Carbon monoxide toxicity

[SH4:p79]

• Degradation of inhalational anaesthetic agents which contains CHF2 (i.e. enflurane, isoflurane, and desflurane) by the strong base in desiccated carbon dioxide absorbents
  --> Formation of carbon monoxide (with trifluoromethane)

Factors influencing carbon monoxide formation

• Dryness of the carbon dioxide absorbent
  * Desiccation encourages formation
  * Hydration prevents formation
• Temperature of the absorbent
  * High temperature encourages formation
• Gas flows
  * High fresh gas flow can dry up the absorbent
• Type of carbon dioxide absorbent
  * Absorbents which contain KOH and/or NaOH favour carbon monoxide formation

Differences between inhalational anaesthetic agents

• Level of CO formation = Desflurane > Enflurane > Isoflurane
• Halothane and sevoflurane does not possess vinyl group
  --> Carbon monoxide production unlikely
  BUT
  * When temperature is > 70degrees
  --> Hexafuoroisopropanol (an intermediate sevoflurane metabolite) breaks down to form carbon monoxides

Suspect carbon monoxide when

• Moderately decreased pulse oximetry despite adequate arterial partial pressure of oxygen
• Gas analyzer detecting enflurane when desflurane or isoflurane is being given

NB:

• Trifluoromethane is produced with carbon monoxide when enflurane, isoflurane, desflurane is degradated
  --> Has similar IR absorption profile as enflurane
• Intraoperative haemolysis can also result in carbon monoxide exposure
  * Heme --> Biliverdin + carbon monoxide
  * Rate-limited enzyme is heme oxygenase-1