Mixed venous O 2 saturation: Measured by co-oximetry versus calculated from P $$\bar v$$ o 2

Abstract

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 \(\dot V\) 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 \(\bar v\) O₂ from P \(\bar v\) O₂). Data used at the time of each S \(\bar v\) O₂ measurement to calculate oxygen consumption ( \(\dot V\) O₂) further was collected.Results. Available for analysis were 118 mixed venous blood samples. Although the S \(\bar v\) O₂ values had a correlation coefficient of 0.807 (95% confidence interval [CI] 0.736 to 0.861, Fisher's z-transform), when \(\dot V\) 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 \(\bar v\) O₂s calculated by a blood gas machine—rather than those co-oximetrically measured—are used to calculate \(\bar v\) 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 measuredPO₂. We were not able to improve mathematically the sensitivity of any of the available regression equations used by blood gas analyzers to calculate S \(\bar v\) O₂ from P \(\bar v\) O₂. Therefore, it remains necessary to use co-oximetrically measured saturations when calculating \(\dot V\) O₂.