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Mapleson Classification — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementMapleson ClassificationFRCA Primary

During spontaneous ventilation through a Mapleson A circuit, fresh-gas flow falls below minute ventilation. Which gas is then most likely to be inspired at the start of the next breath?

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Correct answer: B — Alveolar gas retained in the corrugated tubing

The best answer is “Alveolar gas retained in the corrugated tubing”. With inadequate fresh-gas flow, expired alveolar gas is not fully driven from the corrugated limb during the expiratory pause and is rebreathed on the next inspiration. Dead-space gas can be usefully rebreathed, but retained alveolar gas produces carbon-dioxide rebreathing. The scavenging system does not return gas to the patient.

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