If minute ventilation is held at 5000 mL/min and the respiratory rate increases from 10 to 20 breaths/min with no change in Ve, how would alveolar ventilation change?

If minute ventilation is held at 5000 mL/min and the respiratory rate increases from 10 to 20 breaths/min with no change in Ve, how would alveolar ventilation change?

• A. Increase by 500 mL
• B. Increase by 1000 mL
• C. No change
• D. Decrease by 1500 mL

Answer: (D)

The key idea is that alveolar ventilation (Va) is the portion of ventilation that reaches the gas-exchange areas. It depends on the tidal volume minus dead space, times the respiratory rate: Va = (Vt − Vd) × RR. Since minute ventilation Ve equals Vt × RR, you can also think of Va as Ve minus the ventilation lost to dead space: Va = Ve − (Vd × RR). Start with the given numbers. Ve stays at 5000 mL/min. With a respiratory rate of 10 breaths/min, the tidal volume is Vt = Ve/RR = 5000/10 = 500 mL. Assuming a typical anatomical dead space Vd of 150 mL, the alveolar ventilation is Va1 = (500 − 150) × 10 = 350 × 10 = 3500 mL/min. Now double the rate to 20 breaths/min while Ve remains 5000 mL/min. The new tidal volume is Vt2 = 5000/20 = 250 mL. Alveolar ventilation becomes Va2 = (250 − 150) × 20 = 100 × 20 = 2000 mL/min. So alveolar ventilation falls from 3500 to 2000 mL/min, a decrease of 1500 mL/min. The reason is that increasing the rate with the same overall ventilation increases the portion wasted in dead space (Vd × RR), leaving less air reaching the alveoli.

The key idea is that alveolar ventilation (Va) is the portion of ventilation that reaches the gas-exchange areas. It depends on the tidal volume minus dead space, times the respiratory rate: Va = (Vt − Vd) × RR. Since minute ventilation Ve equals Vt × RR, you can also think of Va as Ve minus the ventilation lost to dead space: Va = Ve − (Vd × RR).

Start with the given numbers. Ve stays at 5000 mL/min. With a respiratory rate of 10 breaths/min, the tidal volume is Vt = Ve/RR = 5000/10 = 500 mL. Assuming a typical anatomical dead space Vd of 150 mL, the alveolar ventilation is Va1 = (500 − 150) × 10 = 350 × 10 = 3500 mL/min.

Now double the rate to 20 breaths/min while Ve remains 5000 mL/min. The new tidal volume is Vt2 = 5000/20 = 250 mL. Alveolar ventilation becomes Va2 = (250 − 150) × 20 = 100 × 20 = 2000 mL/min.

So alveolar ventilation falls from 3500 to 2000 mL/min, a decrease of 1500 mL/min. The reason is that increasing the rate with the same overall ventilation increases the portion wasted in dead space (Vd × RR), leaving less air reaching the alveoli.