Combination of O2 and CO2-derived variables to detect tissue hypoxia in the critically ill patient.

Oxygen-derived parameters have been traditionally used to guide resuscitation during shock states. Nevertheless, normalization of venous oxygen saturation does not exclude the persistence of tissue hypoperfusion and tissue hypoxia. Combination of O2 and CO2-derived variables has consistently demonstrated to be related with clinical outcomes and its variations could anticipate changes in lactate and also predict fluid responsiveness in terms of oxygen consumption. Here we discuss the potential mechanisms leading to increase the venous-to-arterial CO2 (Cv-aCO2) to arterial-to-venous O2 content difference (Ca-vO2), i.e., the Cv-aCO2/Ca-vO2 ratio, its potential clinical application, limitations and uncertainties. Finally, although biologically plausible, the potential applications of the Cv-aCO2/Ca-vO2 ratio in the clinical practice require to be confirmed.