SvO2 and ScvO2

Q: What's the difference between Mixed Venous O2 saturation (SvO2) and Central Venous O2 saturation (ScvO2)?

A: SvO2: "5th vital sign"

1. the oxygen saturation of hemoglobin in pulmonary artery (mixed venous) blood; measures the relationship between O2 consumption and O2 delivery in the body.
   
2. measured using specialized PA catheter (Swan Ganz)
   or in vitro (blood from distal port of PA catheter)
   
3. an indirect marker of cardiac output: (and whole-body tissue oxygenation)
   decrease CO = increase O2 extraction from the capillaries = low SvO2
   

   "but SvO2 is an unreliable index of systemic blood flow if O2 extraction is fixed and does not vary with changes in blood flow (eg Sepsis)"

4. 65-80% (5-10% lower than ScvO2 due to mixing of atrial and coronary sinus blood)

ScvO2:

1. surrogate (?) to SvO2
2. measured using CVP
3. reflects the adequacy of oxygenation in the brain and upper part of the body
4. 5-10% higher than SvO2
   

Both are determinants of global tissue hypoxia. About 50% of patients resuscitated from shock have global tissue hypoxia despite normal vital signs and CVP [Curr Opin Crit Care. 2001; 7(3):204-11.].

In EGDT, the goal is 70% and 65% for ScvO2 and SvO2 respectively.

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Source: ICU Book, 3rd Ed by Marino
This Sepsis learning module link might interest you