ICU · respiratory

Acute Respiratory Distress Syndrome (ARDS) — Berlin Definition, Lung-Protective Ventilation, Proning, ECMO, and Phenotyping

Also known as ARDS · Acute respiratory distress syndrome · Berlin definition ARDS · Non-cardiogenic pulmonary oedema · Diffuse alveolar damage · Lung-protective ventilation · Prone ventilation · VV-ECMO · ARDS subphenotypes · PROSEVA · Baby lung

Acute respiratory distress syndrome (ARDS) — a syndrome of acute, diffuse, inflammatory lung injury causing increased pulmonary vascular permeability, loss of aerated lung, and severe hypoxaemia. The 2012 Berlin Definition replaces AECC: (1) Timing — within 1 week of a known clinical insult or new/worsening respiratory symptoms; (2) Chest imaging — bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules (CT or CXR); (3) Origin of oedema — respiratory failure NOT fully explained by cardiac failure or fluid overload (objective assessment — echo — required if no clear risk factor); (4) Oxygenation — PaO2/FiO2 ratio measured with PEEP/CPAP >=5 cmH2O: MILD 200-300 (inclusive), MODERATE 100-200, SEVERE <100. Causes are DIRECT/pulmonary (pneumonia — most common, aspiration of gastric contents, pulmonary contusion, inhalation injury, near-drowning) or INDIRECT/extrapulmonary (sepsis — most common indirect, severe trauma/shock, acute pancreatitis, massive transfusion/TRALI, severe burns, drug overdose). Pathophysiology — diffuse alveolar damage: exudative phase (type I pneumocyte + capillary endothelial injury → protein-rich alveolar oedema → hyaline membranes), proliferative phase (type II pneumocyte proliferation, fibroblast infiltration), fibrotic phase. The 'baby lung' concept (Gattinoni) — the aerated lung in ARDS is small, not stiff; high tidal volumes overdistend the small healthy portion → volutrauma. Management PILLARS: (1) Treat the cause (antibiotics, source control). (2) Lung-protective ventilation (ARDSNet — Vt 6 mL/kg PREDICTED body weight, plateau pressure <30 cmH2O, driving pressure <15 cmH2O, permissive hypercapnia pH >7.20). (3) PEEP optimisation (PEEP/FiO2 table, best compliance, oesophageal pressure). (4) Prone positioning (PROSEVA — >=16 h/day for moderate-severe ARDS P/F <150, ~50% relative mortality reduction). (5) Conservative fluid strategy (FACTT). (6) Corticosteroids (DEXA-ARDS — dexamethasone improves moderate-severe ARDS). (7) VV-ECMO for severe refractory ARDS (EOLIA). Neuromuscular blockade is NOT routine (ROSE). ARDS subphenotypes (Calfee 2014) — hyperinflammatory vs hypoinflammatory — differ in mortality and treatment response. Mortality ~35% overall (severe ~45%).