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Variable bypass vaporiser temperature compensation — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementVariable bypass vaporiser temperature compensationFRCA Primary

During a long case the theatre temperature falls and the sevoflurane vaporiser is used continuously. The delivered concentration remains close to the dialled setting despite cooling within the vaporising chamber. Which vaporiser feature best explains this?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Temperature-compensated variable bypass design

Modern variable bypass vaporisers use temperature compensation to maintain output as vapour pressure falls with cooling. The copper kettle is a measured-flow device requiring calculation and is not the standard sevoflurane plenum vaporiser. Desflurane requires heating and pressurisation because its boiling point is close to room temperature, which is not the usual design for sevoflurane. The clinical pearl is that vaporiser design compensates for both temperature and flow within limits.

Reference: RCoA Primary FRCA Syllabus 2026; Davis and Kenny Basic Physics and Measurement in Anaesthesia