['To determine the accuracy of continuous (in vivo) measurement of mixed venous oxygen saturation (SvO(2)), using a fibreoptic catheter, in patients having had cardiopulmonary bypass (CPB).', "Using a pulmonary arterial catheter, we prospectively studied 14 patients (age 64 +/- 8) having had cardiopulmonary bypass. Mean hematocrit was 30 +/- 4%. The catheter was calibrated in vitro and in vivo, according to the manufacturer's instructions. Fifty-six simultaneous measurements of continuous SvO(2) (CSvO(2)) and measured SvO(2) (MSvO(2)) were taken with a co-oxymeter and the paired values were analyzed by the linear regression method. To make the two sets of measurements interchangeable, we established, a priori, a maximum limit of 3% (approximately 5% of the measurement), as being an acceptable difference between the two types of measurements.", 'All the measurements were obtained within four hours of the placement of the catheter. CSvO(2) was weakly correlated with MSvO(2), with a correlation coefficient of r(2) = 0.49 (P < 0.001). The Bland-Altman analysis demonstrates an objective mean bias of 0.8 +/- 3%, with 36% of the values measured falling outside clinically acceptable limits. For values of CSvO(2) <or= 65%, the measurements were not correlated (P = NS). The bias was -2 +/- 7%, and 56% of the values measured fell outside clinically acceptable limits.'] Are [ Continuous Svo2 measurements and co-oximetry interchangeable immediately after cardiopulmonary bypass ]?