Lack of correlation between central venous minus arterial PCO2 to arterial minus central venous O2 content ratio and respiratory quotient in patients with septic shock: A prospective observational study

Objective

Central venous-arterial PCO₂ to arterial-central venous O₂ content ratio (P_cv-aCO₂/C_a-cvO₂) is commonly used as a surrogate for respiratory quotient (RQ) and tissue oxygenation. Although P_cv-aCO₂/C_a-cvO₂ might be associated with hyperlactatemia and outcome, neither the interchangeability with RQ nor the correlation with conclusive variables of anaerobic metabolism has never been demonstrated in septic shock. Our goal was to compare P_cv-aCO₂/C_a-cvO₂ and RQ in patients with septic shock.

Design

Prospective, observational study.

Setting

Two adult ICUs.

Patients

Forty-seven patients with septic shock on mechanical ventilation with stable respiratory settings and vasopressor dose after initial resuscitation.

Interventions

None.

Main variables of interest

We measured arterial and central venous gases, Hb, and O₂Hb. P_cv-aCO₂/C_a-cvO₂ and the ratio of central venous-arterial CO₂ content to arterial-central venous O₂ content (C_cv-aCO₂/C_a-cvO₂) were calculated. RQ was determined by indirect calorimetry.

Results

P_cv-aCO₂/C_a-cvO₂ and C_cv-aCO₂/C_a-cvO₂ were not correlated with RQ (R² = 0.01, P = 0.50 and R² = 0.01, P = 0.58, respectively), showing large bias and wide 95 % limits of agreement with RQ (1.09, -1.10−3.27 and 0.42, -1.53−2.37). A multiple linear regression model showed Hb, and central venous PCO₂ and O₂Hb, but not RQ, as P_cv-aCO₂/C_a-cvO₂ determinants (R² = 0.36, P = 0.0007).

Conclusions

In patients with septic shock, P_cv-aCO₂/C_a-cvO₂ did not correlate with RQ and was mainly determined by factors that modify the dissociation of CO₂ from Hb. P_cv-aCO₂/C_a-cvO₂ seems to be a poor surrogate for RQ; therefore, its values should be interpreted with caution.