Venoarterial PCO-to-arteriovenous oxygen content difference ratio is a poor surrogate for anaerobic metabolism in hemodilution: an experimental study.

Background: The identification of anaerobic metabolism in critically ill patients is a challenging task. Observational studies have suggested that the ratio of venoarterial PCO2 (Pv-aCO2) to arteriovenous oxygen content difference (Ca-vO2) might be a good surrogate for respiratory quotient (RQ). Yet Pv-aCO2/Ca-vO2 might be increased by other factors, regardless of anaerobic metabolism. At present, comparisons between Pv-aCO2/Ca-vO2 and RQ have not been performed. We sought to compare these variables during stepwise hemorrhage and hemodilution. Since anemia predictably produces augmented Pv-aCO2 and decreased Ca-vO2, our hypothesis was that Pv-aCO2/Ca-vO2 might be an inadequate surrogate for RQ. Methods: This is a subanalysis of a previously published study. In anesthetized and mechanically ventilated sheep (n = 16), we compared the effects of progressive hemodilution and hemorrhage by means of expired gases analysis. Results: There were comparable reductions in oxygen consumption and increases in RQ in the last step of hemodilution and hemorrhage. The increase in Pv-aCO2/Ca-vO2 was higher in hemodilution than in hemorrhage (1.9 +/- 0.2 to 10.0 +/- 0.9 vs. 1.7 +/- 0.2 to 2.5 +/- 0.1, P < 0.0001). The increase in Pv-aCO2 was lower in hemodilution (6 +/- 0 to 10 +/- 1 vs. 6 +/- 0 to 17 +/- 1 mmHg, P < 0.0001). Venoarterial CO2 content difference and Ca-vO(2) decreased in hemodilution and increased in hemorrhage (2.6 +/- 0.3 to 1.2 +/- 0.1 vs. 2.8 +/- 0.2 to 6.9 +/- 0.5, and 3.4 +/- 0.3 to 1.0 +/- 0.3 vs. 3.6 +/- 0.3 to 6.8 +/- 0.3 mL/dL, P < 0.0001 for both). In hemodilution, Pv-aCO2/Ca-vO2 increased before the fall in oxygen consumption and the increase in RQ. Pv-aCO2/Ca-vO2 was strongly correlated with Hb (R-2 = 0.79, P < 0.00001) and moderately with RQ (R-2 = 0.41, P < 0.0001). A multiple linear regression model found Hb, RQ, base excess, and mixed venous oxygen saturation and PCO2 as Pv-aCO2/Ca-vO2 determinants (adjusted R-2 = 0.86, P < 0.000001). Conclusions: In hemodilution, Pv-aCO2/Ca-vO2 was considerably increased, irrespective of the presence of anaerobic metabolism. Pv-aCO2/Ca-vO2 is a complex variable, which depends on several factors. As such, it was a misleading indicator of anaerobic metabolism in hemodilution.