Capnograph Phase III Slope — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyCapnograph Phase III SlopeFRCA Final

A 55-year-old man (ASA II) is breathing spontaneously through a laryngeal mask airway during elective hernia surgery. The capnograph trace shows a gradually rising plateau phase (phase III) with a steep slope. What does a steep phase III (alveolar plateau) slope indicate?

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Correct answer: A — V/Q mismatch or obstructive airways disease

Phase III (alveolar plateau) of the capnograph represents alveolar gas being exhaled. Normally this is relatively flat. A steep upslope indicates: V/Q mismatch (different lung units emptying at different rates), obstructive airways disease (slow-emptying units with high CO2 continue to add CO2 late in expiration), or inadequate expiratory time. In COPD and asthma, the slope is characteristically steep ('shark fin' pattern in severe obstruction). A completely flat trace that doesn't return to zero baseline suggests rebreathing. An absent plateau suggests a leak or disconnection.

Reference: RCOA – 2023 – Monitoring: capnography interpretation; AAGBI – 2019 – Monitoring standards