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V̇A = (VT − VD) × RR
Normal: 4–5 L/min at rest (vs V̇E of 6–8 L/min — dead space accounts for ~30% of each breath)
Alveolar ventilation is the portion of minute ventilation that actually reaches the alveoli and participates in gas exchange. It equals minute ventilation minus dead space ventilation. PaCO₂ is directly and inversely proportional to alveolar ventilation — only alveolar ventilation eliminates CO₂.
Used when evaluating CO₂ clearance efficiency. Particularly important when dead space is elevated (ARDS, PE, emphysema), as a patient may have normal minute ventilation but inadequate alveolar ventilation due to increased dead space. PaCO₂ can only be normalized by increasing alveolar ventilation, not total minute ventilation.
→ 4.9 L/min
Normal alveolar ventilation
→ 4.0 L/min
Despite increased RR, alveolar ventilation barely maintained due to high VD
VD/VT ratio (dead space fraction) > 0.60 is associated with inability to wean from mechanical ventilation. In ARDS, dead space may increase to 50–70% of each breath, requiring very high minute ventilation to maintain normal PaCO₂. Anatomic dead space ≈ 1 mL/kg IBW (or 150 mL in average adult).