Which expression correctly represents VO2 using the Fick equation?

Which expression correctly represents VO2 using the Fick equation?

• A. VO2 = CO × CaO2
• B. VO2 = CO × (CaO2 − CvO2)
• C. VO2 = HR × SV
• D. VO2 = CO ÷ (CaO2 − CvO2)

Answer: (B)

VO2 follows the Fick principle: oxygen consumption equals how much blood the heart delivers each minute times how much O2 is extracted from that blood by the tissues. The amount of O2 extracted per unit of blood is the difference between arterial O2 content and venous O2 content, CaO2 − CvO2. Multiply that extraction by cardiac output (the volume of blood the heart pumps per minute) to get the total O2 used per minute. So the correct expression is VO2 = CO × (CaO2 − CvO2).

Why the others aren’t right: using CO × CaO2 would assume every bit of arterial O2 delivered is consumed, which isn’t the case because venous O2 content remains after tissue extraction. HR × SV gives cardiac output, not oxygen consumption. Dividing by (CaO2 − CvO2) would invert the relationship, suggesting that greater extraction reduces VO2, which contradicts physiology.