Oxygen Consumption (VO2) is defined by which equation?

Oxygen Consumption (VO2) is defined by which equation?

• A. VO2 = CO × (CaO2 - CvO2) × 10
• B. VO2 = CO × (CaO2 + CvO2) × 10
• C. VO2 = CO × (SaO2 - SvO2) × 10
• D. VO2 = CO × (CaO2 - PvO2) × 10

Answer: (A)

The main idea here is the Fick principle: oxygen consumption equals the amount of blood flowing to the tissues multiplied by how much oxygen is extracted from that blood. In formula terms, VO2 = CO × (CaO2 − CvO2), with a factor of 10 included to make the units consistent. CaO2 is the arterial oxygen content, CvO2 is the mixed venous oxygen content, and the subtraction represents how much oxygen is removed by tissues from every deciliter of blood. Why the factor 10 matters: cardiac output is typically in liters per minute, while CaO2 and CvO2 are in milliliters of O2 per deciliter of blood. Multiplying by 10 converts deciliters to liters, yielding VO2 in mL per minute. It’s important to use content (CaO2 and CvO2) rather than just saturations (SaO2, SvO2) or venous oxygen pressure (PvO2), because content accounts for both the oxygen bound to hemoglobin and the dissolved oxygen in plasma. Saturations or partial pressures alone don’t directly give the amount of oxygen carried in the blood. So the correct form is the arterial minus mixed venous oxygen content difference, times cardiac output, with the 10 for unit conversion.

The main idea here is the Fick principle: oxygen consumption equals the amount of blood flowing to the tissues multiplied by how much oxygen is extracted from that blood. In formula terms, VO2 = CO × (CaO2 − CvO2), with a factor of 10 included to make the units consistent. CaO2 is the arterial oxygen content, CvO2 is the mixed venous oxygen content, and the subtraction represents how much oxygen is removed by tissues from every deciliter of blood.

Why the factor 10 matters: cardiac output is typically in liters per minute, while CaO2 and CvO2 are in milliliters of O2 per deciliter of blood. Multiplying by 10 converts deciliters to liters, yielding VO2 in mL per minute.

It’s important to use content (CaO2 and CvO2) rather than just saturations (SaO2, SvO2) or venous oxygen pressure (PvO2), because content accounts for both the oxygen bound to hemoglobin and the dissolved oxygen in plasma. Saturations or partial pressures alone don’t directly give the amount of oxygen carried in the blood.

So the correct form is the arterial minus mixed venous oxygen content difference, times cardiac output, with the 10 for unit conversion.