Mixed venous O₂ saturation: Measured by co-oximetry versus calculated from P {Mathematical expression}o₂

Objective. The objectives of our study were (1) to compare mixed venous saturations calculated by a blood gas machine with those measured directly by a co-oximeter; and (2) to compare the sensitivities and specificities of {Mathematical expression}o₂s derived from these values. Methods. Charts were retrospectively reviewed of all MICU patients [n=16] between December 1, 1991 and January 31, 1992, who required pulmonary artery catheters for their usual care and who had hemoglobin saturations of mixed venous blood concurrently measured by both a co-oximeter (Co-Ox Model 482, Instrumentation Lab, Lexington, MA) and a blood gas analyzer (Nova Biomedical StatLab5, Waltham, MA) which uses a variant of the Severinghaus equation to calculate S {Mathematical expression}O₂ from P {Mathematical expression}O₂). Data used at the time of each S {Mathematical expression}O₂ measurement to calculate oxygen consumption ( {Mathematical expression}O₂) further was collected. Results. Available for analysis were 118 mixed venous blood samples. Although the S {Mathematical expression}O₂ values had a correlation coefficient of 0.807 (95% confidence interval [CI] 0.736 to 0.861, Fisher's z-transform), when {Mathematical expression}O₂s were calculated, the blood gas analyzer calculated saturations had a sensitivity of only 58.3% and a specificity of 89%, when compared with those calculated using the saturations measured by the co-oximeter. Attempts to mathematically improve upon the Severinghaus equation and upon an additional four regression equations used by other blood gas analyzers resulted in universally worse sensitivity. Conclusion. If S {Mathematical expression}O₂s calculated by a blood gas machine-rather than those co-oximetrically measured-are used to calculate {Mathematical expression}O₂s, 42% of patients with low O₂s will be misclassified as normal and 11% of normals will be misclassified as low. This total error appears to be the result of measurement error by the Po₂ electrode of the blood gas analyzer and shifts of the oxyhemoglobin dissociation curve, which are not accounted for in the equation that is used to calculate saturation from measured PO₂. We were not able to improve mathematically the sensitivity of any of the available regression equations used by blood gas analyzers to calculate S {Mathematical expression}O₂ from P {Mathematical expression}O₂. Therefore, it remains necessary to use co-oximetrically measured saturations when calculating {Mathematical expression}O₂.