Central Venous Oxygen Saturation
As oxygen is extracted from the blood, the oxygen saturation falls. Consequently, the oxygen saturation of blood returning to the lungs (i.e. blood from the pulmonary artery) can give an indication of total body oxygen extraction.
Usually, this mixed venous blood is around 70-75% saturated SvO2 [10]. If it falls below this figure, oxygen extraction has had to increase. In shock states this is usually because oxygen delivery has become inadequate. Demand exceeds supply.
In the ED practice, sampling SvO2 is impracticable. Consequently, interest has focussed on the usefulness of central venous oxygen saturation (ScvO2), which tends to be about 5-7% higher. Rivers’ work on early goal-directed therapy [31] has incorporated ScvO2 as a guide for blood transfusion and inotropic support with a survival benefit and its use is also supported by the Surviving Sepsis campaign.
It should be noted however that in sepsis in particular, ScvO2 can be misleading. As sepsis progresses, oxygen extraction by the tissues becomes less and less efficient and the blood returning to the heart remains oxygenated. In this situation, a normal or high ScvO2 can reflect a worsening clinical picture.
Learning bite
Don’t be falsely reassured by a normal ScvO2 , it may simply represent the tissue’s inability to utilise oxygen.