Capnography Interpretation — FRCA Final MCQ

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EasyAnaesthetic Equipment & SafetyCapnography InterpretationFRCA Final

A 55-year-old woman is undergoing laparoscopic cholecystectomy under general anaesthesia with controlled ventilation via a circle breathing system. The capnograph develops a progressively raised inspiratory baseline that no longer returns to zero between breaths. What does this waveform indicate?

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Correct answer: D — Rebreathing of carbon dioxide

Phase I of the capnogram, the inspiratory baseline, should be zero. A progressively raised baseline means carbon dioxide is present in inspired gas, which by definition is rebreathing. In a circle system the causes include exhausted CO2 absorbent, an incompetent unidirectional valve or inadequate fresh gas flow; the waveform itself does not distinguish between them, so 'rebreathing' is the only defensible interpretation. Oesophageal intubation produces absent or rapidly disappearing exhaled CO2, not a raised baseline. Bronchospasm prolongs expiration, giving a sloping 'shark-fin' upstroke with loss of plateau, while the baseline still returns to zero. Increased CO2 production raises the alveolar plateau and end-tidal value with an intact zero baseline. Hyperventilation lowers end-tidal CO2 and cannot generate inspired CO2.

Reference: Nicolas F et al. Monitoring of anaesthesia breathing systems by capnography — inspired CO2 as the measure of rebreathing (Ann Anesthesiol Fr, 1979). https://pubmed.ncbi.nlm.nih.gov/532995/