Carbon Monoxide Poisoning

What is Carbon Monoxide (CO)?

A colourless, odourless gas that occurs as a result of incomplete combustion of carbon containing substance. It can be created by poorly maintained gas appliances, burning solid fuel inside, house fires or in contaminated air supplies for scuba divers.

Every year there are over 50 accidental deaths from CO poisoning in England and Wales and over 200 non-fatal poisonings which require hospital admission. Evidence suggests a similar number of non-fatal poisonings in people who attend Emergency Departments who are treated for carbon monoxide poisoning, but do not require hospital admission. This is of great concern as CO poisoning can lead to chronic health problems.

How does Carbon Monoxide cause poisoning?

Oxygen is transported around the body, bound to haemoglobin (Hb) molecules within red blood cells. Carbon monoxide (CO) binds to haemoglobin more strongly than oxygen, such that if carbon monoxide is present, oxygen binding is blocked. If there are significant amounts of CO present, oxygen carriage to the tissues is seriously compromised, with the heart and the central nervous system being especially susceptible to the abnormally low levels of oxygen.

CO also causes damage to the energy production system within the cells of the body, inflicting further tissue damage as the cells are then unable to produce energy.

In more detail CO has a further effect within cells, where it is toxic to the cytochrome enzymes which are responsible for intra-cellular energy production. Cells thus suffer a second insult from CO; not only are they deprived of transported oxygen, which is necessary for energy production, but also the energy producing pathway itself is damaged. This second effect is not easily measured, but it explains why carboxyhaemoglobin levels, which are an extra-cellular measurement only, do not correlate with the degree of clinical damage which may result from CO exposure.

What are the symptoms of Carbon Monoxide poisoning?

According to the Department of Health, the commonest symptoms and signs, and their approximate frequency in CO poisoning are:

• Headache – 90%
• Nausea and vomiting – 50%
• Vertigo – 50%
• Alteration in consciousness – 30%
• Weakness  – 20%

What should I do if I suspect Carbon Monoxide poisoning?

Open windows and doors to increase ventilation. Dial for the emergency services or visit your nearest Emergency Department. If appropriate, you may be treated using hyperbaric oxygen (HBO) therapy.

How is Carbon Monoxide poisoning diagnosed?

Levels of carboxyhaemoglobin in the body are detected by a blood test which is taken as soon after CO exposure is suspected. The levels found in this blood test will reduce over time so it is important to note when a person with suspected CO poisoning was removed from the possible source.

How does Hyperbaric Oxygen (HBO) Therapy help in treatment of Carbon Monoxide poisoning?

In the acute instance it may help increase oxygen delivery to areas in the body where insufficient oxygen is being delivered due to the effects of the carbon monoxide (CO) poisoning.HBO speeds up elimination of the carbon monoxide from the body and helps reduce the damage that it causes within the cells.

To be more specific HBO speeds up the dissociation of CO from Hb. The half-life of CO in normobaric air is some 3 to 4 hours, which is reduced to about an hour on 100% oxygen at 1 atmosphere (sea-level). This provides the rationale for the use of prolonged high-flow oxygen in the emergency department. Hyperbaric oxygen at 2.5 atmospheres reduces the half-life further to some 20 minutes, though this benefit is not thought sufficient to justify the transport of the seriously ill for prolonged transfers (National Poisons Information Service).

When should HBO be considered as part of the treatment for CO poisoning?

HBO may be useful if a person has suffered any of the following;

• new neurological deficit or mental status change
• carboxyhaemoglobin of greater than 25% at any time
• pregnancy
• ECG evidence of myocardial involvement (evidence of damage to the heart)
• loss of consciousness, even if subsequently recovered

Current recommendation is that three HBO sessions are undertaken in the first 24 hours after CO poisoning.

The rationale for the use of HBO in CO poisoning stems rather from the suggestion that it reduces the incidence of recognised long-term neurological sequelae (Weaver LK. et al. 2002. Hyperbaric Oxygen for Acute Carbon Monoxide Poisoning. NEJM 347(14):1057-1067).

What are the longer term effects of CO poisoning?

CO poisoning can cause longer term nervous system damage and result in problems with dementia, memory and concentration, as well as causing symptoms similar to Parkinsons Disease. Possible longer term effects include mental health problems such as depression.

What can I do to avoid CO poisoning?

As ever, prevention is better than cure. Many households will have fire alarms or smoke detectors but not carbon monoxide alarms. These are relatively cheap and readily available so ensure you have them fitted.

CO poisoning has become less common since compulsory annual checks of gas appliances were introduced in rental accommodation. Do not use barbeques  or camping stoves inside confined spaces or run a car with the garage door closed.

Make sure boilers, heating systems and chimneys are appropriately serviced and in good working order. Only use properly qualified professionals to undertake this work.

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