MIXED VENOUS O-2 SATURATION - MEASURED BY CO-OXIMETRY VERSUS CALCULATED FROM P(V)OVER-BARO(2)

Objective. The objectives of our study were (1) to compare mixed venou s saturations calculated by a blood gas machine with those measured di rectly by a co-oximeter; and (2) to compare the sensitivities and spec ificities of VO(2)s derived from these values. Methods. Charts were re trospectively 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 venou s blood concurrently measured by both a co-oximeter (Co-Ox Model 482, Instrumentation Lab, Lexington, MA) and a blood gas analyzer (Nova Bio medical StatLab5, Waltham, MA) which uses a variant of the Severinghau s equation to calculate S(v) over barO$(2) from P(v) over barO$(2)). D ata used at the time of each S(v) over barO$(2) measurement to calcula te oxygen consumption (V(O)2) further was collected. Results. Availabl e for analysis were 118 mixed venous blood samples. Although the S(v) over barO$(2) values had a correlation coefficient of 0.807 (95% confi dence interval [CI] 0.736 to 0.861, Fisher's z-transform), when VO(2)s 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. A ttempts to mathematically improve upon the Severinghaus. equation and upon an additional four regression equations used by other blood gas a nalyzers resulted in universally worse sensitivity. Conclusion. If S(v ) over barO$(2)s calculated by a blood gas machine-rather than those c o-oximetrically measured-are used to calculate VO(2)s, 42% of patients with low O(2)s will be misclassified as normal and 11% of normals wil l be misclassified as low. This total error appears to be the result o f measurement error by the PO2 electrode of the blood gas analyzer and shifts of the oxyhemoglobin dissociation curve, which are not account ed for in the equation that is used to calculate saturation from measu red PO2. We were not able to improve mathematically the sensitivity of any of the available regression equations used by blood gas analyzers to calculate S(v) over barO$(2) from P(v) over barO$(2). Therefore, i t remains necessary to use co-oximetrically measured saturations when calculating VO2.