Carbon monoxide has a 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.

Questions may hint at badly maintained housing e.g. student houses.

Pathophysiology

• carbon monoxide binds readily to haemoglobin, forming carboxyhaemoglobin → reduced oxygen-carrying capacity
• in carbon monoxide poisoning the oxygen saturation of haemoglobin decreases leading to an early plateau in the oxygen dissociation curve (Left shift)

Carbon monoxide has ~210 times the affinity for haemoglobin than oxygen. Carboxyhaemoglobin competes with oxygen for binding of haemoglobin. Therefore, an increase in COHb will impair oxygen delivery to the tissues, resulting in a leftward shift of the oxygen-haemoglobin dissociation curve and hypoxia.

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

• 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