ScvO2 values are obtained by measuring the oxygen saturation in venous blood returning to the heart, and reflect the balance between oxygen delivery and oxygen consumption.
Low (<70%) ScvO2 values were targeted by Rivers in his Early Goal Directed Therapy study: by improving the macrocirculation with fluids, vasoactive drugs, and packed red cells the aim is to improve oxygen delivery to tissues, and therefore a higher oxygen saturation is found in the venous blood returning to the heart in adequately resuscitated patients. The story is more complex, however, as mechanisms of oxygen supply (macrocirculatory flow), distribution (microcirculatory flow), and processing (mitochondrial function) must all function at an adequate level to maintain normal physiology.
Although low ScvO2 values may be a marker for macrocirculatory failure, high ScvO2 values may reflect microcirculatory or mitochondrial failure.
A multicentre study demonstrated a higher mortality on patients whose ScvO2 in the ED was high (90-100%) compared with those with a normal ScvO2.
Mortality associated with three groups according to their highest recorded ScvO2 in the ED was:
Hypoxia group (ScvO2 <70%) – 40% mortality (95% CI 29-53)
Normoxia group (ScvO2 71-89%) – 21% mortality (95% CI 17-25)
Hyperoxia group (ScvO2 90-100%) – 34% mortality (95% CI 25-44)
The study design could not control for many potential confounders, but this opens the door for further study, and reminds us that the unthinking pursuit of a single physiological target may miss the bigger clinical picture.
Multicenter Study of Central Venous Oxygen Saturation (ScvO2) as a Predictor of Mortality in Patients With Sepsis
Annals of Emergency Medicine 2010;55(1):40-46