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 CO2 content and tension differences to arteriovenous oxygen content difference ratios (Delta ContCO(2)/Delta ContO(2) and Delta PCO2/Delta 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 (VO2) after an acute increase in oxygen supply (DO2) induced by volume expansion (VO2/DO2 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, VO2, DO2, Delta ContCO(2)/Delta ContO(2) and Delta PCO2/Delta 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 %). DO2 increased significantly (31 +/- 12 %) in these patients. An increase in VO2 = 15 % (VO2-responders) concurrently occurred in 57 % of the 51 fluid-responders (45 +/- 16 %). Compared with VO2-non-responders, VO2-responders were characterized by higher lactate levels and higher Delta ContCO(2)/Delta ContO(2) and Delta PCO2/Delta ContO(2) ratios. At baseline, lactate predicted a fluid-induced increase in VO2 = 15 % with AUC of 0.745. Baseline Delta ContCO(2)/Delta ContO(2) and Delta PCO2/Delta ContO(2) ratios predicted an increase of VO2 = 15 % with AUCs of 0.965 and 0.962, respectively. Baseline ScvO(2) was not able to predict an increase of VO2 = 15 % (AUC = 0.624). Conclusions: Delta ContCO(2)/Delta ContO(2) and Delta PCO2/Delta ContO(2) ratios are more reliable markers of global anaerobic metabolism than lactate. ScvO2 failed to predict the presence of global tissue hypoxia.