Central venous-to-arterial carbon dioxide difference: an additional target for goal-directed therapy in septic shock?

Objective: To test the hypothesis that, in resuscitated septic shock patients, central venous-to-arterial carbon dioxide difference [P(cv-a)CO2] may serve as a global index of tissue perfusion when the central venous oxygen saturation (ScvO(2)) goal value has already been reached. Design: Prospective observational study. Setting: A 22-bed intensive care unit (ICU). Patients: After early resuscitation in the emergency unit, 50 consecutive septic shock patients with ScvO(2) > 70% were included immediately after their admission into the ICU (T0). Patients were separated in Low P(cv-a)CO2 group (Low gap; n = 26) and High P(cv-a)CO2 group (High gap; n = 24) according to a threshold of 6 mmHg at T0. Measurements: Measurements were performed every 6 h over 12 h (T0, T6, T12). Results: At T0, there was a significant difference between Low gap patients and High gap patients for cardiac index (CI) (4.3 +/- 1.6 vs. 2.7 +/- 0.8 l/min/m2, P < 0.0001) but not for ScvO(2) values (78 +/- 5 vs. 75 +/- 5%, P = 0.07). From T0 to T12, the clearance of lactate was significantly larger for the Low gap group than for the High gap group (P < 0.05) as well as the decrease of SOFA score at T24 (P < 0.01). At T0, T6 and T12, CI and P(cv-a)CO2 values were inversely correlated (P < 0.0001). In ICU-resuscitated patients, targeting only ScvO(2) may not be sufficient to guide therapy. When the 70% ScvO(2) goal-value is reached, the presence of a P(cv-a)CO2 larger than 6 mmHg might be a useful tool to identify patients who still remain inadequately resuscitated.