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. The syndrome, initially called acute respiratory distress in adult and is now named as the acute respiratory distress syndrome (ARDS). 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.