Expansion
Minute ventilation includes dead space; alveolar ventilation does not
Expansion
- Minute ventilation = tidal volume x rate, about 6 litres per minute
- Alveolar ventilation = (tidal volume - dead space) x rate, about 4.2 litres per minute
Because dead space is a fixed volume per breath, the breathing pattern determines how much of the minute volume is effective:
| Pattern | Minute volume | Alveolar ventilation |
|---|---|---|
| 1000 ml x 6 | 6 litres | 5.1 litres |
| 500 ml x 12 | 6 litres | 4.2 litres |
| 250 ml x 24 | 6 litres | 2.4 litres |
All three move the same total volume, but the effective ventilation varies more than twofold.
This explains why rapid shallow breathing signals impending respiratory failure: it is inefficient, and carbon dioxide rises even though the patient appears to be ventilating vigorously. The rapid shallow breathing index, respiratory rate divided by tidal volume in litres, is used to predict failure of weaning from ventilation, with a value above 105 suggesting failure.