Mixed venous oxygen saturation (SvO2) is an indirect indicator of cardiac output. Twenty-nine patients undergoing aortic reconstructive surgery were studied to determine whether a sustained 5% or greater change in SvO2 (1) is associated with a similar change in cardiac output, and (2) results in a change in therapy during anesthetic management of the patients. Pulmonary artery cannulation with a fiberoptic oximeter catheter was used for continuous monitoring of SvO2 values in addition to usual hemodynamic parameters. Hemodynamic measurements and therapeutic manipulations were recorded when SvO2 changed by 5% or more for a period exceeding 60 seconds. Any sustained change of 5% or more in SvO2 was plotted against change in cardiac output. The relationship between 5% or greater changes in SvO2 and the initiation of therapeutic interventions was also tested. Results showed a highly significant relationship between changes in SvO2 and initiation of therapeutic intervention. This data suggests that the likelihood of a therapeutic intervention in a patient with a change in SvO2 of 5% or greater is highly significant. The relationship between change in SvO2 and cardiac output was also significant. This suggests that changes in SvO2 are reflective of changes in cardiac output, and thus, the hemodynamic status of the patient.