ARDS was recognized as the most severe form of acute lung injury (ALI), a form of diffuse alveolar injury. The AECC defined ARDS as an acute condition characterized by bilateral pulmonary infiltrates and severe hypoxemia in the absence of evidence for cardiogenic pulmonary edema. The severity of hypoxemia necessary to make the diagnosis of ARDS was defined by the ratio of the partial pressure of oxygen in the patient’s arterial blood (PaO2) to the fraction of oxygen in the inspired air (FiO2). ARDS was defined by a PaO2/FiO2 ratio of less than 200, and in ALI, less than 300.
This definition was further refined in 2011 by a panel of experts and is termed the Berlin definition of ARDS.  ARDS is defined by timing (within 1 wk of clinical insult or onset of respiratory symptoms); radiographic changes (bilateral opacities not fully explained by effusions, consolidation, or atelectasis); origin of edema (not fully explained by cardiac failure or fluid overload); and severity based on the PaO2/FiO2 ratio on 5 cm of continuous positive airway pressure (CPAP). The 3 categories are mild (PaO2/FiO2 200-300), moderate (PaO2/FiO2 100-200), and severe (PaO2/FiO2 ≤100).