Which term in the alveolar ventilation equation represents tidal volume?

• A. VT
• B. RR
• C. HME
• D. ml/lb

Answer: (A)

Tidal volume is the amount of air moved with each breath, and in the alveolar ventilation equation the per-breath air that actually reaches the alveoli is represented by VT. Alveolar ventilation per minute is calculated as (VT minus dead space, VD) multiplied by the respiratory rate (RR). So VT is the term that corresponds to tidal volume. For example, if VT is 500 mL, VD is 150 mL, and RR is 12 breaths/min, alveolar ventilation = (500 − 150) × 12 = 4,200 mL/min. The other terms don’t denote tidal volume: RR is breaths per minute, HME is a humidification device, and ml/lb isn’t the tidal-volume variable in the equation (tidal volume is expressed in milliliters per breath, often related to body weight as ml/kg in clinical settings).