Respiratory quotient estimations as additional prognostic tools in early septic shock

Central venous-to-arterial carbon dioxide difference (P_cvaCO₂), and its correction by the arterial-to-venous oxygen content difference (P_cvaCO₂/C_avO₂) 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 C_cvaCO₂/C_avO₂. The aim of the present study was to explore the factors that might intervene in the difference between P_cvaCO₂/C_avO₂ and C_cvaCO₂/C_avO₂, 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 P_cvaCO₂/C_avO₂ and C_cvaCO₂/C_avO₂ values were correlated, but agreement analysis showed a significant proportional bias. The difference between P_cvaCO₂/C_avO₂ and C_cvaCO₂/C_avO₂ was independently associated with pH, S_cvO₂, baseline C_cvaCO₂/C_avO₂ 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, P_cvaCO₂/C_avO₂, and the difference between P_cvaCO₂/C_avO₂ and C_cvaCO₂/C_avO₂. Initial S_cvO₂, P_cvaCO₂, C_cvaCO₂/C_avO₂, and cardiac index were not different in survivors and non-survivors. In a population of early septic shock patients, simultaneous values of P_cvaCO₂/C_avO₂ and C_cvaCO₂/C_avO₂ were not equivalent, and the main determinant of the magnitude of the difference between these two parameters was pH. The P_cvaCO₂/C_avO₂ ratio was associated with ICU mortality, whereas C_cvaCO₂/C_avO₂ was not.