The Fick principle states that cardiac output (CO) = oxygen consumption (VO2) ÷ arteriovenous oxygen difference. If VO2 = 250 mL/min, arterial O2 content = 200 mL/L, and mixed venous O2 content = 150 mL/L, what is the cardiac output?

• A 2 L/min
• B 5 L/min ✓
• C 3 L/min
• D 8 L/min

Explanation

CO = VO2 / (CaO2 − CvO2) = 250 mL/min ÷ (200 − 150 mL/L) = 250 ÷ 50 = 5 L/min. This is the Fick cardiac output calculation. Normal resting cardiac output is 4–6 L/min (typically ~5 L/min). The A-V O2 difference of 50 mL/L (or 5 mL/dL) is normal at rest; it widens with decreased CO (heart failure) and narrows with increased CO (exercise, anaemia, hyperthyroidism, AV fistula).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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