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Lack of Equivalence Between Central and Mixed Venous Oxygen Saturation^*

^* From the Critical Care Medicine Division (Drs. Chawla, Seneff, and Shah), Department of Anesthesiology, the Division of General Surgery (Dr. Zia), Department of Surgery, the Pulmonary and Critical Care Medicine Division (Dr. Gutierrez), Department of Medicine, and the Cardiothoracic Surgery Division (Dr. Katz), Department of Surgery, The George Washington University Medical Center, Washington, DC.

Correspondence to: Guillermo Gutierrez, MD, PhD, FCCP, Professor of Medicine and Anesthesiology, The George Washington University MFA, 2150 Pennsylvania Ave NW, Suite 5–404, Washington, DC 20037; e-mail: Ggutierrez{at}mfa.gwu.edu

Study objective: We compared paired samples of central venous O₂ saturation (ScvO₂) and mixed venous O₂ saturation (SO₂) to test the hypothesis that ScvO₂ is equivalent to SO₂. We also compared O₂ consumption (O₂) computed with ScvO₂ (O₂cv) to that computed with SO₂ (O₂v).

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: SO₂ was consistently lower than ScvO₂ (p < 0.0001), with a mean (±SD) bias of –5.2 ± 5.1%. Similar differences in ScvO₂ and SO₂ were present within each subgroup (p < 0.001). A lower SO₂ resulted in O₂v values that were higher than the O₂cv values for all patients in the study (mean O₂v, 236.7 ± 103.4 mL/min; mean O₂cv, 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 SO₂ were not equivalent in this sample of critically ill patients. Moreover, substituting ScvO₂ for SO₂ in the calculation of O₂ produced unacceptably large errors. The decrease in SO₂ between ScvO₂ to SO₂ may result from the mixing of atrial and coronary sinus blood. As such, this difference may be a marker of myocardial O₂ consumption.

Key Words: central venous oxygenation • coronary sinus • mixed venous oxygenation • monitoring • myocardial metabolism • oxygen consumption • oxygen delivery • pulmonary artery catheter • resuscitation

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