SvO2 Interpretation Fick Principle — FRCA Final MCQ

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HardIntensive Care MedicineSvO2 Interpretation Fick PrincipleFRCA Final

A patient is found to have a mixed venous oxygen saturation (SvO2) of 40% on a pulmonary artery catheter sample. The cardiac output is 3.0 L/min, Hb 80 g/L, PaO2 12 kPa, and SpO2 99%. Using the Fick principle, what is the primary cause of the low SvO2?

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Correct answer: D — Low cardiac output

SvO2 depends on the balance between oxygen delivery (DO2) and oxygen consumption (VO2). DO2 = CO × CaO2. Low SvO2 (<65%) indicates that oxygen extraction is increased, meaning delivery is inadequate relative to consumption. In this patient: adequate PaO2 and SpO2 exclude hypoxaemia as the primary cause; Hb is low but 80 g/L alone would not cause SvO2 of 40%; the cardiac output of 3.0 L/min is low and is the primary driver. The combination of low CO and mild anaemia drastically reduces DO2, causing critically increased extraction.

Reference: RCOA – 2023 – Final FRCA curriculum: oxygen delivery physiology; ICS – 2023 – Haemodynamic monitoring