The American-European Consensus Conference (AECC) and the Berlin definitions of the Acute Respiratory Distress Syndrome (ARDS) could be adequate for epidemiologic studies, but it is not adequate for inclusion of patients into therapeutic clinical trials. Despite recent reports on the effects of standardized ventilator settings on PaO2/FIO2 and fulfillment of AECC and Berlin definitions of ARDS, it is still a matter of debate whether the assessment of hypoxemia at 24 hours is the most appropriate tool for stratifying lung severity in patients with ARDS. The investigators will perform an observational, multicenter, prospective audit in a network of intensive care units in Spain and China for validating and confirming that the assessment of hypoxemia at 24 hours after ARDS onset is the most valuable tool for stratifying and predicting outcome in patients with ARDS.

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 PaO2/FiO2 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 PaO2/FiO2 cut-off values at ARDS onset: severe (≤100 mmHg), moderate (>100-≤200), and mild (>200-≤300) on PEEP≥5 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.

The PaO2/FIO2 can be easily manipulated. Alterations in PEEP and FIO2 can dramatically change the PaO2/FIO2. Despite recent reports on the effects of standardized ventilator settings on PaO2/FIO2 and fulfillment of AECC definitions of ARDS, it is still a matter of debate whether the assessment of hypoxemia at 24 hours is the most appropriate tool for stratifying lung severity in patients with ARDS.

The investigators will examine whether the assessment of hypoxemia at 24 h after ARDS diagnosis under standard guidelines for ventilatory management have an impact on the stratification of lung severity and on predicting mortality of ARDS patients in the intensive care unit.

Inclusion Criteria:

• Patients meeting the AECC definition for ARDS on PEEP greater or equal than 5 cmH2O (or the Berlin criteria for moderate and severe ARDS).

Exclusion Criteria:

• None

All consecutive ARDS patients