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.