What is the difference between Mixed Venous Oxygen Saturation (SvO2) and Central Venous Oxygen Saturation (ScvO2)?

From the Guidelines

SvO2 and ScvO2 are both useful measurements of oxygen saturation in venous blood, but ScvO2 is more commonly used in clinical practice due to its less invasive nature, and it can generally substitute for SvO2 in many clinical scenarios, with a target value of ≥ 70% as recommended by the Surviving Sepsis Campaign guidelines 1.

Key Differences Between SvO2 and ScvO2

• SvO2 (mixed venous oxygen saturation) is measured from the pulmonary artery using a pulmonary artery catheter and represents the oxygen saturation of blood returning from all body tissues.
• ScvO2 (central venous oxygen saturation) is measured from the superior vena cava via a central venous catheter and primarily reflects oxygen saturation from the upper body.
• Normal SvO2 values range from 65-75%, while ScvO2 is typically 3-5% higher (70-80%) because blood from the coronary sinus, which has very low oxygen content, mixes in after the superior vena cava.

Clinical Implications

• Both measurements help assess the balance between oxygen delivery and consumption, with low values indicating either increased oxygen extraction or decreased delivery, which can guide resuscitation efforts in critically ill patients, particularly those with sepsis or shock.
• The relationship between SvO2 and ScvO2 may become unpredictable during shock states, so understanding their differences is important for proper interpretation.
• The Surviving Sepsis Campaign guidelines recommend a target ScvO2 value of ≥ 70% as part of the initial resuscitation strategy for sepsis-induced hypoperfusion or septic shock 1.

Recommendations

• Use ScvO2 as a target value for resuscitation in sepsis-induced hypoperfusion or septic shock, with a target value of ≥ 70% 1.
• Consider using dynamic over static variables to predict fluid responsiveness, where available 1.
• Guide resuscitation to normalize lactate in patients with elevated lactate levels as a marker of tissue hypoperfusion 1.

From the Research

Comparison of Svo2 and ScvO2

• Svo2 (mixed venous oxygen saturation) and ScvO2 (central venous oxygen saturation) are both used to monitor oxygen saturation in critically ill patients 2, 3.
• ScvO2 is a useful therapeutic target when treating septic shock, with a goal of achieving ScvO2 ≥ 70% 2.
• Studies have shown that combining ScvO2 and central venous-to-arterial partial pressure of carbon dioxide difference (△Pco2) may provide additional information about survival in septic shock patients 2.
• The incidence of low ScvO2 values is low in critically ill patients, especially in those with sepsis or septic shock 3.

Clinical Applications

• Fluid resuscitation guided by lactate clearance rate and ScvO2 may be more effective than guidance by ScvO2 alone in patients with septic shock 4.
• A combination of lactate clearance rate and ScvO2 may serve as an index for confirming the endpoint of fluid resuscitation in septic shock patients 4.
• ScvO2 > 70% or SvO2 > 65% is recommended for both septic and non-septic patients, but physiological and pathological features can affect these values 5.

Physiological Considerations

• SvO2 and ScvO2 are adaptive variables with large inter-patient variability, and optimizing one to three of the four S(c)vO2 components can homogenize patients and yield a clear dependency with the fourth one 5.
• Appropriate intensive care mandates determination of the best compromise between S(c)vO2 and its four components, taking into account the specific constraints of each individual patient 5.