Range of accuracy of two wavelength oximetry

Earlier reports of a two wavelength oximeter suggested a tendency toward overestimation of arterial oxygen saturation (SaO2) at the lowest values examined. To investigate this possible inaccuracy, we compared oximeter readings to SaO2 over a wider range of laboratory-induced steady-state hypoxia than has previously been reported. For values of SaO2 greater than 75 percent, oximeter readings were linearly related to SaO2 such that y = 0.94 SaO2 + 8.7 (r = 0.96) and the mean difference between oximeter reading and SaO2 was 0.09 percent. However, when SaO2 was less than 75 percent, the instrument readings were progressively higher than the SaO2 so that when SaO2 was approximately 50 percent, the oximeter reading was approximately 65 percent. These data show that the two wavelength oximeter displays falsely elevated readings when the arterial saturation falls below 75 percent.