Relation between Mixed Venous Oxygen Saturation and Cardiac Index: Nonlinearity and Normalization for Oxygen Uptake and Hemoglobin

Abstract

The ability of mixed venous oxygen saturation (SvO₂) monitoring to reflect changes in cardiac index (CI) with therapy in critically ill patients is unclear. To this end, SvO₂and CI were measured before and during an infusion of enoximone and/or dobutamine in 30 patients with advanced heart failure. A nonlinear relationship was observed between SvO₂ and CI with the nonlinear correlation coefficient being 0.52. On normalizing for individual differences in hemoglobin and oxygen consumption, this correlation coefficient became 0.90. Further analysis of individual data was performed using linear regression, and the slopes and correlation coefficients were found to span a wide range (slope: −10.0 to 30.9 min-mVL, r: −0.27 to 0.99). However, the mean slope and correlation coefficient for patients with baseline CI and SvO₂ <21/min/m* and <55 percent were 18.3 min-m²/L and 0.87, respectively, while those for the remainder of patients were only 3.1 min-m²/L and 0.42, respectively. Thus, the nonlinear correlation coefficient of the SvO₂-CI relationship in a group of patients is dependent on the homogeneity of their oxygen consumption and hemoglobin concentration. Furthermore, the ability of SvO₂to serve as a therapeutic indicator in any given patient is dependent on baseline SvO₂ and CI.