Tissue hypoxia is the central pathophysiological process in shock and an important co-factor in the development of organ dysfunction. Hemodynamic parameters, usually used to assess the perfusion of organs and tissues, like arterial blood pressure, heart rate, urine output and blood gases can be normal in the presence of tissue hypoxia and cannot rule out imbalances between global oxygen supply and demand. Mixed venous oxygen saturation (SvO2) is a sensitive indicator of the adequacy of whole-body tissue oxygenation. However, it requires the placement of a pulmonary artery catheter, which is an invasive procedure with the possibility of numerous complications and is increasingly questioned due to the lack of evidence that it improves outcome. Central venous oxygen saturation (ScvO2) requires the insertion of a central venous catheter, which is routinely used in most critically ill patients, but it reflects the adequacy of oxygenation in the brain and upper part of the body and differs from SvO2. Still, it can be used as a surrogate for mixed venous oxygen saturation because the changes and trends of both variables parallel each other. Both variables are used extensively in the treatment of patients with severe sepsis, shock and trauma. In combination with other hemodynamic and biochemical parameters, they have diagnostic and prognostic value and allow for rational treatment of critically ill patients.
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