3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.2. Inhalational anaesthetic agents
                      3.1.3.2.5. Comparisons of inhalational agents
 3.1.3.2.5.8. Carbon monoxide toxicity with inhalational anaesthetic agents 

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