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Oxygen Content Equation — FRCA Primary MCQ

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HardPhysiology - RespiratoryOxygen Content EquationFRCA Primary

At 37°C, haemoglobin concentration is expressed in g dL−1, arterial oxygen saturation (SaO2) as a fraction from 0 to 1, and arterial oxygen partial pressure (PaO2) in kPa. Which equation gives arterial oxygen content (CaO2) in mL O2 dL−1, including both haemoglobin-bound and physically dissolved oxygen?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — CaO2 = (Hb × 1.34 × SaO2) + (0.023 × PaO2)

Explanation lettering: D = shown as C · C = shown as D

A is correct. Arterial oxygen content is the sum of oxygen bound to haemoglobin and oxygen physically dissolved in plasma. With Hb in g dL−1 and SaO2 expressed as a fraction, the bound component is Hb × 1.34 × SaO2. When PaO2 is measured in kPa, the dissolved component is 0.023 × PaO2; the equivalent coefficient is approximately 0.003 when PaO2 is in mmHg. B omits dissolved oxygen. C incorrectly divides a fractional saturation by 100; division is required only when saturation is entered as a percentage. D uses the theoretical Hüfner value rather than the conventional effective clinical coefficient specified here. E mixes a mmHg-based solubility coefficient with PaO2 measured in kPa.

Reference: Rachmale S, Li G, Wilson G, Malinchoc M, Gajic O. Balancing the risks and benefits of oxygen therapy in critically ill adults. Chest. 2013;143(4):1151–1162. https://pubmed.ncbi.nlm.nih.gov/23546490/