Poor agreement in the calculation of venoarterial PCO 2 to arteriovenous O 2 content difference ratio using central and mixed venous blood samples in septic patients

Central venous minus arterial PCO to arterial minus central venous O content difference ratio (P CO /C O ) 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 (P CO /C O ). 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 CO /C O and P CO /C O was evaluated by Bland and Altman analysis. We also performed linear regression analysis with P CO /C O as the dependent variable. 95% limits of agreement between P CO /C O and P CO /C O were 1.48. P CO /C O was significantly correlated with hemoglobin and lactate (R = 0.48 and 0.31, respectively, P