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Capnograph Phase III — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementCapnograph Phase IIIFRCA Primary

In a mechanically ventilated adult, the time capnogram consistently shows an abnormally steep upward slope during phase III. Which physiological abnormality best explains this finding?

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Correct answer: B — Uneven alveolar emptying from lung units with differing time constants

Phase III predominantly represents alveolar gas. An increased upward slope indicates sequential emptying of lung units with unequal expiratory time constants. In airflow obstruction or bronchospasm, relatively unobstructed units empty first, whereas slowly emptying obstructed units contribute later, progressively increasing expired CO2 and producing the characteristic sloping or “shark-fin” plateau. Circuit disconnection causes abrupt loss of the waveform rather than isolated phase III steepening. Increased alveolar dead space primarily increases the PaCO2–end-tidal CO2 difference and may reduce end-tidal CO2; it is not the specific explanation for this waveform. Hyperventilation lowers end-tidal CO2. Homogeneous ventilation and similar regional time constants produce a relatively flat phase III plateau.

Reference: Balogh AL et al. Capnogram slope and ventilation dead space parameters: comparison of mainstream and sidestream techniques. British Journal of Anaesthesia. 2016;117(1):109–117. https://pubmed.ncbi.nlm.nih.gov/27317710/