Vaporiser Output Calculation — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyVaporiser Output CalculationFRCA Final

A 45-year-old man is undergoing general anaesthesia. The vaporiser is delivering sevoflurane at 2%. The fresh gas flow is 2 L/min O2. What is the approximate output of sevoflurane vapour from the vaporiser in mL/min?

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Correct answer: A — 40 mL/min

At 2% sevoflurane with a fresh gas flow of 2 L/min (2000 mL/min), the vaporiser outputs: 2% of 2000 = 40 mL/min of sevoflurane vapour. This is calculated as: vaporiser output = (dial setting / (100 – dial setting)) × fresh gas flow. For 2%: (2/98) × 2000 = 40.8 mL/min. At higher dial settings, the more accurate formula becomes important, but at low settings the simple percentage calculation is close enough.

Reference: RCOA – 2023 – Anaesthetic equipment: vaporiser physics; Davis PD – 2019 – Basic Physics and Measurement in Anaesthesia