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

Objective: Central venous-arterial PCO to arterial-central venous O content ratio (PCO/CO) is commonly used as a surrogate for respiratory quotient (RQ) and tissue oxygenation. Although PCO/CO 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 PCO/CO 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 OHb. PCO/CO and the ratio of central venous-arterial CO content to arterial-central venous O content (CCO/CO) were calculated. RQ was determined by indirect calorimetry.

Results: PCO/CO and CCO/CO were not correlated with RQ (R = 0.01, P = 0.50 and R = 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 PCO and OHb, but not RQ, as PCO/CO determinants (R = 0.36, P = 0.0007).

Conclusions: In patients with septic shock, PCO/CO did not correlate with RQ and was mainly determined by factors that modify the dissociation of CO from Hb. PCO/CO seems to be a poor surrogate for RQ; therefore, its values should be interpreted with caution.