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Sudden low end-tidal CO2 — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementSudden low end-tidal CO2FRCA Primary

A 36-year-old patient in theatre has a sudden fall in end-tidal CO2 from 5.1 to 2.4 kPa with hypotension and increased airway pressure unchanged. Oxygen saturation remains 98% initially. What is the most likely explanation?

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Correct answer: E — Acute reduction in pulmonary blood flow

A sudden fall in end-tidal CO2 with hypotension and unchanged airway pressure suggests reduced pulmonary blood flow, such as severe embolism or circulatory collapse. Exhausted absorber causes inspired CO2 and usually raises end-tidal CO2. Bronchospasm tends to increase airway pressure and produce a sloping waveform. Capnography is a real-time monitor of ventilation, metabolism and pulmonary perfusion.

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