Acute respiratory distress syndrome (ARDS) is a clinical syndrome which includes severe dyspnea, hypoxia and invasive pulmonary infiltration. While there is no gold standard test for diagnosing ARDS, its recognition is relied on a valid definition [1,2].

In 1967, Ashbaugh and Petty defined ARDS as a clinical founding which is including long lasting tachypnea, hypoxemia, decreased lung compliance, bilateral opacity in chest X-ray with high mortality [3]. In 1988, John Murray, suggests lung injury score. Which includes: 1) opacities on chest X-ray 2) Hypoxemia 3) Applied PEEP 4) elasticity of respiratory system [4].

In 1994, American European consensus conference (AECC) described ARDS as a syndrome which includes: 1) acute onset of respiratory failure, 2) sever hypoxemia as defined by PaO₂/FIO₂<200 mmHg, 3) bilateral infiltrate on chest X-ray, 4) no evidence of left ventricle failure and pulmonary capillary wedge pressure<18 [5] In this definition, risk factors were not included, interobserver reliability was moderate and there was no clear definition of acute phase [6]. Berlin definition introduced with 3 categories: 1) mild (200 mm Hg2/ FIO₂<300 mmHg), 2) moderate (100 mmHg2/FIO₂<200 mmHg) and 3) severe (PaO₂/FIO₂<100 mmHg). Acute onset defines when the respiratory symptoms complicate or patient exposes to defined risk factors during a week. In chest X-ray, bilateral infiltration which is involving 3 quadrants or more and is not compatible with pleural effusion, nodules or atelectasis is important [7].

Berlin definition is not a sufficient and complete definition but in compare with AECC definition did better at predicting mortality [8].