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, p2- 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%, pConclusion: 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.