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Capnography in bronchospasm — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementCapnography in bronchospasmFRCA Primary

During controlled ventilation, a 44-year-old patient develops an end-tidal CO2 trace with a prolonged sloping expiratory upstroke and an elevated phase III plateau. Peak airway pressure has risen and wheeze is audible. What is the most likely explanation?

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Correct answer: D — Rebreathing due to exhausted soda lime

Bronchospasm and airway obstruction produce slow, uneven alveolar emptying, giving a sloping capnographic plateau and increased airway pressure. Exhausted soda lime tends to raise inspired CO2 rather than produce a classic obstructive shark-fin waveform. Pulmonary embolism usually lowers end-tidal CO2 by increasing dead space. The capnogram is a physiological waveform, not merely a tube-position monitor.

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