On a Monday morning, the first case is maintained with desflurane using an anaesthesia machine whose carbon dioxide absorber was left flushed with fresh gas over the weekend and is completely dry. Two hours into surgery, co-oximetry of an arterial sample shows carboxyhaemoglobin of 15%. What is the most likely explanation?

• A Nitrous oxide contamination of the fresh gas supply displaced oxygen, causing endogenous CO retention
• B Desflurane undergoes hepatic oxidative metabolism releasing free carbon monoxide
• C Compound A formed from desflurane binds haemoglobin, spuriously reading as carboxyhaemoglobin
• D Desflurane reacts with dry, hot soda lime to generate carbon monoxide ✓

Explanation

Strong base plus heat degrades inhaled anaesthetics to carbon monoxide, and desflurane produces the greatest amount because its difluoromethoxy group is most susceptible to abstraction by hydroxide. Completely dried absorbent, typically after a long gas flow over a weekend, is the classic setting. Hepatic metabolism contributes negligible CO, and Compound D formation is specific to sevoflurane, so options B and C are incorrect.

Reference: Miller's Anesthesia, 9th ed.

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