Cardiac index estimation using central venous oxygen saturation (Scv O₂) in critical illness

Background

The use of PA catheter is associated with increased costs and risks to the patients. Central venous (superior vena cava) oxygen saturation obtained by a less risky and costly manner can be used to estimate Sv O₂ with great accuracy up to 92% according to our results.

Objective

The aim of this study was to find whether estimated values of Sv O₂ can estimate with accuracy cardiac index using Fick method in comparison with thermodilution in critically ill patients.

Material and methods

Sixty-one critically ill patients were catheterized (Opticath PA Catheter P7110 Abbot) upon their admission and the values of Sv O₂, Scv O₂ and CI were simultaneously measured.

Results

The value of Sv O₂ was 68.6% ± 1.1 (X ± SEM), the value of Scv O₂ was 69.4% ± 1.1, pearson correlation coefficient was r = 0.95 and r ² = 0.89 with standard error 3.01. The power model of regression analysis which has the expression Sv O₂ = b0 (Scv O₂)^b1 [in our patients Sv O₂ = 1.1612 (Scv O₂)^0.9617] was found to better describe the relation between the two variables with value of r ² = 0.92. That means we can estimate the value of Sv O₂ from the value of Scv O₂ with accuracy 92%. We used estimated values of Sv O₂ with power model in Fick equation to estimate CI values and we correlated the estimated CI values with thermodilution CI values. Thermodilution CI values were 3.93 ± 0.17 (range = 1.85-9.3) and Fick method CI values were 3.88 ± 0.18 (range = 1.3-9.76). Pearson correlation coefficient between them was r = 0.86 and r ² = 0.73 with high significance (P < 0.001).

Discussion-conclusion

Scv O₂ can be used as a mirror of Sv O₂ for the initial evaluation of critically ill patients with value of r = 0.95 and r ² = 0.89. These results are in accordance with other authors [1,2]. However, some others found that there is poor correlation and questioned the usefulness of Scv O₂ measurement [3]. Power model of regression analysis which has the expression Sv O₂ = 1.1612 (Scv O₂)^0.9617 can estimate the value of Sv O₂ from the value of Scv O₂ with accuracy 92% (r ² = 0.92). The use of estimated (with power model) value of Sv O₂ in Fick equation can estimate the value of CI with accuracy 86% in comparison with thermodilution (value of r = 0.86).