Carbon monoxide toxicity

[Ref: SH4:p79]

Degradation of AA which contains CHF2 (i.e. ENF, ISO, and DES) 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

Differences between AAs

• DES produces the highest level of carbon monoxide
  * Then ENF, then ISO
• HAL and SEVO does not possess vinyl group
  --> Carbon monoxide production unlikely
• When temperature is > 70degrees
  --> Hexafuoroisopropanol (an intermediate SEVO 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 ENF when DES or ISO is being given

NB:

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