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ScvO2 Interpretation — FRCA Final MCQ

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ModerateIntensive Care MedicineScvO2 InterpretationFRCA Final

A 50-year-old woman with septic shock has been fluid resuscitated and is no longer fluid responsive. Her mean arterial pressure is 68 mmHg while receiving noradrenaline. Her haemoglobin is 120 g/L, arterial oxygen saturation is 97%, and cardiac output measured by pulse contour analysis is 3.2 L/min. Repeated co-oximetry from a correctly positioned superior vena caval catheter shows an ScvO2 of 55%. Which is the best physiological interpretation?

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Correct answer: C — Oxygen delivery is inadequate for metabolic demand

ScvO2 reflects the balance between global oxygen delivery and oxygen consumption. By the Fick principle, VO2 = cardiac output × (CaO2 − CvO2). With a normal arterial saturation, normal haemoglobin and a confirmed venous sample, an ScvO2 of 55% indicates increased oxygen extraction because delivery is inadequate relative to metabolic demand; the low cardiac output is a likely contributor. It does not, by itself, identify a specific treatment, so clinical perfusion, lactate, cardiac function and the components of oxygen delivery must also be assessed. Adequate delivery would not explain the low value. Cyanide inhibits cellular oxygen utilisation and therefore tends to leave venous oxygen saturation abnormally high. Carbon monoxide poisoning requires an appropriate exposure history and co-oximetry evidence. Sampling or analytical error is made unlikely by the correctly positioned catheter and repeated measurement.

Reference: Shepherd SJ, Pearse RM. Role of central and mixed venous oxygen saturation measurement in perioperative care. Anesthesiology. 2009;111:649-656. https://pubmed.ncbi.nlm.nih.gov/19672190/