A patient undergoing cardiac catheterisation has an oxygen consumption of 250 mL/min. Mixed venous oxygen content is 14 mL/dL and arterial oxygen content is 19 mL/dL. Using the Fick principle, the cardiac output is approximately:

• A 2.5 L/min
• B 5.0 L/min ✓
• C 6.25 L/min
• D 8.3 L/min

Explanation

The Fick principle states cardiac output equals oxygen consumption divided by the arteriovenous oxygen content difference. Here the difference is 19 minus 14, which is 5 mL/dL. Converting units, 250 mL/min divided by 50 mL/L gives 5.0 L/min. Option A results from forgetting to convert dL to L, and option C comes from dividing by 40 mL/L, a common arithmetic slip under exam pressure.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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