Study objective: We compared paired samples of central venous O2 saturation (Scvo2) and mixed venous O2 saturation (Sv̄o2) to test the hypothesis that Scvo2 is equivalent to Sv̄o2. We also compared O2 consumption (V̇o2) computed with Scvo2 (V̇o2cv) to that computed with Sv̄o2 (V̇o2v).
Design: Prospective, sequential, observational study.
Setting: Combined medical-surgical ICU.
Patients: Fifty-three individuals > 18 years of age of either sex who required a pulmonary artery catheter (PAC) to guide fluid therapy. Subjects were identified as postsurgical (32 patients) or medical (21 patients) according to their ICU admission diagnosis.
Interventions: A PAC was inserted through the internal jugular or subclavian veins. Care was taken to place the PAC proximal port approximately 3 cm above the tricuspid valve. Blood samples were drawn from the proximal and distal ports in random order. An arterial blood sample also was drawn.
Measurements: Cardiac output in triplicate, systemic pressure, and central pressure. We analyzed blood samples for hemoglobin concentration and O2 saturation (So2). Data were compared by correlation analysis and by the method of Bland and Altman.
Results: Sv̄o2 was consistently lower than Scvo2 (p < 0.0001), with a mean (±SD) bias of −5.2 ± 5.1%. Similar differences in Scvo2 and Sv̄o2 were present within each subgroup (p < 0.001). A lower Sv̄o2 resulted in V̇o2v values that were higher than the V̇o2cv values for all patients in the study (mean V̇o2v, 236.7 ± 103.4 mL/min; mean V̇o2cv, 191.1 ± 84.0 mL/min; p < 0.001) as well as for patients within each subgroup (p < 0.001).
Conclusions: Measurements of Scvo2 and Sv̄o2 were not equivalent in this sample of critically ill patients. Moreover, substituting Scvo2 for Sv̄o2 in the calculation of V̇o2 produced unacceptably large errors. The decrease in So2 between Scvo2 to Sv̄o2 may result from the mixing of atrial and coronary sinus blood. As such, this difference may be a marker of myocardial O2 consumption.