What are mixed venous oxygen saturations (SvO2)?

From the Guidelines

Mixed venous oxygen saturation (SvO2) should be targeted to ≥70% as part of a treatment protocol for sepsis-induced tissue hypoperfusion, as recommended by the Surviving Sepsis Campaign guidelines 1. Mixed venous oxygen saturation (SvO2) is a crucial measurement of the percentage of hemoglobin saturated with oxygen in blood returning to the right side of the heart from the body's tissues.

• Normal SvO2 values range from 60-80%, reflecting the balance between oxygen delivery and consumption in the body.
• This measurement is obtained by drawing blood from a pulmonary artery catheter (Swan-Ganz catheter) positioned in the pulmonary artery, which contains mixed blood from the superior and inferior vena cava and coronary sinus.
• SvO2 serves as an important clinical indicator of global tissue oxygenation and cardiac output adequacy, and is a key component of the initial resuscitation protocol for sepsis-induced hypoperfusion, along with central venous pressure (CVP), mean arterial pressure (MAP), and urine output 1.
• Low values (below 60%) suggest tissues are extracting more oxygen due to either decreased delivery (from low cardiac output, anemia, or hypoxemia) or increased consumption (as in sepsis, fever, or seizures).
• High values (above 80%) may indicate reduced tissue oxygen extraction, often seen in sepsis, cyanide toxicity, or arteriovenous shunting.
• Continuous SvO2 monitoring is particularly valuable in critically ill patients to guide interventions aimed at optimizing oxygen delivery, including fluid resuscitation, blood transfusions, inotropic support, or mechanical ventilation adjustments, with the goal of normalizing lactate levels in patients with elevated lactate as a marker of tissue hypoperfusion 1.

From the Research

Definition and Purpose of Mixed Venous Saturations

• Mixed venous saturations reflect the balance between oxygen requirement and oxygen delivery, and thus may be used to assess the adequacy of tissue oxygenation 2.
• The principal task of acute critical care is to avoid or correct oxygen debt by increasing oxygen delivery (DO2) and/or decreasing oxygen consumption (VO2), with mixed venous oxygen saturation and its surrogate, central venous oxygen saturation (ScvO2), being commonly used methods to assess this relationship 3.

Measurement and Clinical Use

• Mixed and central venous oxygen saturations are commonly used to ascertain the degree of systemic oxygenation in critically ill patients 4.
• Central venous saturation (ScvO2) is a less invasive alternative to mixed venous saturation (SvO2) for determining the balance of oxygen delivery and consumption, although it is inferior as it does not cover the lower part of the body, including splanchnic perfusion 5.
• Studies have shown that central venous saturation is a reliable marker for goal-directed therapy in intensive care medicine, especially in patients with septic or hemorrhagic shock 5.

Diagnostic Accuracy and Limitations

• The diagnostic accuracy of central venous oxygen saturation (ScvO2) in estimating mixed venous oxygen saturation (SvO2) and cardiac index in critically ill patients has been evaluated, with moderate reliability for ScvO2 in predicting SvO2, but limited evidence on the independent use of ScvO2 in predicting cardiac index 6.
• The Quality Assessment of Diagnostic Accuracy Studies-2 and Grading Recommendations Assessment, Development, and Evaluation tools have been used to assess the risk of bias and certainty of evidence for the use of ScvO2 in clinical practice 6.

Prognostic Significance

• Central venous saturation may have prognostic significance following major surgery, with further evaluation of peri-operative trends in central venous saturation required 2.
• Not only decreased venous saturations but also elevated venous saturations are associated with poor outcome, highlighting the importance of monitoring and interpreting venous saturation values in clinical practice 5.