Respiratory quotient estimations as additional prognostic tools in early septic shock.

Central venous-to-arterial carbon dioxide difference (PcvaCO2), and its correction by the arterial-to-venous oxygen content difference (PcvaCO2/CavO2) have been proposed as additional tools to evaluate tissue hypoxia. Since the relationship between pressure and content of CO2 (CCO2) might be affected by several factors, some authors advocate for the use of CcvaCO2/CavO2. The aim of the present study was to explore the factors that might intervene in the difference between PcvaCO2/CavO2 and CcvaCO2/CavO2, and to analyze their association with mortality. Observational study in a 30-bed mixed ICU. Fifty-two septic shock patients within the first 24 h of ICU admission were studied. After restoration of mean arterial pressure, hemodynamic and metabolic parameters were evaluated. A total of 110 sets of measurements were performed. Simultaneous PcvaCO2/CavO2 and CcvaCO2/CavO2 values were correlated, but agreement analysis showed a significant proportional bias. The difference between PcvaCO2/CavO2 and CcvaCO2/CavO2 was independently associated with pH, ScvO2, baseline CcvaCO2/CavO2 and hemoglobin. A stepwise regression analysis showed that pH was the single best predictor for the magnitude of such difference, with very limited effect of other variables. At inclusion, variables associated with ICU-mortality were lactate, pH, PcvaCO2/CavO2, and the difference between PcvaCO2/CavO2 and CcvaCO2/CavO2. Initial ScvO2, PcvaCO2, CcvaCO2/CavO2, and cardiac index were not different in survivors and non-survivors. In a population of early septic shock patients, simultaneous values of PcvaCO2/CavO2 and CcvaCO2/CavO2 were not equivalent, and the main determinant of the magnitude of the difference between these two parameters was pH. The PcvaCO2/CavO2 ratio was associated with ICU mortality, whereas CcvaCO2/CavO2 was not.