Oxygen saturation calculation procedures: a critical analysis of six equations for the determination of oxygen saturation.

Photometrically measured values of O2-saturation (SO2) were compared with SO2 predictions on the basis of the equations proposed by Heck, Kelman, Lutz, Marsoner, Severinghaus, and Siggaard-Andersen on 1350 occasions. Capillary, venous and mixed venous blood samples from 23 healthy subjects and 30 patients, suffering from cardio-pulmonary diseases were analyzed at rest, during exhaustion, limited exercise and during the recovery phase of ergometric tests. Overall the best agreements with the measured values were found for the equations of Kelman, Severinghaus and Siggaard-Andersen. The results of Heck's, Lutz', and Marsoner's predictions were in reasonable agreement with the measured SO2 only under special physiological conditions. No calculation mode can be performed with constant accuracy and reliability when covering a wide range of acid-base values. If SO2 values are used for further calculations, e.g. for determination of cardiac output, measured values are preferred.