Inhaled poisons Carbon monoxide

Carbon monoxide poisoning should be considered in anyone found in a smoke filled, enclosed space. Carbon monoxide displaces oxygen from haemoglobin, to which it has 200 times greater affinity and thus prevents oxygen carriage. There is also a direct toxic effect on mitochondrial oxidative phosphorylation as it competes with oxygen for the same binding site on cytochrome oxidase.

Clinical features

• Fatigue, headache, vomiting, dizziness, confusion, dyspnoea.

• A cherry red appearance of the skin and mucosae are classical but not common.

• PaO2 will be normal unless there is respiratory depression and pulse oximetry is misleading.

• The half life of carboxyhaemoglobin is 4h when breathing room air and 50min when breathing 100% oxygen.


• Carboxyhaemoglobin levels should be measured by a co-oximeter and treatment started immediately with oxygen at the maximum concentration that can be delivered (FIO2 1.0 if ventilated and 0.6-1.0 if self-ventilating).

• If carboxyhaemoglobin levels >25% or carbon monoxide poisoning is associated with mental disturbance, the optimal treatment is hyperbaric oxygen at 3 atmospheres for 30min, repeated 6-hrly if levels remain >25%. Death is likely with carboxyhaemoglobin levels >60%.

• High concentration oxygen treatment should continue until carboxyhaemoglobin levels <10%.

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