Venoarterial PCO2-to-arteriovenous oxygen content difference ratio is a poor surrogate for anaerobic metabolism in hemodilution: an experimental study

Arnaldo Dubin; Gonzalo Ferrara; Vanina Siham Kanoore Edul; Enrique Martins; Héctor Saúl Canales; Carlos Canullán; Gastón Murias; Mario Omar Pozo; Elisa Estenssoro

doi:10.1186/s13613-017-0288-z

Venoarterial PCO₂-to-arteriovenous oxygen content difference ratio is a poor surrogate for anaerobic metabolism in hemodilution: an experimental study

Ann Intensive Care. 2017 Dec;7(1):65. doi: 10.1186/s13613-017-0288-z. Epub 2017 Jun 12.

Authors

Arnaldo Dubin¹, Gonzalo Ferrara², Vanina Siham Kanoore Edul², Enrique Martins², Héctor Saúl Canales², Carlos Canullán², Gastón Murias², Mario Omar Pozo², Elisa Estenssoro²

Affiliations

¹ Cátedra de Farmacología Aplicada, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, Calle 42 Nº 577, 60 y 120, 1900, La Plata, Argentina. arnaldodubin@gmail.com.
² Cátedra de Farmacología Aplicada, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, Calle 42 Nº 577, 60 y 120, 1900, La Plata, Argentina.

Abstract

Background: 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.

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-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).

Conclusions: 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.

Keywords: Anaerobic metabolism; Carbon dioxide; Hemodilution; Hemorrhage; Oxygen; Respiratory quotient.