Carbon Monoxide Poisoning

Overview

Carbon monoxide is a colourless, odourless, and tasteless gas that is produced due to the incomplete combustion of organic material. Carbon monoxide binds to haemoglobin more strongly, forming carboxyhaemoglobin, leading to tissue hypoxia. Carboxyhaemoglobin can take several hours to dissociate, meaning blood cannot carry as much oxygen in this timeframe.

Sources of Carbon Monoxide

• Poorly-maintained housing (e.g. blocked chimneys, lack of ventilation, gas ovens)
• Smoke in burning buildings
• Aerosols
• Running petrol/diesel engines in confined spaces, even if the windows are open

Epidemiology

• Around 40 deaths and 200 non-fatal poisonings are reported each year
• The true incidence of carbon monoxide poisoning may be higher due to the signs and symptoms being non-specific and misdiagnosed as another condition

Presentation

A thorough history is essential. Questions to ask can be remembered using the mnemonic COMA:

• Cohabitees – is anyone else in the property affected? – including pets
• Outdoors – do the symptoms improve outside the building?
• Maintenance – are any fuel-burning appliances and vents properly maintained?
• Alarm – does the house have a carbon monoxide alarm?

Features seen in carbon monoxide poisoning are non-specific, making it difficult to diagnose:

• Headaches – most common:
  • Often described as tension-type headaches 
• Nausea and vomiting
• Vertigo
• Altered consciousness
• Confusion
• Fatigue
• Non-specific pain – usually the chest or stomach
• Weakness

Features of severe toxicity may be:

• Pink skin and mucosae
• Personality changes
• Arrhythmia
• Parkinsonism
• Coma
• Death
• Classic ‘cherry red’ skin is rarely seen in life and is usually seen post-mortem

Investigations

All patients

• Venous or arterial blood gas – gold standard:
  • Pulse oximetry may be falsely normal – it cannot differentiate oxyhaemoglobin and carboxyhaemoglobin
  • Shows increased carboxyhaemoglobin levels:
    • <3% is normal for non-smokers
    • <10% is normal for smokers
    • >30% indicates severe exposure
• Serum lactate:
  • May be elevated in severe poisoning
• ECG:
  • May show cardiac ischaemia or arrhythmia
• Serum troponin:
  • May show cardiac ischaemia
• Serum glucose:
  • Always test in any patient with reduced consciousness

Management

All patients

• 1^st-line: 100% high-flow O₂ via non-rebreather mask
  • Evidence suggests that this reduces the half-life of carboxyhaemoglobin
• Evidence surrounding hyperbaric oxygen is controversial
• Any patient who has suffered smoke inhalation should be assumed to have carbon monoxide poisoning and should also be treated as such

Monitoring

• Patients are generally followed up around 1-2 months after treatment, or sooner, to assess for delayed or persistent symptoms
• The local Health Protection Team should be contacted who coordinate services for the person and provides further guidance

Patient Advice

• Patients should fit a carbon monoxide alarm
• Rooms should be kept well-ventilated when using gas appliances
• Patients should never use gas appliances that may be faulty. Features may be:
  • Yellow or orange flames
  • Soot or stains around the appliance
• Patients should never burn charcoal indoors, this always produces carbon monoxide
• It is the legal duty of landlords to check gas appliances annually and provide a copy of the latest safety certificate to their tenants

Complications

• Myocardial infarction
• Neuropsychiatric complications:
  • Personality changes
  • Memory impairment
  • Parkinsonism
  • Impaired coordination
• Coma
• Death

Prognosis

• The prognosis can vary significantly
• The prognosis is poorer if there are any of the following:
  • High carboxyhaemoglobin levels
  • Metabolic acidosis
  • Cardiac arrest
  • Coma