A note on mixed venous oxygen saturation (SVO2)

A true mixed venous sample (called SvO2) should includes all of the venous blood returning from

1. the head and arms (via superior vena cava),

2. the gut and lower extremities (via the inferior vena cava) and

3. the coronary veins (via the coronary sinus).

All venous blood gets "mixed" in right ventricle. Venous blood obtained from tip of Pulmonary artery catheter reflect the overall amount of oxygen remaining after all tissues in the body have removed oxygen from the hemoglobin. Another important minute detail often forgotten, the mixed venous sample also captures the blood before it is re-oxygenated in the pulmonary capillary.

In contrast, ScVO2 is not the true reflection of overall oxygen extraction as it represents collection of blood only via superior vena cava.