Oxygen delivery to carbon dioxide production ratio for continuously detecting anaerobic metabolism in trauma patients
© BioMed Central Ltd. 2007
Published: 22 March 2007
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.
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.
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+.
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.