Accuracy of indirect estimation of oxyhaemoglobin saturation in patients with respiratory failure

The indirect estimate of oxyhaemoglobin saturation (Sa,O2) is largely used in the management of patients with respiratory failure. The aim of this study was to evaluate the accuracy of the Sa,O2 values calculated by using both the Siggaard-Andersen (SA) and the Severinghaus (SE) equation, in comparison with the real Sa,O2 values measured by a co-oximeter. A total of 558 arterial blood samples were analysed. On average, only a slight overestimation was found for Sa,O2 estimated by the SA (2.35 +/- 5.75%) and the SE (2.37 +/- 5.65%) equations in comparison with the real values. However, a difference higher than 5% between estimated and real Sa,O2 values was demonstrated in more than 20% of the blood samples evaluated. This difference was higher than 10% in 9% of the samples, with similar results obtained with the two equations. The discrepancy between real and estimated values, probably due, at least in part, to errors in measurement of arterial oxygen tension (Pa,O2) and pH, was maximally evident for Sa,O2 values lower than 70%. An indirect estimate of Sa,O2 is not homogeneously accurate, and the clinical consequence of this finding might be especially dangerous in monitoring patients with severe respiratory failure.