Poor agreement in the calculation of venoarterial PCO2 to arteriovenous O2 content difference ratio using central and mixed venous blood samples in septic patients

Abstract

Purpose: Central venous minus arterial PCO₂ to arterial minus central venous O₂ content difference ratio (P_cv-aCO₂/C_a-cvO₂) has been proposed as a clinical surrogate for respiratory quotient. Our goal was to assess its interchangeability with mixed venous minus arterial PCO₂ to arterial minus mixed venous O₂ content difference ratio (P_mv-aCO₂/C_a-mvO₂).

Materials and methods: This is a subanalysis of a previously published study. We studied 23 septic patients who had an indwelling Swan-Ganz catheter. The agreement between P_cv-aCO₂/C_a-cvO₂ and P_mv-aCO₂/C_a-mvO₂ was evaluated by Bland and Altman analysis. We also performed linear regression analysis with P_mv-aCO₂/C_a-mvO₂ as the dependent variable.

Results: 95% limits of agreement between P_cv-aCO₂/C_a-cvO₂ and P_mv-aCO₂/C_a-mvO₂ were 1.48. P_mv-aCO₂/C_a-mvO₂ was significantly correlated with hemoglobin and lactate (R² = 0.48 and 0.31, respectively, P < 0.01 for both).

Conclusions: In this study, P_cv-aCO₂/C_a-cvO₂ and P_mv-aCO₂/C_a-mvO₂ were not interchangeable. In addition, P_mv-aCO₂/C_a-mvO₂ is a composite variable, which depends on several determinants. Values of P_cv-aCO₂/C_a-cvO₂ should be cautiously interpreted in the assessment of critically ill patients.

Keywords: Arteriovenous oxygen content difference; Central venous blood; Hemoglobin; Mixed venous blood; Venoarterial PCO(2).