

First moves

100% oxygen by nonrebreather nowt½ 4.5 h → 1.5 h on O₂

→

Measure COHb by CO-oximetry; SpO₂ is falsely normal

→

Fire victim with lactate ≥ 10: treat cyanidelactate ≥ 10 mmol/L

→

Hydroxocobalamin 5 g IV over 15 minutes5 g IV over 15 min

→

Consider hyperbaric oxygen if COHb > 25%COHb > 25%

Why each step

100% oxygen by nonrebreather nowRemove the patient from the source and give 100% oxygen by nonrebreather: COHb half-life falls from about 4.5 hours on room air to 1.5 hours, and to about 20 minutes with hyperbaric oxygen.

Measure COHb by CO-oximetry; SpO₂ is falsely normalDiagnose CO poisoning with a carboxyhemoglobin level by CO-oximetry (venous is fine). Pulse oximetry and ABG-calculated saturation read falsely normal, and pulse CO-oximetry should not be used for diagnosis.

Fire victim with lactate ≥ 10: treat cyanideSuspect cyanide in smoke inhalation from an enclosed-space fire with soot in the airway and altered mental status; hypotension or lactate ≥ 10 mmol/L strongly suggests it. Treat empirically without waiting for a cyanide level.

Hydroxocobalamin 5 g IV over 15 minutesTreat known or suspected cyanide poisoning with hydroxocobalamin 5 g IV over 15 minutes; a second 5 g (total 10 g) may follow, depending on severity and response.

Consider hyperbaric oxygen if COHb > 25%Consider hyperbaric oxygen, with poison center or hyperbaric input, for any loss of consciousness, altered mental status, ongoing chest pain or cardiopulmonary complications, COHb > 25%, or pregnancy at lower levels.

Red flags & don'ts

Avoid harm

Delayed neuropsychiatric sequelae days to weeks later complicationAfter severe CO poisoning, delayed neuropsychiatric sequelae (dementia, psychosis, parkinsonism, amnesia) can appear days to weeks later and may be permanent.

Hydroxocobalamin: red skin and urine, hypertension, AKI complicationExpect red skin and chromaturia; blood pressure can rise substantially, and acute kidney injury with oxalate crystals occurs — monitor creatinine for 7 days.

The rules

Know these

A normal COHb does not exclude COCOHb falls quickly once exposure ends, especially after prehospital oxygen, so a low level does not exclude significant poisoning.

Carbon monoxide or cyanide?

Commonly confused

How it injures	Displaces O₂ from hemoglobin, left-shifts the curve, inhibits mitochondria
Key test	Carboxyhemoglobin by CO-oximetry
Treatment	100% oxygen; hyperbaric oxygen if severe
Speed	Symptoms track COHb; delayed neuropsychiatric sequelae

Commonly confused

Tell apart

Hydroxocobalamin, not nitrites, when CO coexistsHydroxocobalamin binds cyanide to form cyanocobalamin, excreted in urine, without causing methemoglobinemia — so it is preferred in fire victims who also have CO poisoning. Nitrites induce methemoglobin.

More detail

Reference

Show 5 reference facts

CO binds hemoglobin more avidly than oxygen, shifts the oxyhemoglobin curve left and inhibits mitochondrial respiration; cyanide binds cytochrome a3 of cytochrome c oxidase, halting aerobic metabolism.

CO sources: house fires, poorly vented vehicles, generators, furnaces, gas and kerosene heaters; methylene chloride (paint stripper) is metabolized to CO and causes delayed poisoning.

Cyanide sources besides smoke: potassium or sodium cyanide, bitter almond oil, and nitroprusside.

If hydroxocobalamin is unavailable, give sodium nitrite 300 mg IV (10 mL of 3%), then sodium thiosulfate 12.5 g IV (50 mL of 25%); nitrite-induced methemoglobin and hypotension make it riskier when CO coexists.

In a 2002 randomized trial, three hyperbaric sessions within 24 hours lowered cognitive sequelae at 6 weeks (25% vs 46%) versus normobaric oxygen.

Pearl

The pulse oximeter cannot see carbon monoxide — a normal SpO₂ never rules it out; send a co-oximetry COHb.

Sources · Merck Manual Professional — Carbon Monoxide Poisoning · ACEP Clinical Policy: acute carbon monoxide poisoning. Ann Emerg Med 2017;69:98–107 (recommendations as summarized by ACEP Now) · ACEP Clinical Policy (critical issue update): hyperbaric oxygen in adult ED patients with acute carbon monoxide poisoning. Ann Emerg Med 2025;85:e45–e59 (confirmed via JournalFeed summary, https://journalfeed.org/article-a-day/2025/new-acep-policy-hyperbaric-oxygen-for-co-poisoning/) · Cyanokit (hydroxocobalamin for injection) prescribing information — DailyMed · Merck Manual Professional — Symptoms and Treatment of Specific Poisons (cyanides) · Intravenous hydroxocobalamin for cyanide poisoning from smoke inhalation: a comprehensive scoping review. J Am Coll Emerg Physicians Open 2026 · Weaver LK et al. Hyperbaric oxygen for acute carbon monoxide poisoning. N Engl J Med 2002;347:1057–1067 (abstract confirmed via Europe PMC) · Lavonas EJ et al. 2023 AHA focused update on the management of patients with cardiac arrest or life-threatening toxicity due to poisoning. Circulation 2023;148:e149–e184 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).