A true mixed venous sample (called SvO2) is drawn from the tip of the pulmonary artery catheter, and includes all of the venous blood returning from the head and arms (via superior vena cava), the gut and lower extremities (via the inferior vena cava) and the coronary veins (via the coronary sinus).issues begin to remove or extract a higher percentage of oxygen from the arterial blood. This results in a reduced amount of oxygen remaining in the blood as it returns to the right side of the heart (decreased SvO 2).