Is Central Venous Oxygen Saturation Higher Than Mixed Venous Oxygen Saturation?
Yes, central venous oxygen saturation (ScvO₂) consistently runs approximately 5% higher than true mixed venous oxygen saturation (SvO₂) measured from the pulmonary artery. 1
Physiological Basis for the Difference
The difference between ScvO₂ and SvO₂ exists because they sample blood from different anatomical locations that reflect different oxygen extraction patterns:
Mixed venous saturation (SvO₂) is measured from the pulmonary artery and represents true mixed venous blood from the entire body—combining blood from the superior vena cava, inferior vena cava, and coronary sinus 2
Central venous saturation (ScvO₂) is measured from the superior vena cava or right atrium and does not capture venous return from the lower body, including splanchnic circulation 3
The decrease in oxygen saturation from ScvO₂ to SvO₂ likely results from mixing of atrial blood with coronary sinus blood, which has very low oxygen saturation due to high myocardial oxygen extraction 4
Magnitude of the Difference
Multiple studies have quantified this difference with consistent findings:
Mean bias of 5-7%: ScvO₂ overestimates SvO₂ by approximately 5.2-6.9% 5, 4
Wide limits of agreement: The 95% limits of agreement range from -5.0% to 18.8%, indicating substantial variability between individual measurements 5
Greater divergence at lower values: The difference between ScvO₂ and SvO₂ becomes more pronounced when SvO₂ is <70%, making ScvO₂ less reliable in critically ill patients with poor oxygen delivery 5
Clinical Implications for Target Values
Because ScvO₂ runs higher than SvO₂, therapeutic targets reflect this physiological difference:
ScvO₂ target: ≥70% in septic shock and critically ill patients 6, 1
SvO₂ target: ≥65% when using pulmonary artery catheter monitoring 6, 1
These targets are designed to be equivalent despite the 5% difference in absolute values 1
Important Caveats
The two measurements are not interchangeable numerically despite both reflecting oxygen delivery-consumption balance 5:
Poor correlation exists between ScvO₂ and SvO₂ values (r = 0.48-0.62), particularly during therapeutic interventions 7
Differences ≥5% occur in approximately 50% of measurements, even during periods of hemodynamic stability 7
Substituting ScvO₂ for SvO₂ in oxygen consumption calculations produces unacceptably large errors 4
ScvO₂ does not capture lower body perfusion, including splanchnic circulation, making it inferior to SvO₂ for comprehensive assessment of tissue oxygen delivery 3
Despite these limitations, ScvO₂ remains a reliable marker for goal-directed therapy in septic and hemorrhagic shock when pulmonary artery catheterization is not feasible 3, and protocol-driven therapy targeting ScvO₂ >70% improves survival in critically ill patients 1.