Venoarterial PCO(2)-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 PCO(2) (P(v–a)CO(2)) to arteriovenous oxygen content difference (C(a–v)O(2)) might be a good surrogate for respiratory quotient (RQ). Yet P(v–a)CO(2)/C(a–v)O(2) might be increased by other factors, regardless of anaerobic metabolism. At present, comparisons between P(v–a)CO(2)/C(a–v)O(2) and RQ have not been performed. We sought to compare these variables during stepwise hemorrhage and hemodilution. Since anemia predictably produces augmented P(v–a)CO(2) and decreased C(a–v)O(2), our hypothesis was that P(v–a)CO(2)/C(a–v)O(2) 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 P(v–a)CO(2)/C(a–v)O(2) 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 P(v–a)CO(2) was lower in hemodilution (6 ± 0 to 10 ± 1 vs. 6 ± 0 to 17 ± 1 mmHg, P < 0.0001). Venoarterial CO(2) content difference and C(a–v)O(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, P(v–a)CO(2)/C(a–v)O(2) increased before the fall in oxygen consumption and the increase in RQ. P(v–a)CO(2)/C(a–v)O(2) 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 PCO(2) as P(v–a)CO(2)/C(a–v)O(2) determinants (adjusted R (2) = 0.86, P < 0.000001). CONCLUSIONS: In hemodilution, P(v–a)CO(2)/C(a–v)O(2) was considerably increased, irrespective of the presence of anaerobic metabolism. P(v–a)CO(2)/C(a–v)O(2) is a complex variable, which depends on several factors. As such, it was a misleading indicator of anaerobic metabolism in hemodilution.