O2 Extraction Ratio Formula — FRCA Primary MCQ

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HardPhysiology - CardiovascularO2 Extraction Ratio FormulaFRCA Primary

Arterial and true mixed-venous blood samples from a patient give the following results: haemoglobin 10.0 g/dL, SaO2 0.98, PaO2 13.3 kPa, SvO2 0.70 and PvO2 5.3 kPa. Calculate the whole-body oxygen extraction ratio. Use CO2 = (1.34 × haemoglobin × oxygen saturation) + (0.003 × PO2), where oxygen content is in mL/dL, PO2 is in mmHg and 1 kPa = 7.5 mmHg.

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Correct answer: D — 0.29

The oxygen extraction ratio is the fraction of delivered oxygen consumed: O2ER = VO2/DO2 = (CaO2 − CvO2)/CaO2. The calculated arterial and mixed-venous oxygen contents are 13.43 and 9.50 mL/dL, respectively, giving O2ER = 3.93/13.43 = 0.29; therefore D is correct. Option B divides the arteriovenous content difference by venous rather than arterial content. Option C is CaO2/CvO2. Option A is the reciprocal, DO2/VO2. Option E is merely the arterial–venous saturation difference; it neglects dissolved oxygen and, more importantly, is not expressed as a fraction of arterial oxygen content.

Reference: Schumacker PT, Rowland J, Saltz S, Nelson DP, Wood LDH. Effects of hyperthermia and hypothermia on oxygen extraction by tissues during hypovolemia. Journal of Applied Physiology. 1987;63(3):1246–1252. https://pubmed.ncbi.nlm.nih.gov/3654470/