Alveolar minute ventilation subtracts out the portion of each breath that never reaches gas-exchanging alveoli (dead space), giving a truer picture of effective ventilation than minute ventilation alone.
Typical effective ventilation.
Slightly below typical.
Low — CO₂ likely to rise; check tidal volume and dead space.
Elevated effective ventilation.
Given: Vt 500 mL, Vd 150 mL, rate 14/min.
350 mLDead space fills first; the rest reaches the alveoli and does the gas exchange.
Ventilation that only fills dead space doesn't participate in gas exchange, so two patients with identical minute ventilation can have very different effective ventilation depending on their dead space.
About 4–5 L/min — roughly 60–70% of total minute ventilation, since dead space is typically about a third of each tidal volume.
A common bedside estimate is about 2.2 mL per kg of ideal body weight, which often works out to roughly 150 mL in an average adult. It rises with conditions like pulmonary embolism.
Small tidal volumes mean dead space eats up a larger fraction of each breath, so even a fast rate moves little fresh gas to the alveoli — the basis of the RSBI weaning index.