What is the alveolar minute volume for a patient with a tidal volume of 500 mL, dead space of 150 mL, and a respiratory rate of 16 breaths per minute?

• A. 8,000 mL
• B. 6,000 mL
• C. 5,600 mL
• D. 7,400 mL

Answer: (C)

Alveolar minute volume measures the air actually reaching the gas-exchanging portion of the lungs each minute. To get it, remove the air that fills dead space from each breath, then multiply by how many breaths occur each minute.

Per breath, alveolar volume = tidal volume minus dead space = 500 mL − 150 mL = 350 mL. Multiply by the respiratory rate: 350 mL × 16 breaths/min = 5,600 mL/min, which is about 5.6 L/min.

So the alveolar minute volume is 5,600 mL per minute. Using the full tidal volume would give 8,000 mL/min, which overestimates ventilated air because it includes air that merely fills dead space.