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Soda lime carbon dioxide absorption — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementSoda lime carbon dioxide absorptionFRCA Primary

An anaesthetic assistant checks a circle system before a case. The soda lime is exhausted and fresh gas flow is low. Inspired CO2 is detected during ventilation. Which process should normally prevent this?

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

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Correct answer: B — Physical adsorption of CO2 by activated charcoal

Circle absorbers remove carbon dioxide by chemical reaction with strong bases in the absorbent, producing carbonates, water and heat. Activated charcoal adsorption is not the main process in soda lime. Carbon monoxide production is a hazard with desiccated absorbent and volatile agents, not the intended function. The pearl is that inspired CO2 monitoring is the practical safeguard against absorber failure.

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