Mapleson D Best Controlled — FRCA Primary MCQ
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Correct answer: D — Positive-pressure inspiration preferentially vents alveolar gas before fresh gas
The best answer is “Positive-pressure inspiration preferentially vents alveolar gas before fresh gas”. During controlled ventilation, pressure opens the expiratory valve and preferentially vents expired alveolar gas while fresh gas near the patient is retained for inspiration. During spontaneous breathing, mixing and rebreathing require higher flows. The reservoir bag does not absorb carbon dioxide and efficiency is not created by intrinsic PEEP.
Reference: Kaul and Mittal, Mapleson's breathing systems: https://pmc.ncbi.nlm.nih.gov/articles/PMC3821268/