Venoarterial PCO2-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-vO2 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.