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PH-RS07

Pulmonary oedema

How the lungs keep their air spaces dry, why high pressure in the pulmonary capillaries (as in left heart failure) or a damaged, leaky barrier (as in ARDS) floods them with fluid, and how that causes breathlessness, crackles and low oxygen.

By the end, you can

  • Describe the alveolar-capillary barrier and the forces that move fluid across it.
  • Distinguish cardiogenic (high-pressure) from non-cardiogenic (leaky) pulmonary oedema.
  • Explain the sequence from interstitial to alveolar oedema and its effect on gas exchange.
  • Relate orthopnoea, paroxysmal breathlessness at night, crackles and frothy sputum to the mechanism.
  • Recognise the main causes of acute respiratory distress syndrome.
  • Recognise the classic lung appearances, X-ray stages, ARDS severity grades and tests of pulmonary oedema, and explain how oxygen, positive pressure, diuretics and nitrates act on its mechanism.

What it is

Pulmonary oedema is abnormal accumulation of fluid in the lung interstitium and alveoli, impairing gas exchange. Cardiogenic (hydrostatic) oedema follows raised pressure in the pulmonary capillaries, usually from left heart failure. Non-cardiogenic oedema follows injury to the alveolar-capillary barrier, as in the acute respiratory distress syndrome (ARDS).

2 types to tell apart
Cardiogenic (high-pressure)
Left ventricular failure, mitral valve disease, severe hypertension or fluid overload raise pulmonary capillary pressure.
Non-cardiogenic (increased permeability)
Sepsis, pneumonia, aspiration, trauma or pancreatitis damage the barrier; capillary pressure may be normal.

US spelling: oedema → edema.

Normal function

Keeping the air spaces dry

Each is wrapped in a dense net of . The barrier between air and blood is extremely thin, a fraction of a micrometre in places, which makes rapid gas exchange possible.

Fluid crosses capillaries according to the balance of hydrostatic pressure (pushing out) and oncotic pressure from plasma proteins (pulling in). Pulmonary capillary pressure is low, so only a little fluid filters into the interstitium, and lung lymphatics carry it away. Tight junctions between alveolar cells keep it out of the air spaces.

secrete surfactant, which lowers surface tension and helps keep the alveoli both open and dry, and they pump salt and water out of the air spaces.

Point to it on the model
Forces across the alveolar-capillary barrierSchematic
NormalPulmonary capillaryLow hydrostatic pressure; plasma proteins
  • Small filtration, removed by lymph (from Pulmonary capillary to Interstitium)
InterstitiumDrained by lymphatics
  • Normally blocked (from Interstitium to Alveolus)
NormalAlveolusKept dry by tight junctions and pumps

Schematic. Raised pressure or a damaged barrier overwhelms drainage.