Low-Flow Anaesthesia — FRCA Final MCQ

Correct answer: E — Sevoflurane degradation by a highly desiccated carbon dioxide absorbent may generate carbon monoxide

Explanation lettering: D = shown as A · C = shown as B · A = shown as C · E = shown as D · B = shown as E

B is correct. Sevoflurane can undergo chemical degradation when exposed to a highly desiccated carbon dioxide absorbent, particularly one containing strong alkalis. Reported products include formaldehyde, methanol, Compound A and carbon monoxide; suspected desiccated absorbent should therefore be replaced. This is primarily an absorbent-desiccation hazard rather than a consequence of low flow alone, although lower fresh gas flow can increase degradation-product concentrations. At low flow, circuit volume, rebreathing and continuing patient uptake mean that inspired sevoflurane need not equal the vaporiser setting. Rebreathing conserves heat and moisture, so inspired humidity is generally higher than with high flow. Air continuously introduces nitrogen, so nitrogen accumulation is not impossible. Sevoflurane is not chemically inert: it interacts with carbon dioxide absorbents and can form Compound A.

Reference: Piramal Critical Care Ltd. Sevoflurane 100% Inhalation Vapour, liquid: Summary of Product Characteristics, sections 4.4, 6.2 and 6.6. Revised March 2026. https://www.medicines.org.uk/emc/product/9780/smpc