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Bain Circuit Mapleson D — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementBain Circuit Mapleson DFRCA Primary

During controlled ventilation with a Bain circuit, inspired carbon dioxide rises abruptly despite high fresh-gas flow and a patent expiratory valve. Which fault is most likely?

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Correct answer: B — Disconnection of the inner fresh-gas limb

The best answer is “Disconnection of the inner fresh-gas limb”. The Bain is a coaxial Mapleson D circuit. If the inner fresh-gas limb disconnects, fresh gas enters at the machine end and the outer limb effectively becomes apparatus dead space, causing substantial rebreathing. There is no carbon-dioxide absorber in this circuit, and a high nominal flow cannot compensate reliably for the disconnection.

Reference: Kaul and Mittal, Mapleson's breathing systems: https://pmc.ncbi.nlm.nih.gov/articles/PMC3821268/