Similar to other syndromes, patients are defined as having acute respiratory distress syndrome (ARDS) when they meet prespecified diagnostic criteria. These criteria have evolved over time, having gained and lost complexity, but the core principles have remained remarkably similar over the past 45 years. The specific diagnostic criteria allow clinicians and investigators to reliably identify patients with the syndrome of ARDS. ARDS is a form of acute diffuse lung injury occurring in patients with a predisposing risk factor. Lung injury is characterized by inflammation leading to increased endothelial and epithelial permeability and loss of aerated lung tissue resulting in hypoxemia and bilateral radiographic opacities on chest radiography. Additional physiological derangements include increased venous admixture and physiological dead space along with decreased respiratory system compliance. The corresponding pathological findings are lung edema, inflammation, hyaline membranes, and alveolar hemorrhage (i.e., diffuse alveolar damage). This article discusses the evolution of the definition of ARDS to the new Berlin definition of ARDS proposed in 2012 and its novel iterative refinement.
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