Correlation between three methods of calculating oxygen extraction ratio (OER)

Introduction

OER can be calculated using OER = (SaO₂-SvO₂)/SaO₂. SaO₂ and SvO₂ 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

Sixty-five sets of measurements from 16 patients were studied. Arterial and continuous cardiac output with SvO₂ (SvO₂–CCO, Baxter) catheters were inserted. Simultaneous blood samples were taken for arterial and mixed venous blood gas measurements using co-oximetry. Continuous SaO₂ from pulse oxymetry (SpO₂) was recorded. OER was calculated using three methods for each set of measurements: Conventional blood sampling method: OER1 = (SaO₂–SvO₂)/SaO₂; Partial blood sampling method: OER 2 = (SaO₂–SvO₂–CCO)/SaO₂; Non-blood sampling method: OER 3 = (SpO₂–SvO₂–CCO/SpO₂. Simple linear regression with 95% confidence intervals was applied to test the correlation between the three methods.

Results

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

Discussion

These three methods can be used to calculate OER. However, the conventional method (OER1) appears to have more variability than OER2 and OER3. OER3 (SpO₂ and SvO₂–CCO), in contrast, is the simplest and most accurate method for continuous monitoring of OER in intensive care management.