Central venous minus arterial carbon dioxide pressure to arterial minus central venous oxygen content ratio as an indicator of tissue oxygenation: a narrative review

ABSTRACT

The central venous minus arterial carbon dioxide pressure to arterial minus central venous oxygen content ratio (P_cv-aCO₂/C_a-cvO₂) has been proposed as a surrogate for respiratory quotient and an indicator of tissue oxygenation. Some small observational studies have found that a P_cv-aCO₂/C_a-cvO₂ > 1.4 was associated with hyperlactatemia, oxygen supply dependency, and increased mortality. Moreover, P_cv-aCO₂/C_a-cvO₂ has been incorporated into algorithms for tissue oxygenation evaluation and resuscitation. However, the evidence for these recommendations is quite limited and of low quality. The goal of this narrative review was to analyze the methodological bases, the pathophysiologic foundations, and the experimental and clinical evidence supporting the use of P_cv-aCO₂/C_a-cvO₂ as a surrogate for respiratory quotient. Physiologically, the increase in respiratory quotient secondary to critical reductions in oxygen transport is a life-threatening and dramatic event. Nevertheless, this event is easily noticeable and probably does not require further monitoring. Since the beginning of anaerobic metabolism is indicated by the sudden increase in respiratory quotient and the normal range of respiratory quotient is wide, the use of a defined cutoff of 1.4 for P_cv-aCO₂/C_a-cvO₂ is meaningless. Experimental studies have shown that P_cv-aCO₂/C_a-cvO₂ is more dependent on factors that modify the dissociation of carbon dioxide from hemoglobin than on respiratory quotient and that respiratory quotient and P_cv-aCO₂/C_a-cvO₂ may have distinct behaviors. Studies performed in critically ill patients have shown controversial results regarding the ability of P_cv-aCO₂/C_a-cvO₂ to predict outcome, hyperlactatemia, microvascular abnormalities, and oxygen supply dependency. A randomized controlled trial also showed that P_cv-aCO₂/C_a-cvO₂ is useless as a goal of resuscitation. P_cv-aCO₂/C_a-cvO₂ should be carefully interpreted in critically ill patients.

Keywords:
Anaerobiosis; Respiration; Oxygenation; Carbon dioxide; Respiratory quotient; Critical illness