Venoarterial PCO₂-to-arteriovenous oxygen content difference ratio is a poor surrogate for anaerobic metabolism in...

The identification of anaerobic metabolism in critically ill patients is a challenging task. Observational studies have suggested that the ratio of venoarterial PCO₂ (P_v-aCO₂) to arteriovenous oxygen content difference (C_a-vO₂) might be a good surrogate for respiratory quotient (RQ). Yet P_v-aCO₂/C_a-vO₂ might be increased by other factors, regardless of anaerobic metabolism. At present, comparisons between P_v-aCO₂/C_a-vO₂ and RQ have not been performed. We sought to compare these variables during stepwise hemorrhage and hemodilution. Since anemia predictably produces augmented P_v-aCO₂ and decreased C_a-vO₂, our hypothesis was that P_v-aCO₂/C_a-vO₂ might be an inadequate surrogate for RQ.

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.

There were comparable reductions in oxygen consumption and increases in RQ in the last step of hemodilution and hemorrhage. The increase in P_v-aCO₂/C_a-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 P_v-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 C_a-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, P_v-aCO₂/C_a-vO₂ increased before the fall in oxygen consumption and the increase in RQ. P_v-aCO₂/C_a-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 P_v-aCO₂/C_a-vO₂ determinants (adjusted R ² = 0.86, P < 0.000001).

In hemodilution, P_v-aCO₂/C_a-vO₂ was considerably increased, irrespective of the presence of anaerobic metabolism. P_v-aCO₂/C_a-vO₂ is a complex variable, which depends on several factors. As such, it was a misleading indicator of anaerobic metabolism in hemodilution.