Which expression represents the alveolar ventilation calculation as described?

• A. RR x (VT - (ml/lb + HME))
• B. RR x (VT - Dead space)
• C. (VT x RR) - HME
• D. RR + VT + HME

Answer: (A)

Alveolar ventilation is the amount of air per minute that actually reaches the gas-exchanging areas of the lungs. It’s found by taking the tidal volume that gets to the alveoli (tidal volume minus the air that stays in the airways, i.e., dead space) and multiplying by the respiratory rate. Dead space includes the anatomical portion (often estimated as a certain mL per pound of ideal body weight) plus any extra dead space from equipment like an HME. So the total dead space per breath is (ml per lb × weight) plus HME. Therefore, the expression to describe alveolar ventilation is RR × [VT − (ml per lb × weight + HME)]. This matches the described method because you subtract all dead space from the tidal volume on a per-breath basis before scaling by breaths per minute. The other forms either mix ventilation and dead space incorrectly or treat dead space as a simple subtraction from the total minute ventilation, which doesn’t reflect the actual alveolar ventilation calculation.