Central venous and mixed venous oxygen saturation in critically ill patients

Although mixed venous O2 saturation (SvO2) accurately indicates the balance of O2 supply/demand and provides an index of tissue oxygenation, the use of a pulmonary artery (PA) catheter is associated with significant costs, risks and complications. Central venous O2 saturation (ScvO2), obtained in a less risky and costly manner, can be an attractive alternative to SvO2.

To investigate whether the values of ScvO2 and SvO2 are well correlated and interchangeable in the evaluation of critically ill ICU patients and to create an equation that could estimate SvO2 from ScvO2.

Sixty-one mechanically ventilated patients were catheterized upon admission and ScvO2 and SvO2 values were simultaneously measured in the lower part of the superior vena cava and PA respectively.

SvO2 was 68.6 +/- 1.2% (mean +/- SEM) and ScvO2 was 69.4 +/- 1.1%. The difference is statistically significant (p < 0.03). The correlation coefficient r is 0.945 for the total population, 0.937 and 0.950 in surgical and medical patients, respectively. In 90.2% of patients the difference was <5%. When regression analysis was performed, among 11 models tested, power model [SvO2 = b0(ScvO2)b1] best described the relationship between the two parameters (R2 = 0.917).

ScvO2 and SvO2 are closely related and are interchangeable for the initial evaluation of critically ill patients even if cardiac indices are different. SvO2 can be estimated with great accuracy by ScvO2 in 92% of the patients using a power model.