BACKGROUND: Mixed venous oxygen saturation (SO2) has been considered as an important parameter during therapy of critically ill patients. But pulmonary artery catheterization for monitoring of SO2 involves risks and sometimes may be contraindicated. The purpose of this study was to evaluate whether SO2 could be replaced by oxygen saturation of superior vena cava (SvcO2). METHODS: We inserted pulmonary artery catheter in 38 patients scheduled for undergoing mitral valve replacement. Blood samples for SO2 were drawn from PA ports of pulmonary artery catheter, blood samples for distal SvcO2 were drawn from CVP ports of pulmonary artery catheter and blood samples for proximal SvcO2 were drawn from distal ports of sheat introducer. RESULTS: There was no significant difference between SO2 and SvcO2 (79.6% and 79.9%, 78.0%), and also no significant difference between distal and proximal SvcO2 (79.9%, 78.0%). Correlation of SO2 and SvcO2 was good (r=0.69: PA and SVCD, r=0.62: PA and SVCP). CONCLUSION: We conclude that mixed venous oxygen saturation might be replaced by oxygen saturation of superior vena cava during mitral valve replacement.