A patient undergoing cardiac catheterization has an oxygen consumption of 250 mL/min, systemic arterial oxygen content of 200 mL O2 per litre of blood, and mixed venous oxygen content of 160 mL O2 per litre. Using the Fick principle, the cardiac output is:

• A 4.0 L/min
• B 5.0 L/min
• C 6.25 L/min ✓
• D 8.0 L/min

Explanation

The Fick principle states that oxygen consumption equals cardiac output multiplied by the arteriovenous oxygen content difference. Cardiac output therefore equals 250 divided by (200 minus 160), which is 250 divided by 40, giving 6.25 L/min. Option B would result if the arteriovenous difference were wrongly taken as 50 rather than 40, and option D results from dividing by an incorrect difference of about 31, a common arithmetic slip when candidates subtract in the wrong order.

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

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