In 1967, Ashbaugh et al first described 12 patients with a syndrome characterized by the acute onset dyspnea, severe hypoxemia, diffuse lung infiltrates on the chest radiography and decreased lung compliance. Moreover, in 1988, Murray et al proposed a lung injury scoring system based on the level of positive end-expiratory pressure (PEEP), PaO2-to-FiO2 ratio, static lung compliance, degree of infiltrates on the chest radiograph, and clinical cause. A score of 2.5 or greater was considered to be consistent with a diagnosis of ARDS. The current definition of ARDS was proposed by Bernard et al and the American-European Consensus Conference in 1994. The criteria of ARDS included: 1) acute onset; 2) a PaO2/FIO2 ratio, or hypoxia score, of < 200, regardless of positive endexpiratory pressure; 3) bilateral infiltrates on chest radiograph; and 4) a pulmonary artery occlusion pressure of <18 mm Hg or the absence of clinical evidence of left atrial hypertension.