Shock is a life-threatening condition, and its timely recognition is essential for adequate management. Pediatric patients with congenital heart disease admitted to a cardiac intensive care unit (CICU) after surgical corrections are particularly at risk of low cardiac output syndrome (LCOS) and shock. Blood lactate levels and venous oxygen saturation (ScVO2) are usually used as shock biomarkers to monitor the efficacy of resuscitation efforts, but they are plagued by some limitations. Carbon dioxide (CO2)-derived parameters, namely veno-arterial CO2difference (ΔCCO2) and the VCO2/VO2ratio, may represent a potentially valuable addition as sensitive biomarkers to assess tissue perfusion and cellular oxygenation and may represent a valuable addition in shock monitoring. These variables have been mostly studied in the adult population, with a strong association between ΔCCO2or VCO2/VO2ratio and mortality. In children, particularly in CICU, few studies looked at these parameters, while they reported promising results on the use of CO2-derived indices for patients’ management after cardiac surgeries. This review focuses on the physiological and pathophysiological determinants of ΔCCO2and VCO2/VO2ratio while summarizing the actual state of knowledge on the use of CO2-derived indices as hemodynamical markers in CICU.