Pulmonary oedema develops when pulmonary capillary hydrostatic pressure exceeds the sum of plasma oncotic pressure and lymphatic drainage capacity. At what approximate pulmonary capillary wedge pressure (PCWP) does cardiogenic pulmonary oedema typically begin?
- A 8–12 mmHg (normal range)
- B 14–18 mmHg (vascular congestion, no oedema)
- C Above 25 mmHg ✓
- D Above 35 mmHg
Correct answer: C. Above 25 mmHg
Explanation
Normal PCWP is 6–12 mmHg and plasma oncotic pressure is approximately 25–28 mmHg. The safety factor against oedema includes lymphatic drainage (capable of increasing 7–10× under stress). Frank pulmonary oedema typically develops when PCWP exceeds approximately 25 mmHg, at which point the lymphatics are overwhelmed and fluid transudates into the alveolar interstitium and eventually alveoli. At 18–25 mmHg, pulmonary venous congestion and interstitial oedema occur (Kerley B lines, vascular redistribution) but alveolar flooding has not yet occurred.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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