SvO2

SvO2, or mixed venous oxygen saturation, is the percentage of hemoglobin that is bound to oxygen in mixed venous blood returning to the right heart from the entire systemic circulation. It serves as an index of the balance between systemic oxygen delivery (DO2) and consumption (VO2). In healthy adults, normal SvO2 is typically around 65 to 75 percent.

Measurement and source are key to interpretation. SvO2 is normally measured from blood sampled in the pulmonary artery, usually via a right heart or pulmonary artery catheter, which yields mixed venous blood from the entire body. By contrast, central venous oxygen saturation (ScvO2) is measured from a venous site in the upper body and may differ from SvO2 because it does not sample the whole circulation. Thus SvO2 reflects global oxygen balance, whereas ScvO2 reflects a regional venous return.

Clinical interpretation hinges on SvO2 values and the clinical context. Low SvO2 (<60–65%) suggests inadequate oxygen delivery relative to consumption or markedly increased metabolic demand, with possible etiologies including anemia, hypoxemia, hypotension, reduced cardiac output, sepsis, fever, or shunting. High SvO2 (>75–80%) can occur with impaired tissue oxygen extraction (for example, profound sepsis with mitochondrial dysfunction), cyanide poisoning, liver failure, or conditions of very high cardiac output where delivery far exceeds tissue needs. However, SvO2 is a global measure and may not detect regional or microcirculatory dysfunction.

Applications and limitations are important considerations. SvO2 can guide resuscitation and monitoring in critical illness, particularly when used alongside other hemodynamic parameters. It requires an invasive catheter and is influenced by multiple factors (CO, hemoglobin, temperature, arterial oxygen content). Therefore, SvO2 should be interpreted within the overall clinical and laboratory context.