What is the difference between central venous oxygen saturation (ScvO2) and mixed venous oxygen saturation (SvO2)?

From the Guidelines

Central venous and mixed venous blood samples have distinct differences in terms of location, oxygen content, and clinical significance, with mixed venous samples providing a more accurate assessment of overall tissue oxygen extraction and cardiac output. The primary difference between central venous and mixed venous blood samples lies in their location and oxygen content. Central venous blood is obtained from the superior vena cava or right atrium through a central venous catheter, reflecting oxygen saturation from the upper body only, typically ranging from 60-80% 1. On the other hand, mixed venous blood is collected from the pulmonary artery via a pulmonary artery catheter (Swan-Ganz), representing truly mixed blood from all body regions after tissue oxygen extraction, with saturations typically 65-75% 1.

Key Differences

• Central venous oxygen saturation (ScvO2) can be used as a surrogate when pulmonary artery catheterization isn't available, but it generally reads 3-5% higher than mixed venous oxygen saturation (SvO2) due to preferential blood flow to the brain during shock states 1.
• Mixed venous samples provide a more accurate assessment of overall tissue oxygen extraction and cardiac output, making them clinically significant for guiding resuscitation efforts in sepsis or cardiogenic shock.
• A low ScvO2 value is an important warning sign of the inadequacy of systemic oxygen delivery to meet oxygen demands, but it does not provide information about the reason for this inadequacy, nor does it provide guidance as to the optimal therapeutic approach 1.

Clinical Implications

• Understanding the difference between central venous and mixed venous blood samples is essential for proper interpretation of oxygen delivery and consumption parameters in critically ill patients.
• The Surviving Sepsis Campaign protocol recommends a central venous oxygen saturation (ScvO2) of greater than or equal to 70% or mixed venous oxygen saturation (SvO2) of greater than or equal to 65% as a resuscitation goal 1.
• However, the use of ScvO2 as a therapeutic goal has been questioned due to its potential limitations in guiding resuscitation in septic patients, particularly in the ICU setting 1.

From the Research

Central Venous and Mixed Venous Oxygen Saturation

• Central venous oxygen saturation (ScvO2) and mixed venous oxygen saturation (SvO2) are two related but distinct parameters used to assess the adequacy of cardiac output and tissue oxygenation 2, 3, 4, 5.
• ScvO2 is measured from the tip of a central venous catheter placed close to, or within, the right atrium, while SvO2 is measured using a pulmonary artery catheter 4, 5.
• Studies have shown that ScvO2 values closely parallel SvO2 values, but average about 7% higher 5.
• ScvO2 can be used as a surrogate for SvO2 measurement, but its accuracy and clinical utility depend on various factors, including the position of the central venous catheter and the presence of physiological variables such as hematocrit, hemoglobin, pH, and temperature 3, 4, 5.

Comparison of Central Venous and Mixed Venous Oxygen Saturation

• A study found that a central venous oxygen saturation <70% predicted a mixed venous oxygen saturation <60% with a sensitivity of 84%, specificity of 70%, positive predictive value of 31%, and negative predictive value of 96% 2.
• Another study showed that ScvO2 values changed in parallel with SvO2 values in 90% of instances where SvO2 changed more than 5% 5.
• The use of ScvO2 as a guide for therapy has been proposed, particularly in scenarios where tissue oxygenation is a clinical concern 4.

Clinical Utility and Limitations

• The clinical utility of ScvO2 and SvO2 measurements depends on their accurate interpretation and application 4, 6.
• Limitations of ScvO2 measurement include its potential inaccuracy in certain clinical situations, such as changes in hematocrit, hemoglobin, pH, or temperature 5.
• The use of pulmonary artery catheters, which provide SvO2 measurements, has diminished in recent years due to advances in less invasive monitoring techniques 6.