Investigations

• carbon monoxide in expired air - it is possible to measure from expired air; there is no point in measuring carbon monoxide concentrations in this way if the patient has spent hours away from the source of CO
  • CO can be measured in expired air and as such is used in smoking cessation clinics
    • monitors are available that convert CO concentration into COHb concentration from the standard equilibration curve
      • if such devices are used, they must be used quickly: there is no point in taking a measurement if the patient has spent hours away from the source of CO
      • measurements taken the next day at the surgery may be misleading
• COHb can be measured in blood by any clinical chemistry laboratory. Venous blood should be taken into anti-coagulant and sent to the laboratory
  • COHb should be measured directly: measuring PO2 and calculating the percentage saturation of haemoglobin with oxygen will be misleading as the PO2 in CO poisoning may well be normal
  • blood levels of carbon monoxide can be undertaken; levels greater than 5% indicate systemic inhalation but not the severity of any injury (2)
• pulse oximetry
  • advances in pulse oximeters allow them to take readings for CO levels. Whilst there are some concerns if a traditional pulse-oximeter is used, owing to the similar light absorbance of COHb and oxyhaemoglobin and the likely display of false high oxygen saturations, those designed for use in detecting blood CO levels (pulse CO-oximeters) are a useful aid to diagnosis
• rapid measurement of expired air CO and pulse CO-oximetry are useful in diagnosis

Reference:

• 1. PL/CMO/2008/8, PL/CNO/2008/8: Recognising Carbon Monoxide Poisoning – 'Think CO'
• 2. : PL/CMO/2002/2, PL/CNO/2002/2 Carbon Monoxide: the Forgotten Killer.