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Carbon monoxide

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  • affinity for Hb 200-250 times that of oxygen
  • combines with Hb to form carboxy-Hb and the affinity of the remaining haem for oxygen is increased
  • toxicity caused by tissue anoxia and possibly also inhibition of cellular respiration as a result of binding to other haem proteins
  • elimination t1/2 of CO reduced from 250 mins when breathing air to 59 mins breathing 100% O2 and 22 min when breathing 100% O2 at 2.2 atm



  • acute exposure: depends on level of carboxy-Hb. Generally no symptoms if level < 10% while levels > 60% associated with coma and cardiorespiratory arrest
  • late sequelae include neuropsychiatric complications which may develop weeks after exposure. Include memory loss, impaired intellect, signs of cerebellar and mid-brain damage
  • signs of poisoning include: cherry red skin, coma, hyper-reflexia, convulsion, arrhythmias


  • remove from site of exposure
  • 100% O2 by facemask
  • if there is respiratory depression or airway compromise intubate and ventilate
  • in severe poisoning assume cerebral oedema and treat accordingly
  • indications for hyperbaric oxygen:
    • conscious patient with carboxy-Hb > 20%
    • depressed level of consciousness but able to maintain airway
    • recovery of consciousness after initial carboxy-Hb > 40%
  • contraindications (single person chamber)
    • artificial ventilation
    • inability to maintain airway
    • hypovolaemia or dependence on inotropes
    • arrhythmias potentially requiring urgent treatment
    • asthma


  • fetal haemoglobin binds CO with greater affinity than maternal Hb
    • T1/2 fetal carboxyhaemoglobin 3.5 longer than maternal
    • fetal carboxyhaemoglobin concentration 15% higher than maternal

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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