Oxygen consumption (VO2) is calculated using which formula?

• A. VO2 = QT (CaO2 - CvO2) x 10
• B. VO2 = QT x (CaO2 x 10)
• C. VO2 = QT x (CvO2 x 10)
• D. VO2 = QT (CaO2 - CvO2)

Answer: (A)

The main concept is the Fick principle: oxygen consumption equals the amount of blood flow delivering oxygen to the tissues times how much oxygen that blood is carrying. In clinical practice, CaO2 and CvO2 are expressed in ml of O2 per deciliter of blood, while cardiac output (QT) is in liters per minute. To get VO2 in ml per minute, you multiply the arterial-venous O2 content difference by the cardiac output and then convert units from deciliters to liters. That conversion is the factor of 10, so VO2 = QT × (CaO2 − CvO2) × 10. This accounts for the typical values (for example, CaO2 around 20 ml/dl and CvO2 around 15 ml/dl, with QT around 5 L/min leading to about 250 ml/min VO2).

The other forms miss or misplace that unit conversion or the subtraction itself. Omitting the 10 would underrepresent VO2 by a factor of ten, and using only CaO2 or only CvO2 inside the calculation ignores the essential arterial-venous difference that reflects how much oxygen is extracted by tissues.