Introduction

Carbon monoxide has high affinity for haemoglobin and myoglobin resulting in a left-shift of the oxygen dissociation curve and tissue hypoxia. There are approximately 50 per year deaths from accidental carbon monoxide poisoning in the UK

Epidemiology

  • Incidence: 3.00 cases per 100,000 person-years
  • Most commonly see in infants
  • Sex ratio: 1:1
Condition Relative
incidence
Tension-type headache5000.00
Influenza50.00
Carbon monoxide poisoning1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Pathophysiology

Pathophysiology
  • in carbon monoxide poisoning the oxygen saturation of haemoglobin decreases leading to an early plateau in the oxygen dissociation curve

Clinical features

Features of carbon monoxide toxicity
  • headache: 90% of cases
  • nausea and vomiting: 50%
  • vertigo: 50%
  • confusion: 30%
  • subjective weakness: 20%
  • severe toxicity: 'pink' skin and mucosae, hyperpyrexia, arrhythmias, extrapyramidal features, coma, death

Investigations

Investigations
  • pulse oximetry may be falsely high due to similarities between oxyhaemoglobin and carboxyhaemoglobin
  • therefore a venous or arterial blood gas should be taken
  • typical carboxyhaemoglobin levels
    • < 3% non-smokers
    • < 10% smokers
    • 10 - 30% symptomatic: headache, vomiting
    • > 30% severe toxicity
  • an ECG is a useful supplementary investgation to look for cardiac ischaemia

Management

Management
  • patients with suspected carbon monoxide poisoning should be assessed in the emergency department
  • 100% high-flow oxygen via a non-rebreather mask
    • from a physiological perspective, this decreases the half-life of carboxyhemoglobin (COHb)
    • should be administered as soon as possible, with treatment continuing for a minimum of six hours
    • target oxygen saturations are 100%
    • treatment is generally continued until all symptoms have resolved, rather than monitoring CO levels
  • hyperbaric oxygen
    • due to the small number of cases the evidence base is limited, but there is some evidence that long-term outcomes may be better than standard oxygen therapy for more severe cases
    • therefore, discussion with a specialist should be considered for more severe cases (e.g. levels > 25%)
    • in 2008, the Department of Health publication 'Recognising Carbon Monoxide Poisoning' also listed loss of consciousness at any point, neurological signs other than headache, myocardial ischaemia or arrhythmia and pregnancy as indications for hyperbaric oxygen