Combination of venoarterial PCO₂ difference with arteriovenous O₂ content difference to detect anaerobic metabolism in patients

Abstract.

Objective: Under conditions of tissue hypoxia total CO₂ production (VCO₂) should be less reduced than O₂ consumption (VO₂) since an anaerobic CO₂ production should occur. Thus the VCO₂/VO₂ ratio, and hence the venoarterial CO₂ tension difference/arteriovenous O₂ content difference ratio (ΔPCO₂/C_(a-v)O₂), should increase. We tested the value of the ΔPCO₂/C_(a-v)O₂ ratio in detecting the presence of global anaerobic metabolism as defined by an increase in arterial lactate level above 2 mmol/l (Lac⁺). Design and setting: Retrospective study over a 17-month period in medical intensive care unit of a university hospital. Patients: We obtained 148 sets of measurements in 89 critically ill patients monitored by a pulmonary artery catheter. Results: The ΔPCO₂/C_(a-v)O₂ ratio was higher in those with increased (n=73) than in the normolactatemic group (2.0±0.9 vs. 1.1±0.6, p<0.0001). Among all the O₂- and CO₂-derived parameters the ΔPCO₂/C_(a-v)O₂ ratio had the highest correlation with the arterial lactate level (r=0.57). Moreover, for a threshold value of 1.4 the ΔPCO₂/C_(a-v)O₂ ratio predicted significantly better than the other parameters (receiver operating characteristic curves) the presence of hyperlactatemia (positive and negative predictive values of 86% and 80%, respectively). The overall survival estimate at 1 month was greater when the ΔPCO₂/C_(a-v)O₂ ratio was less than 1.4 on the first set of measurements (38±10% vs. 20±8%, p<0.01). Conclusion: The ΔPCO₂/C_(a-v)O₂ ratio seems a reliable marker of global anaerobic metabolism. Its calculation would be helpful for a better interpretation of pulmonary artery catheter data.