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

PURPOSE

Central venous minus arterial PCO to arterial minus central venous O content difference ratio (PCO/CO) has been proposed as a clinical surrogate for respiratory quotient. Our goal was to assess its interchangeability with mixed venous minus arterial PCO to arterial minus mixed venous O content difference ratio (PCO/CO).

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 PCO/CO and PCO/CO was evaluated by Bland and Altman analysis. We also performed linear regression analysis with PCO/CO as the dependent variable.

RESULTS

95% limits of agreement between PCO/CO and PCO/CO were 1.48. PCO/CO was significantly correlated with hemoglobin and lactate (R = 0.48 and 0.31, respectively, P < 0.01 for both).

CONCLUSIONS

In this study, PCO/CO and PCO/CO were not interchangeable. In addition, PCO/CO is a composite variable, which depends on several determinants. Values of PCO/CO should be cautiously interpreted in the assessment of critically ill patients.