Which measure describes the percentage of oxygen remaining in the blood returning to the lungs, typically sampled from the pulmonary artery via a pulmonary artery catheter?

• A. Arterial oxygen saturation (SaO2)
• B. Venous oxygen saturation (SvO2)
• C. Central venous oxygen saturation (ScvO2)
• D. Mixed venous oxygen saturation (SmvO2)

Answer: (D)

The main concept here is mixed venous oxygen saturation, which tells you how much oxygen is left in blood after tissues throughout the body have extracted what they need. This value is obtained from blood returning to the right heart and then to the lungs, and when you pull it from the pulmonary artery with a catheter, you’re sampling truly mixed venous blood from all parts of the body. It reflects the balance between oxygen delivery (amount reaching the tissues) and oxygen consumption (how much the tissues use). If delivery is adequate and tissues don’t extract aggressively, the mixed venous saturation stays higher; if delivery falls or tissue extraction rises, it drops. The normal range is roughly 65–75%.

This is distinct from arterial oxygen saturation, which measures oxygen in the blood going to tissues, not what remains after tissue use. Central venous saturation comes from a central line and represents blood returning mainly from the upper body, which may not perfectly mirror the global mixed venous value. The term mixed venous saturation is sometimes denoted SmvO2, but the concept is the same: it’s the oxygen reserve in venous blood returning to the lungs, sampled from the pulmonary artery.