Expansion
Raised hydrostatic pressure or increased permeability floods the interstitium then alveoli
Expansion
Cardiogenic (hydrostatic)
- Raised pulmonary capillary pressure, usually from left heart failure or mitral disease
- Oedema fluid is protein-poor
- Wedge pressure high, typically above 18 to 20 mmHg
Non-cardiogenic (permeability)
- Damaged alveolar-capillary membrane: acute respiratory distress syndrome, sepsis, aspiration, near drowning, high altitude
- Oedema fluid is protein-rich
- Wedge pressure normal
The sequence of accumulation explains the clinical progression:
- Interstitial oedema first, since the interstitium is the most compliant space. Stimulates J receptors, giving rapid shallow breathing and breathlessness; radiologically gives Kerley B lines and peribronchial cuffing
- Alveolar flooding once the interstitium and lymphatics are overwhelmed, giving crackles, frothy sputum and shunt
Lymphatic drainage can increase up to tenfold, which is why chronic elevation of pressure is tolerated far better than an acute rise.