A ventilated patient has measured tidal volume 500 mL, respiratory rate 12/min and anatomical dead space estimated at 150 mL. The alveolar ventilation per minute is approximately:

• A 4.2 L/min ✓
• B 5.0 L/min
• C 6.0 L/min
• D 7.2 L/min

Explanation

Alveolar ventilation equals (tidal volume minus dead space) multiplied by respiratory rate: (500 minus 150) times 12 equals 4200 mL/min, that is 4.2 L/min. Total minute ventilation is 6.0 L/min, which is option C and the common trap. Only the portion of each breath reaching perfused alveoli contributes to gas exchange; the 150 mL per breath remaining in conducting airways does not.

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

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