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Septic shock oxygen extraction — FRCA Primary MCQ

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HardPhysiologySeptic shock oxygen extractionFRCA Primary

A patient with septic shock has warm peripheries, bounding pulses, lactate 5.8 mmol L-1 and mixed venous oxygen saturation 82%. Which physiological abnormality best explains the high mixed venous saturation despite shock?

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

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Correct answer: B — Reduced tissue oxygen extraction with maldistributed flow

Septic shock can involve maldistributed microcirculatory flow and impaired extraction, so venous oxygen saturation may be high despite tissue hypoxia and lactate. A low inspired oxygen concentration would reduce arterial saturation and likely venous saturation. Low cardiac output shock usually lowers mixed venous saturation because extraction rises. The clinical pearl is that lactate and venous saturation must be interpreted in context.

Reference: RCoA Primary FRCA Syllabus 2026; Ganong's Review of Medical Physiology; West's Respiratory Physiology