ARDS and Pulmonary Oedema
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Direct answer
Acute respiratory distress syndrome is the clinical consequence of diffuse alveolar damage — widespread injury to the alveolar-capillary barrier from sepsis, trauma, aspiration, pneumonia, shock or pancreatitis, making the membrane leaky. Protein-rich oedema floods alveoli, necrotic type I pneumocytes and fibrin deposit as hyaline membranes, and refractory hypoxaemia with stiff, non-compliant lungs follows; type II pneumocyte proliferation and later organisation or fibrosis complete the pathology. It contrasts with hydrostatic pulmonary oedema of left heart failure, in which an intact barrier is flooded by pressure-driven, protein-poor fluid.
What you must remember
- Definition: acute-onset hypoxaemia with bilateral infiltrates, no cardiac cause, and reduced PaO2/FiO2 ratio — the clinical face of diffuse alveolar damage.
- Causes: sepsis is the commonest trigger; others are major trauma, aspiration of gastric contents, severe pneumonia, shock, pancreatitis, burns, near-drowning and fat embolism.
- Acute (exudative) phase: interstitial and intra-alveolar oedema with protein-rich fluid, hyaline membranes lining alveoli — eosinophilic bands of fibrin and necrotic type I cell debris — plus interstitial inflammation and thrombi.
- Proliferative (organising) phase: type II pneumocytes proliferate to re-line alveoli and produce surfactant; interstitial oedema organises with fibroblast ingrowth, and some patients progress to honeycomb fibrosis.
- Physiology: markedly reduced compliance, refractory hypoxaemia from shunt and low functional residual capacity; management is lung-protective, low-tidal-volume ventilation with appropriate positive end-expiratory pressure per current guidance.
- Neonatal parallel: neonatal respiratory distress syndrome shares hyaline membranes but stems from surfactant deficiency from immature type II cells, linked to prematurity and maternal diabetes.
- Hydrostatic oedema contrast: left heart failure and mitral stenosis raise pulmonary venous pressure, producing perihilar bat-wing opacities with protein-poor transudate and small haemosiderin-laden macrophages in chronicity ("brown induration").
Common confusion
The two oedemas differ by barrier integrity: hydrostatic oedema is a transudate that clears rapidly with diuresis, whereas permeability oedema of ARDS is an exudate that organises into membranes and fibrosis. Hyaline membranes appear in both adult diffuse alveolar damage and neonatal surfactant deficiency — in neonates the cause is developmental, in adults injurious. Finally, ARDS is a syndrome, not a diagnosis; every stem hides a trigger, most often sepsis, that must be named and treated.
Exam-focused takeaway
Questions pair a trigger — sepsis, near-drowning, pancreatitis — with bilateral infiltrates and a normal heart, then ask for the pathology (hyaline membranes of diffuse alveolar damage) or the management principle. Photomicrographs of hyaline membranes are classics; the neonatal-versus-adult comparison and transudate-versus-exudate split are frequent one-liners. Remember sepsis as the commonest cause and lung-protective ventilation as the evidence-based support.
Frequently asked questions
What are hyaline membranes?
Eosinophilic bands of fibrin and necrotic type I pneumocyte debris lining alveolar walls — the histological signature of diffuse alveolar damage.
What is the commonest cause of ARDS?
Sepsis, followed by major trauma, aspiration and severe pneumonia as frequent triggers.
How does hydrostatic pulmonary oedema differ from ARDS oedema?
Heart-failure oedema is a protein-poor transudate through an intact barrier; ARDS oedema is protein-rich exudate through a damaged barrier that organises into membranes and fibrosis.
What happens in the proliferative phase of diffuse alveolar damage?
Type II pneumocytes proliferate to cover denuded basement membranes, and fibroblasts organise the exudate, sometimes progressing to interstitial fibrosis.
Why does neonatal respiratory distress syndrome show hyaline membranes?
Surfactant deficiency from immature lungs causes atelectasis and membrane formation from barotrauma and hypoxia, rather than barrier injury from sepsis or trauma.