What is the oxygen extraction fraction commonly used in CPB calculations?

• A. The fraction of delivered oxygen that is extracted by tissues, calculated as (CaO2 − CvO2)/CaO2.
• B. The ratio of VO2 to DO2.
• C. The oxygen content difference between arterial and venous blood.
• D. The product of CO and CaO2.

Answer: (A)

The main idea is that oxygen extraction fraction measures what portion of the oxygen delivered to the tissues is actually used. It is the ratio of oxygen consumed to oxygen delivered. In CPB, delivery of oxygen is DO2 = CO × CaO2 and consumption is VO2 = CO × (CaO2 − CvO2). So OEF = VO2/DO2 = (CaO2 − CvO2)/CaO2. This form is standard in CPB because it directly uses arterial oxygen content in the denominator, making the calculation straightforward from measured CaO2 and CvO2. The ratio VO2/DO2 is equivalent in meaning, but expressing it as (CaO2 − CvO2)/CaO2 is the most practical form for CPB calculations.