Mixed Venous Oxygen Saturation — FRCA Primary MCQ

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EasyPhysiology - CardiovascularMixed Venous Oxygen SaturationFRCA Primary

A healthy resting adult has an arterial oxygen saturation of 100% and a whole-body oxygen extraction ratio of 25%. Ignoring dissolved oxygen, the expected mixed venous oxygen saturation is approximately:

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Correct answer: A — 75%

The correct answer is A, 75%. The oxygen extraction ratio is (CaO2 − CvO2)/CaO2. An extraction ratio of 25% therefore leaves 75% of the arterial oxygen content in mixed venous blood. When dissolved oxygen is ignored and haemoglobin concentration is unchanged, oxygen content is approximately proportional to haemoglobin saturation, giving an SvO2 of 75%. Normal SvO2 is commonly described as approximately 65–75% and is measured in pulmonary arterial blood. It reflects the global balance between oxygen delivery and consumption. A value of 65% is near the lower limit rather than the value calculated here; 55% suggests substantially increased extraction. Values of 85% or 95% imply unusually low extraction, excessive delivery, impaired tissue utilisation, or sampling of more highly oxygenated blood.

Reference: Chan YL et al. Transfusion of Red Blood Cells to Patients with Sepsis, Figure 1, 2017. https://pubmed.ncbi.nlm.nih.gov/28891973/