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Carbon monoxide poisoning
Carbon monoxide binds haemoglobin with far higher affinity than oxygen and impairs oxygen delivery to the tissues; an ordinary pulse oximeter cannot detect it, the diagnosis is a carboxyhaemoglobin (COHb) level on co-oximetry, and treatment is 100% oxygen — and in severe cases hyperbaric oxygen.
Why it matters in the exam
The typical question describes several family members with headache, nausea and dizziness in winter, near a stove, generator or faulty water heater, or a patient rescued from a fire. The standard trap is a normal pulse-oximetry saturation — the device reads COHb as if it were oxygenated haemoglobin, so a normal saturation does not exclude poisoning.
The indications for hyperbaric oxygen are also tested: loss of consciousness, seizures or a neurological deficit, COHb above 25%, above 15% in pregnancy, cardiac ischaemia or severe acidosis. Pregnancy matters especially because fetal haemoglobin binds carbon monoxide more tightly, and the fetus is harmed even when the mother looks well.
Remember the delayed damage as well: some patients develop, days to weeks later, delayed neurocognitive injury — memory loss, behavioural change and parkinsonism. The half-life of COHb is about 4–6 hours in room air, about an hour and a half on 100% oxygen and about 20–30 minutes in a hyperbaric chamber. In fire victims with a very high unexplained lactate, also suspect cyanide poisoning from burning synthetic materials and give hydroxocobalamin.
Key points
- Suspect from the context: winter, stove or generator, several victims, fire.
- Diagnosis by co-oximetry; an ordinary pulse oximeter misleads.
- 100% oxygen at once greatly shortens the half-life.
- Hyperbaric treatment: loss of consciousness, neurological deficit, COHb above 25%, pregnancy above 15%, ischaemia.
- Remember delayed neurological damage and combined cyanide in fire victims.
Summary table
| COHb level | Typical symptoms | Treatment | Note |
|---|---|---|---|
| Up to 3% (smokers up to 10%) | None | Not needed | Baseline value |
| 10–25% | Headache, nausea, dizziness, fatigue | 100% oxygen by mask | Flu-like illness in several household members |
| Above 25% | Confusion, syncope, chest pain | 100% oxygen; consider hyperbaric treatment | ECG and troponin |
| Any level with loss of consciousness or a neurological deficit | Seizures, coma | Hyperbaric treatment | The level correlates poorly with severity |
| Pregnancy, above 15% | Sometimes few symptoms | Hyperbaric treatment | The fetus is more sensitive |
A free exam question
שימוש בביו- דיזל גורם לשינוי בהרכב הפליטות ממנוע דיזל. איזה מהבאים מתאר נכונה את השינוי? ( PM – particulate matter, CO – carbon monoxide, NOx – nitrous oxides ) (the question as examined, in Hebrew)
- א. עליה ב- PM , ירידה ב- CO , ירידה ב- Hydrocarbons , ירידה ב- NOx
- ב. ירידה ב- PM , ירידה ב- CO , עליה ב- Hydrocarbons , עליה ב- NOx
- ג. עליה ב- PM , עליה ב- CO , ירידה ב- Hydrocarbons , ירידה ב- NOx
- ד. ירידה ב- PM , ירידה ב- CO , ירידה ב- Hydrocarbons , עליה ב- NOx
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Sources
- ACEP Clinical Policy: critical issues in the evaluation and management of adult patients presenting with suspected acute carbon monoxide poisoning, 2017 — Hyperbaric oxygen and cardiac testing
- Undersea and Hyperbaric Medical Society, Hyperbaric Oxygen Therapy Indications, 15th edition (2023) — Carbon monoxide poisoning
- Goldfrank's Toxicologic Emergencies, 11th edition (2019) — Carbon monoxide
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.