During a low-flow anaesthetic at 0.5 L/min with sevoflurane, a nurse bumps the CO2 absorber canister and desiccated soda lime (not replaced for 4 days) is present. Which dangerous compound can form from the interaction of sevoflurane with desiccated soda lime?

• A Compound A (fluoromethyl vinyl ether)
• B Formaldehyde
• C Chloroform
• D Carbon monoxide ✓

Explanation

While Compound A forms in all conditions with soda lime and sevoflurane (a concern at ultra-low flows), desiccated CO2 absorbent specifically reacts with sevoflurane to produce significant amounts of carbon monoxide. The reaction is exothermic and can cause canister fires with desiccated absorbents. CO binds haemoglobin with 250× the affinity of oxygen causing carboxyhaemoglobinaemia; pulse oximetry reads falsely normal. Desflurane and isoflurane produce more CO than sevoflurane with desiccated absorbents, but sevoflurane can still produce clinically significant CO with very dry absorbent. Regular replacement and prevention of desiccation (avoiding sustained low or zero fresh gas flow over weekends) are critical.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.