Medical School Lesson Plan – Carbon Monoxide Poisoning

Picture the scene…

A family of 5 are in the waiting room. The gasman came to service their living room fire and said it was faulty. They are worried they might have Carbon Monoxide poisoning and want some blood tests.

Learning Objective

In this session we will discuss potential Carbon Monoxide (CO) poisoning symptoms, when and how to test for CO poisoning and how to treat a patient with CO poisoning

Listen to two EM physicians, Anand Swaminathan and Jenny Beck Esmay discuss Carbon Monoxide poisoning on the Core EM podcast

Read through the information on CO Poisoning from NICE – starting with Background Information and finishing with Management.

This part of the teaching session should be lead by an experienced clinican. The cases provided are merely examples and if possible the learners should be encouraged to discuss patients they have seen in their clinical practice.

A family of 5 are in the waiting room. The gasman came to service their living room fire and said it was faulty. They are worried they might have Carbon Monoxide poisoning although they have no obvious symptoms.

Incomplete combustion of hydrocarbon fuels (gas, coal, wood, petrol) produces CO.

It is usually caused by an accidental lack of oxygen in the environment due to a blocked flue (gas boilers, wood stoves) or burning fuel in an enclosed space (indoor BBQ, car engine running in a garage).

CO Poisoning can affect all systems but classically you get

  • Neuro symptoms (headache, nausea, dizziness, visual disturbance, ataxia, seizures);
  • Psychocognitive symptoms (memory loss, concentration problems, confusion, personality change);
  • Cardiorespiratory symptoms (SOB, chest pain, arrhythmias, hypertension)

Whilst CO Poisoning seems unlikely given a lack of symptoms, occasionally onset of symptoms is delayed and so advice on seeking help if any problems do occur would be wise.

This is also a good moment for some safety advice about having a CO alarm and ensuring heating appliances are regularly serviced, particularly in the autumn, as ventilation pathways can become blocked when appliances have been dormant during the summer months.

A 25 year old woman attends with a severe headache and vomiting. She lives in a small bedsit and decided to use a camping stove to warm her room up today. A friend went round to see her and called an ambulance because she seemed a bit drowsy. She was given oxygen by the ambulance crew initially and has been sat in the waiting room for the last two hours with her symptoms gradually improving.

CO binds with haemoglobin (HbCO) with over 200 times the affinity of oxygen. This leads to impairment of oxygen delivery to tissues and cellular hypoxia. Dissolved CO in the blood also reaches the tissues and binds even more strongly to mitochondrial cytochromes, impairing cellular respiratory function further.

High concentration oxygen therapy is the mainstay of treatment in the ED.

A well fitted, non rebreather mask with reservoir bag at 10-15 litres per minute of oxygen can potentially reduce the half-life of HbCO from about 6 hours to only 90mins.

For most people with mild symptoms, high flow oxygen for 4-6hrs or until their symptoms resolve, is sufficient medical therapy.

Whilst HbCO levels can be useful for ruling in CO poisoning, unless a sample is taken very early after an acute exposure, they are not good for ruling out CO toxicity.

This is because the HbCO level begins falling immediately the patient is removed from the toxic environment, more quickly if they have been given oxygen therapy (perfectly appropriately). 

A similar issue occurs with chronic, intermittent, low dose exposure, where cellular CO binding can build up whilst HbCO is constantly cleared away. A clue to this may be that although the HbCO level is normal, a lactic acidosis might be present on the blood gas.

Ultimately, this means that CO poisoning is usually a clinical diagnosis, based on a history of potential exposure, appropriate symptomatology and resolution of symptoms with oxygen therapy.

A 66 year old man is brought in from an apparent suicide attempt. He was found unconscious in his car, in the garage with the engine running.

He has no obvious injuries and remains neurologically impaired.

  • Pulse: 106 beats per minute,
  • Blood Pressure: 157/98
  • Respiratory rate: 24 breaths per minute
  • Saturations: 99% on high flow oxygen
  • GCS 9/15: E3 M3 V3 (9/15)
  • Pupils: equal and reactive to light

Whilst the diagnosis appears obvious, it is important to consider other causes for an unconscious patient and investigate appropriately.

Thinking specifically of CO Poisoning, a blood gas with HbCO will be necessary, as will an ECG to look for cardiac ischaemia.

A CT of the head may be part of the general work up but is unlikely to show any changes related to CO poisoning at this time.

General supportive care alongside oxygen delivery at the highest concentration available is the management strategy. In this setting intubation and ventilation with 100% oxygen is needed.

A higher ventilation rate can aid CO expulsion but needs to be balanced against the dangers of a low pCO2 on cerebral perfusion.

Whilst this patient clearly has severe CO Poisoning the use of hyperbaric oxygen (HBO) therapy remains controversial.

In theory the best chance of it working is in the initial ‘washout’ phase of treatment (<6hrs) but the inevitable delays of accessing HBO and the logistical problems associated with managing a patient in a hyperbaric chamber, means it is rarely used outside specialist centres. In pregnancy there may be some benefit in using HBO to potentially increase oxygen delivery to the foetus, so cases should be discussed with a Poisons Centre for consideration of treatment. 

Carbon monoxide poisoning can present with a myriad of often non-specific symptoms. Making the connection between the symptoms and a potential exposure is key to getting the diagnosis right.

Testing for HbCO can be helpful but needs to be put in context of the time it was taken since the exposure and if any oxygen treatment has been started. For this reason it is much more useful for ruling in a diagnosis than ruling out.

The best treatment for CO Poisoning is high concentration of inspired oxygen. Most patients have mild poisoning and can be managed with high flow oxygen via a mask until their symptoms settle, whilst in severe cases, ventilation with 100% oxygen is recommended. Hyperbaric oxygen remains controversial but may still be appropriate if easily available in severe cases and in pregnancy.

In order to embed today’s learning further, reflect on what you have learnt and record in your portfolio whether it has had any impact (or is expected to have any impact) on your performance and practice.

Was this a topic that you were confident you knew already? Which parts were new to you? Were there elements that you will use on your next clinical shift.

Dscuss this session with your colleagues – were there people who missed it who you can share the highlights with?

References and Further Reading

  1. Swaminathan A, Beck Emsay J. Episode 96.0 – Carbon Monoxide Poisoning. Core EM Podcast. May 2017
  2. Carbon Monoxide Poisoning. NICE Guidance. October 2018
  3. Farkas, J. Carbon Monoxide Poisoning. EMCrit. January 2rd 2017

Cite this article as: Simon McCormick, "Medical School Lesson Plan – Carbon Monoxide Poisoning," in St.Emlyn's, September 25, 2020,

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