Trends but not individual values of central venous oxygen saturation agree with mixed venous oxygen saturation during varying hemodynamic conditions

Background: Previous studies found contradictory results regarding the question whether mixed venous oxygen saturation (Svo(2)) and central venous oxygen saturation (Scvo(2)) are equivalent. The inconsistency of study results may result from different study designs and different, partly questionable, statistical approaches. Methods: The authors performed a prospective clinical trial comparing individual oxygen saturation values as well as the trend of values in blood from the superior vena cava (Scvo(2)), the right atrium (Srao(2)), and the pulmonary artery (Svo(2)) during varying hemodynamic situations. The subjects were 70 patients scheduled to undergo elective neurosurgical operations in the sitting position. Oxygen saturation was measured photospectrometrically in blood samples simultaneously taken at four different time points during supine and sitting positions. Statistical analysis was performed following the recommendations of Bland and Altman. Results: Five hundred two comparative sets of measurements were obtained. Ninety-five percent limits of agreement ranging from +/- 6.83 to +/- 9-30% for single values were interpreted as clinically unacceptable. in contrast, correlations between changes of Svo(2) and Scvo(2) as well as of Svo(2) and Srao(2) were interpreted as clinically acceptable (R >= 0.755, Pearson correlation coefficient; P <= 0.0001). Conclusions: In this sample of patients, exact numerical values of Scvo(2) and Srao(2) are not equivalent to those of Svo(2) in varying hemodynamic conditions. However, for clinical purposes, the trend of Scvo(2) may be substituted for the trend of Svo(2). In addition, previous studies investigating the agreement between Svo(2) and Scvo(2) were found to be lacking in their chosen statistical approaches.