Expansion
Alveolar pressure minus pleural pressure
Expansion
- Transpulmonary pressure = alveolar pressure - pleural pressure
- It is the distending pressure of the lung, and it is always positive in health
Through a quiet breath:
| Point | Pleural | Alveolar | Transpulmonary |
|---|---|---|---|
| End expiration | -5 cmH2O | 0 | +5 |
| Mid inspiration | -6 | -1 | +5 |
| End inspiration | -8 | 0 | +8 |
Airflow depends on the difference between alveolar and atmospheric pressure, while inflation depends on the transpulmonary pressure. Confusing these is a common error.
In pneumothorax the pleural space communicates with the atmosphere, pleural pressure rises to zero, transpulmonary pressure falls to zero, and the lung collapses under its own recoil while the chest wall springs outwards.
In ventilated patients, oesophageal pressure is used to estimate pleural pressure so that transpulmonary pressure can be calculated, guiding PEEP so that alveoli remain open without overdistension.