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–aCO_2) to arteriovenous oxygen content difference (C_a–vO_2) might be a good surrogate for respiratory quotient (RQ). Yet P_v–aCO_2/C_a–vO_2 might be increased by other factors, regardless of anaerobic metabolism. At present, comparisons between P_v–aCO_2/C_a–vO_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–aCO_2 and decreased C_a–vO_2, our hypothesis was that P_v–aCO_2/C_a–vO_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–aCO_2/C_a–vO_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–aCO_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–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, P_v–aCO_2/C_a–vO_2 increased before the fall in oxygen consumption and the increase in RQ. P_v–aCO_2/C_a–vO_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–aCO_2/C_a–vO_2 determinants (adjusted R ^2 = 0.86, P  < 0.000001)

Conclusions

In hemodilution, P_v–aCO_2/C_a–vO_2 was considerably increased, irrespective of the presence of anaerobic metabolism. P_v–aCO_2/C_a–vO_2 is a complex variable, which depends on several factors. As such, it was a misleading indicator of anaerobic metabolism in hemodilution.