ObjectiveTo determine the accuracy of in vivo mixed venous hemoglobin saturation (S&OV0456;o2) measurements with a fiber optic thermodilution catheter during acute changes in hematocrit.DesignComparison of fiberoptic in vivo S&OV0456;o2values with in vitro S&OV0456;o2values obtained with a multiwavelength spectrophotometer.SettingOperating room in a university hospital.PatientsSix consecutive patients who are Jehovah's Witnesses.Measurements and Main ResultsBefore and after each step of hypervolemic hemodilution and after every 500 mL of blood loss, blood gases were analyzed and hemodynamic, hemoglobin, hematocrit, and in vitro and in vivo S&OV0456;o2measurements were made. Hematocrit values were measured in the range of 40% to 18%. Plotting all in vivo S&OV0456;o2values (n = 74) against the in vitro S&OV0456;o2measurements obtained during the entire study period gives r2= .86. The accuracy of in vivo S&OV0456;o2measurements was not affected by changes in hematocrit or cardiac output. The S&OV0456;o2catheter value at the beginning of the study differed from the in vitro S&OV0456;o2value by −0.86 ± 2.56% and at the end of the study period of 8 to 10 hrs by 0.71 ± 3.04%.ConclusionsThe accuracy of the studied fiberoptic continuous measuring S&OV0456;o2system was not affected by changes in hematocrit or cardiac output. No significant drift in the in vitro S&OV0456;o2measurements was observed.