Ratios of central venous-to-arterial carbon dioxide content or tension to arteriovenous oxygen content are better markers of global anaerobic metabolism than lactate in septic shock patients

Background

To evaluate the ability of the central venous-to-arterial CO_2 content and tension differences to arteriovenous oxygen content difference ratios (∆ContCO_2/∆ContO_2 and ∆PCO_2/∆ContO_2, respectively), blood lactate concentration, and central venous oxygen saturation (ScvO_2) to detect the presence of global anaerobic metabolism through the increase in oxygen consumption (VO_2) after an acute increase in oxygen supply (DO_2) induced by volume expansion (VO_2/DO_2 dependence)

Methods

We prospectively studied 98 critically ill mechanically ventilated patients in whom a fluid challenge was decided due to acute circulatory failure related to septic shock. Before and after volume expansion (500 mL of colloid solution), we measured cardiac index, VO_2, DO_2, ∆ContCO_2/∆ContO_2 and ∆PCO_2/∆ContO_2 ratios, lactate, and ScvO_2. Fluid-responders were defined as a ≥15 % increase in cardiac index. Areas under the receiver operating characteristic curves (AUC) were determined for these variables

Results

Fifty-one patients were fluid-responders (52 %). DO_2 increased significantly (31 ± 12 %) in these patients. An increase in VO_2 ≥ 15 % (“VO_2-responders”) concurrently occurred in 57 % of the 51 fluid-responders (45 ± 16 %). Compared with VO_2-non-responders, VO_2-responders were characterized by higher lactate levels and higher ∆ContCO_2/∆ContO_2 and ∆PCO_2/∆ContO_2 ratios. At baseline, lactate predicted a fluid-induced increase in VO_2 ≥ 15 % with AUC of 0.745. Baseline ∆ContCO_2/∆ContO_2 and ∆PCO_2/∆ContO_2 ratios predicted an increase of VO_2 ≥ 15 % with AUCs of 0.965 and 0.962, respectively. Baseline ScvO_2 was not able to predict an increase of VO_2 ≥ 15 % (AUC = 0.624)

Conclusions

∆ContCO_2/∆ContO_2 and ∆PCO_2/∆ContO_2 ratios are more reliable markers of global anaerobic metabolism than lactate. ScvO_2 failed to predict the presence of global tissue hypoxia.