Intended for healthcare professionals

Practice Easily Missed?

Carbon monoxide poisoning

BMJ 2019; 365 doi: (Published 13 June 2019) Cite this as: BMJ 2019;365:l2299
  1. James Ashcroft, academic foundation programme doctor1 2,
  2. Emma Fraser, fire, water and carbon monoxide safety and senior development officer3,
  3. Sanjay Krishnamoorthy, consultant in acute medicine2 ,
  4. Sue Westwood-Ruttledge, patient adviser and campaigner for carbon monoxide poisoning awareness4
  1. 1Department of Surgery and Cancer, St Mary’s Hospital, London, UK
  2. 2Department of Acute Medicine, West Middlesex University Hospital, Isleworth, UK
  3. 3London Fire Brigade, London, UK
  4. 4Sale, Cheshire, UK
  1. Correspondence to J Ashcroft james.ashcroft17{at},

What you need to know

  • Carbon monoxide (CO) poisoning can present in patients as a wide range of acute and chronic symptoms

  • Diagnosing CO poisoning relies on taking a thorough history exploring the relationship of symptoms to environment

  • Emission of abnormally high CO levels can be detected by certified carbon monoxide alarms and can be avoided by routine maintenance of fuel burning appliances

A 23 year old man with no medical history presents to the emergency department with a three day history of headache, transient visual disturbance, dizziness, and hypertension. On clinical examination the patient is flushed and drowsy with redness in the sclera, with no further visual or systemic symptoms. A venous blood gas on air shows a carboxyhaemoglobin level (COHb) grossly elevated at 26.0 (0-3), which leads to the diagnosis of carbon monoxide poisoning.

What is carbon monoxide poisoning?

Carbon monoxide (CO) is a poisonous gas produced through burning fuel. Sources of CO in the home include boilers and central heating systems, cookers and barbecues, and fireplaces and chimneys (figure).1 Owners of such appliances are required to have them serviced by registered technicians as per recommended individual guidance (usually once per year).1 If outlets from these appliances (such as flues and chimneys) become blocked or if they are operated as normal in a closed environment with no ventilation, dangerous levels of CO can build up in living spaces.1 When inhaled in high quantities, CO enters the bloodstream and binds to haemoglobin molecules with a much greater affinity (230 times higher) than oxygen, creating carboxyhaemoglobin.2 The binding of CO results in reduced delivery of oxygen to tissues, which leads to tissue ischaemia.2


Common domestic sources of CO

Why is it missed?

Presentation of CO poisoning is not frequent enough for the condition to prioritised in medical training, diagnostic decision making, initial patient testing, and publicity and public …

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