Introduction
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OER can be calculated using OER = (SaO~2~-SvO~2~)/SaO~2~. SaO~2~ and SvO~2~ can be measured by blood gas analysis or by continuous non-blood sampling display. The aim of this study was to compare the three methods of calculating OER.

Methods
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Sixty-five sets of measurements from 16 patients were studied. Arterial and continuous cardiac output with SvO~2~ (SvO~2~--CCO, Baxter) catheters were inserted. Simultaneous blood samples were taken for arterial and mixed venous blood gas measurements using co-oximetry. Continuous SaO~2~ from pulse oxymetry (SpO~2~) was recorded. OER was calculated using three methods for each set of measurements: Conventional blood sampling method: OER1 = (SaO~2~--SvO~2~)/SaO~2~; Partial blood sampling method: OER 2 = (SaO~2~--SvO~2~--CCO)/SaO~2~; Non-blood sampling method: OER 3 = (SpO~2~--SvO~2~--CCO/SpO~2~. Simple linear regression with 95% confidence intervals was applied to test the correlation between the three methods.

Results
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There was a significant positive correlation (*P* \< 0.001) between the three methods.

![](cc516-1){#F1}

![](cc516-2){#F2}

![](cc516-3){#F3}

Discussion
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These three methods can be used to calculate OER. However, the conventional method (OER1) appears to have more variability than OER2 and OER3. OER3 (SpO~2~ and SvO~2~--CCO), in contrast, is the simplest and most accurate method for continuous monitoring of OER in intensive care management.
