Expected PaO2 100% Oxygen — FRCA Final MCQ

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HardGeneral AnaesthesiaExpected PaO2 100% OxygenFRCA Final

A patient breathing 100% O2 at sea level has a PaO2 of 40 kPa (300 mmHg). The expected PaO2 breathing 100% O2 at sea level is approximately:

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Correct answer: A — 80 kPa

Using the alveolar gas equation at FiO2 1.0: PAO2 = 1.0 × (101.3 – 6.3) – (5.3/0.8) = 95 – 6.6 = 88.4 kPa. Normal A-a gradient on 100% O2 is approximately 2-5 kPa, giving expected PaO2 of ~80-86 kPa. A PaO2 of only 40 kPa on 100% O2 indicates a significant intrapulmonary shunt (approximately 25-30%).

Reference: RCOA – 2023 – Respiratory physiology: shunt calculation