Anatomical Dead Space — FRCA Primary MCQ

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HardPhysiology - RespiratoryAnatomical Dead SpaceFRCA Primary

A patient has tidal volume 500 mL, anatomical dead space 150 mL and respiratory rate 12/min. Assuming negligible alveolar dead space, what is alveolar ventilation?

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Correct answer: E — 4.2 L/min

The best answer is “4.2 L/min”. Alveolar ventilation equals (tidal volume − dead space) × respiratory rate: (500 − 150) mL × 12 = 4200 mL/min, or 4.2 L/min. Five litres per minute is total minute ventilation. The larger answers result from ignoring dead space or multiplying the wrong volume; this distinction is crucial when tidal volume is reduced.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; The respiratory system and acid–base disorders: https://pmc.ncbi.nlm.nih.gov/articles/PMC10201398/