PaO2 on 100% Oxygen — FRCA Primary MCQ

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HardPhysiology - RespiratoryPaO2 on 100% OxygenFRCA Primary

At sea level (barometric pressure 101 kPa), a patient breathes FiO2 0.60. PaCO2 is 5.3 kPa and respiratory quotient is 0.8. Ignoring a small correction for inspired CO2, which alveolar oxygen tension is closest?

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Correct answer: B — 50 kPa

The best answer is “50 kPa”. The alveolar gas equation gives PAO2 ≈ FiO2 × (PB − PH2O) − PaCO2/R. Thus 0.60 × (101 − 6.3) − 5.3/0.8 ≈ 56.8 − 6.6 = 50.2 kPa, so 50 kPa is the closest option. The alternatives reflect common errors in water-vapour or carbon-dioxide correction.

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/