How does O2ER relate to mixed venous oxygen saturation?

O2ER = 1 − SvO2 (approximately, assuming SaO2 ≈ 100%). Normal O2ER is 0.20–0.30, corresponding to SvO2 of 70–80%. When DO2 is inadequate or VO2 increases, tissues extract more oxygen, raising O2ER and lowering SvO2. An O2ER >0.50 (SvO2 <50%) indicates critical supply-demand imbalance with imminent anaerobic metabolism and lactic acidosis.

Can O2ER be elevated without true tissue hypoxia?

Yes, in states of increased metabolic demand such as fever, septic hypermetabolism, or vigorous muscle activity. In septic shock, however, O2ER is often paradoxically low due to microcirculatory dysfunction and mitochondrial impairment — tissues cannot extract oxygen efficiently despite low DO2. This 'pathological supply dependence' explains why lactate may remain elevated even when global hemodynamic parameters appear normal.

Oxygen Extraction Ratio

Calculate oxygen extraction ratio (O2ER = VO2/DO2) to assess the balance between oxygen delivery and consumption, with normal O2ER of 20–30% rising during circulatory shock.

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DO2

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VO2

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ScvO2

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Oxygen Extraction Ratio

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Frequently Asked Questions

How does O2ER relate to mixed venous oxygen saturation?

Can O2ER be elevated without true tissue hypoxia?