Central venous-to-arterial PCO₂ difference as a marker to identify fluid responsiveness in septic shock

Defining the hemodynamic response to volume therapy is integral to managing critically ill patients with acute circulatory failure, especially in the absence of cardiac index (CI) measurement. This study aimed at investigating whether changes in central venous-to-arterial CO₂ difference (Δ-ΔPCO₂) and central venous oxygen saturation (ΔScvO₂) induced by volume expansion (VE) are reliable parameters to define fluid responsiveness in sedated and mechanically ventilated septic patients. We prospectively studied 49 critically ill septic patients in whom VE was indicated because of circulatory failure and clinical indices. CI, ΔPCO₂, ScvO₂, and oxygen consumption (VO₂) were measured before and after VE. Responders were defined as patients with a > 10% increase in CI (transpulmonary thermodilution) after VE. We calculated areas under the receiver operating characteristic curves (AUCs) for Δ-ΔPCO₂, ΔScvO₂, and changes in CI (ΔCI) after VE in the whole population and in the subgroup of patients with an increase in VO₂ (ΔVO₂) ≤ 10% after VE (oxygen-supply independency). Twenty-five patients were fluid responders. In the whole population, Δ-ΔPCO₂ and ΔScvO₂ were significantly correlated with ΔCI after VE (r =  - 0.30, p = 0.03 and r = 0.42, p = 0.003, respectively). The AUCs for Δ-ΔPCO₂ and ΔScvO₂ to define fluid responsiveness (increase in CI > 10% after VE) were 0.76 (p < 0.001) and 0.68 (p = 0.02), respectively. In patients with ΔVO₂ ≤ 10% (n = 36) after VE, the correlation between ΔScvO₂ and ΔCI was 0.62 (p < 0.001), and between Δ-ΔPCO₂ and ΔCI was - 0.47 (p = 0.004). The AUCs for Δ-ΔPCO₂ and ΔScvO₂ were 0.83 (p < 0.001) and 0.73 (p = 0.006), respectively. In these patients, Δ-ΔPCO₂ ≤ -37.5% after VE allowed the categorization between responders and non-responders with a positive predictive value of 100% and a negative predictive value of 60%. In sedated and mechanically ventilated septic patients with no signs of tissue hypoxia (oxygen-supply independency), Δ-ΔPCO₂ is a reliable parameter to define fluid responsiveness.