Oxygen delivery to carbon dioxide production ratio for continuously detecting anaerobic metabolism in trauma patients

Introduction

Lactate levels have been shown to correlate with tissue hypoxia. Unfortunately, due to their slow clearance, lactate levels may not reflect the actual metabolic condition. Under tissue hypoxia the carbon dioxide production (VCO₂) should be less reduced than the oxygen consumption, and the respiratory quotient (RQ) should increase. The oxygen delivery (DO₂)/VCO₂ ratio could be used as an indicator of anaerobic metabolism (AM) since it reflects the oxygen demand and delivery, and the tissue oxygenation. We tested the DO₂/VCO₂ ratio as a potential predictor of AM in trauma patients.

Methods

Eighty consecutive adult trauma patients were prospectively studied. The DO₂, VCO₂, RQ, DO₂/VCO₂ ratio, SvO₂, and arterial lactate (Lac) values were collected at ICU admission. The DO₂ was calculated using the cardiac index measured by a pulse contour system (Hemoscan). The VCO₂ was measured under steady-state conditions using a CO₂ analyzer (930 Siemens Elema). The presence of AM (for example, hyperlactatemia, Lac+) was defined by an increase in Lac >2 mmol/l. Correlation analysis and the ROC test were applied.

Results

For a threshold value of DO₂ > 9 ml/kg, Lac showed an inverse relationship with SvO₂ (R = -0.84; P < 0.01) and DO₂/VCO₂ ratio (R = -0.73; P < 0.01). Conversely, a direct correlation between Lac and RQ (R = 0.66; P < 0.01) was found. For a DO₂ < 9 ml/kg, SvO₂ did not correlate with Lac. Opposite, a direct correlation between Lac and RQ (R = 0.81; P < 0.01) was found. The DO₂/VCO₂ ratio showed an inverse relationship with Lac (R = -0.75; P < 0.01). ROC curves to predict Lac+ were constructed. The areas under the ROC curves were 0.40, 0.74, and 0.81 for SvO2, RQ, and DO2/VCO2 ratio, respectively. An optimal cutoff value of 3.1 (sensitivity = 0.70, specificity = 0.77) was determined for the DO₂/VCO₂ ratio predicting the presence of Lac+.

Conclusion

Our findings showed that, for a DO₂ > 9 ml/kg, the SVO₂, RQ, and DO₂/VCO₂ ratio may be used interchangeably. For a DO₂ < 9 ml/kg, the DO₂/VCO₂ ratio seems a more reliable predictor of AM than SvO₂ and RQ. The DO₂/VCO₂ ratio can be simply and quickly calculated at the bedside because pulse wave analysis allows the DO₂ to be frequently calculated, and because the CO₂ analyzer provides VCO₂ values continuously. Combined gas exchange and pulse wave monitoring might be a valuable and a useful approach to detect AM in trauma patients.