 In 1994, the American-European Consensus Conference AECC defined ARDS as follows: 1 acute and sudden onset of severe respiratory distress, 2 bilateral infiltrates on frontal chest radiograph, 3 absence of left atrial hypertension or no clinical signs of left heart failure, and 4 severe hypoxemia, as PaO2FiO2 ratio 200 mmHg, regardless of FIO2 or PEEP levels. Although this definition could be adequate for epidemiologic studies, it is not adequate for inclusion of patients into therapeutic clinical trials. In 2012, a proposal for updating the ARDS definition the Berlin criteria was published and an empirical classification was proposed according to three PaO2FiO2 cut-off values at ARDS onset: severe 100 mmHg, moderate 100-200, and mild 200-300 on PEEP5 cmH2O. However, despite that there is sufficient evidence about the interactions between PEEP and FiO2, the Berlin criteria did not mandate the assessment of hypoxemia at 24 hours under standarized guidelines. 