Comparison of calculated and measured mixed venous oxygen saturation in critically ill patients.

Objective: According to the Fick principle the calculation of cardiac output and shunts is based on the exact determination of oxygen saturation. In this study an intraindividual comparison of two frequently applied methods was done.

Design and setting: The calculated saturation by measuring partial oxygen pressure was compared with a gold standard, the reflectometrically measured saturation.

Patients: 75 patients of an intensive care unit in whom a Swan-Ganz catheter was introduced to answer clinical questions.

Results: Overall, most calculated values were higher than the measured ones, the mean difference was 5.9% (+/- 8.03%), ranging from -10.4 to +37%. In lower saturations (< 65%) the difference between both estimated values was larger than in the range above 65%.

Conclusions: Especially when handling with lower oxygen saturations, calculation of saturation with an automatic gas analyzer leads to overestimated values. The Fick principle can only be applied for hemodynamic calculations when oxygen saturation can be determined by an easy and correct method.