How do you calculate alveolar ventilation rate?

• A. (Tidal volume − dead space) × respiratory rate
• B. Tidal volume × respiratory rate
• C. Tidal volume divided by respiratory rate
• D. Tidal volume minus dead space

Answer: (A)

Alveolar ventilation rate measures the amount of air per minute that actually reaches the gas-exchange surfaces. Each breath moves a tidal volume, but a portion stays in dead space (airways and nonexchange areas) and doesn’t participate in gas exchange. So the volume that can participate per breath is tidal volume minus dead space. Multiply that by the number of breaths you take per minute to get how much air per minute reaches the alveoli: alveolar ventilation rate = (tidal volume − dead space) × respiratory rate. For example, with a tidal volume of 500 mL, dead space of 150 mL, and 12 breaths per minute, AVR = (500 − 150) × 12 = 350 × 12 = 4,200 mL/min (4.2 L/min). The other options don’t reflect both the dead-space subtraction and the per-minute rate, so they don’t represent alveolar ventilation.