Overview
Carbon Monoxide Poisoning
Topic Overview
Summary
Carbon monoxide (CO) poisoning is caused by inhalation of CO, which binds haemoglobin with 200x greater affinity than oxygen, forming carboxyhaemoglobin (COHb) and causing tissue hypoxia. Sources include faulty gas appliances, fires, car exhaust, and enclosed spaces with combustion. Symptoms are non-specific (headache, nausea, confusion) and easily missed. Classic "cherry-red skin" is rare. Diagnosis is by COHb level on blood gas. Treatment is 100% oxygen; hyperbaric oxygen for severe cases.
Key Facts
- Mechanism: CO binds Hb → COHb → tissue hypoxia
- Sources: Faulty gas appliances, fires, car exhaust, BBQs indoors
- Symptoms: Headache, nausea, dizziness, confusion (non-specific)
- Diagnosis: COHb level on blood gas (over 3% non-smoker, over 10% smoker)
- Treatment: 100% high-flow oxygen; hyperbaric oxygen if severe
Clinical Pearls
"Flu-like symptoms in winter with faulty boiler" = think CO poisoning
Pulse oximetry may be NORMAL (reads COHb as OxyHb) — use co-oximetry
Multiple family members with headache at home = CO poisoning until proven otherwise
Why This Matters Clinically
CO poisoning is commonly missed because symptoms are non-specific. It causes significant morbidity (delayed neurological sequelae) and mortality. A high index of suspicion saves lives.
Visual Summary
Visual assets to be added:
- CO pathophysiology diagram
- COHb-oxygen dissociation curve shift
- CO poisoning signs and symptoms
- Management algorithm
Epidemiology
Incidence
- 4,000 ED attendances/year in UK
- 40-50 deaths/year in UK
- Under-diagnosed due to non-specific symptoms
Demographics
- All ages
- Peak in winter (heating appliances)
- Suicide (car exhaust — decreasing with catalytic converters)
Sources
| Source | Notes |
|---|---|
| Faulty gas appliances | Boilers, heaters — most common |
| Fires | House fires, smoke inhalation |
| Car exhaust | Enclosed garages |
| Portable generators | Poor ventilation |
| BBQs indoors | |
| Shisha pipes |
Pathophysiology
Mechanism
- CO inhaled → binds haemoglobin
- COHb formed (200-250x affinity of O2)
- Oxygen-carrying capacity reduced
- Oxygen dissociation curve shifts LEFT → oxygen not released to tissues
- CO binds cytochrome oxidase → cellular toxicity
- Tissue hypoxia → organ damage
Why SpO2 is Misleading
- Pulse oximetry cannot distinguish COHb from OxyHb
- SpO2 may read 100% despite severe CO poisoning
- Need co-oximetry (blood gas)
Delayed Neurological Sequelae
- Cognitive impairment, personality change
- Parkinsonism
- Can occur days to weeks after exposure
Clinical Presentation
Symptoms — Non-Specific
| Symptom | Notes |
|---|---|
| Headache | Most common |
| Nausea, vomiting | |
| Dizziness | |
| Weakness, fatigue | |
| Confusion | |
| Chest pain | Cardiac ischaemia |
| Syncope |
Signs
Clues to Diagnosis
| Clue | Significance |
|---|---|
| Multiple household members affected | Strong indicator |
| Symptoms resolve when leaving building | |
| Winter, faulty heating | |
| Fire/smoke exposure |
Red Flags
| Finding | Significance |
|---|---|
| Loss of consciousness | Severe — hyperbaric O2 |
| Cardiac ischaemia | High mortality |
| Pregnancy | Fetal toxicity |
| COHb over 25% | Severe |
Tachycardia
Common presentation.
Confusion
Common presentation.
Reduced consciousness
Common presentation.
Cherry-red skin (rare — late sign)
Common presentation.
Clinical Examination
General
- Confusion
- Reduced GCS
- Tachycardia
Skin
- Cherry-red discolouration (rare)
- Often normal
Cardiovascular
- Tachycardia
- Hypotension (severe)
- Signs of ischaemia
Investigations
Blood Gas with Co-Oximetry — ESSENTIAL
| COHb Level | Interpretation |
|---|---|
| Under 3% | Normal (non-smoker) |
| 3-10% | Heavy smoker baseline |
| 10-20% | Mild poisoning |
| 20-40% | Moderate poisoning |
| Over 40% | Severe poisoning |
| Over 60% | Usually fatal |
Other Tests
| Test | Purpose |
|---|---|
| Lactate | Tissue hypoxia |
| Troponin | Cardiac ischaemia |
| ECG | Ischaemia, arrhythmias |
| Glucose |
Pulse Oximetry
- May be falsely normal — do NOT rely on it
Classification & Staging
By Severity
| Severity | COHb | Features |
|---|---|---|
| Mild | 10-20% | Headache, nausea |
| Moderate | 20-40% | Confusion, chest pain |
| Severe | Over 40% | LOC, seizures, cardiac ischaemia |
Management
Immediate
| Action | Details |
|---|---|
| Remove from source | Fresh air |
| 100% oxygen | High-flow via non-rebreather mask |
| Check COHb | Blood gas with co-oximetry |
| Monitor | ECG, cardiac enzymes |
Oxygen Therapy
- 100% O2 reduces COHb half-life from 5h to 1h
- Continue until COHb under 3% and asymptomatic
Hyperbaric Oxygen (HBO)
Indications:
- Loss of consciousness at any time
- Neurological symptoms
- Cardiac ischaemia
- COHb over 25-30%
- Pregnancy (regardless of COHb)
Mechanism:
- Increases dissolved O2
- Accelerates CO dissociation
- May reduce delayed neurological sequelae
Supportive Care
- Airway management if needed
- Treat seizures
- Treat cardiac ischaemia
Public Health
- Investigate source
- Check other household members
- Environmental health/gas safe engineer
Complications
Acute
- Cardiac ischaemia/MI
- Arrhythmias
- Cerebral oedema
- Death
Delayed Neurological Sequelae
- Cognitive impairment
- Memory problems
- Parkinsonism
- Personality change
- May occur 2-40 days after exposure
Prognosis & Outcomes
Prognosis
- Good if treated early
- Delayed neurological sequelae in 10-30%
- Higher risk with severe exposure
Mortality
- 1-3% with treatment
- Higher with cardiac involvement
Evidence & Guidelines
Key Guidelines
- TOXBASE (UK National Poisons Information Service)
- Undersea and Hyperbaric Medical Society Guidelines
Key Evidence
- Hyperbaric oxygen may reduce delayed neurological sequelae (controversial)
- 100% oxygen is mainstay of treatment
Patient & Family Information
What is Carbon Monoxide Poisoning?
Carbon monoxide is a poisonous gas you can't see or smell. It is produced by faulty gas appliances, fires, or running engines in enclosed spaces.
Symptoms
- Headache
- Feeling sick
- Dizziness
- Confusion
- Collapse
What Should I Do?
- Get fresh air immediately
- Call 999
- Do not go back into the building
Prevention
- Install CO alarms
- Have gas appliances serviced regularly
- Never use BBQs or generators indoors
Resources
References
Key Reviews
- Rose JJ, et al. Carbon Monoxide Poisoning: Pathogenesis, Management, and Future Directions of Therapy. Am J Respir Crit Care Med. 2017;195(5):596-606. PMID: 27753502
- Weaver LK. Carbon monoxide poisoning. N Engl J Med. 2009;360(12):1217-1225. PMID: 19297574
Guidelines
- TOXBASE. Carbon Monoxide Poisoning Management. 2023.