Acute Respiratory Distress Syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) is a severe, life-threatening form of respiratory failure characterized by diffuse bilateral pulmonary infiltrates, severe hypoxemia, and non-cardiogenic pulmonary edema. ARDS is defined by the Berlin criteria: acute onset (within 1 week of clinical insult), bilateral opacities on chest imaging (not explained by effusions, atelectasis, or nodules), respiratory failure not fully explained by cardiac failure or fluid overload, and PaO2/FiO2 ratio categorizing severity (mild 200-300, moderate 100-200, severe below 100 mmHg with PEEP 5+ cmH2O). The pathophysiology involves three phases: (1) Exudative phase (days 1-7): an inciting event (pneumonia, sepsis, aspiration, trauma, pancreatitis, transfusion) triggers diffuse alveolar damage (DAD) with neutrophil infiltration, release of inflammatory mediators (TNF-alpha, IL-1, IL-6, IL-8), endothelial and epithelial injury, increased alveolar-capillary permeability, and flooding of alveoli with protein-rich edema fluid. This inactivates surfactant, causes alveolar collapse, and creates intrapulmonary shunting (perfused but unventilated alveoli). (2) Proliferative phase (weeks 1-3): type II alveolar cells proliferate to restore the alveolar epithelium; fibroblasts deposit collagen; some patients improve while others develop progressive fibrosis. (3) Fibrotic phase (after week 3): extensive fibrosis and remodeling;...