Current definitions of the Acute Respiratory Distress Syndrome (ARDS) could be adequate for epidemiological studies but are not adequate for inclusion of patients into therapeutic clinical trials. It is a matter of debate whether the assessment of hypoxemia at ARDS onset is appropriate for stratifying lung severity and risk of death in ARDS patients. The investigators will perform an observational, non-interventional, multicenter, prospective audit in a network of intensive care units in Spain for evaluating the severity and risk of death based on the assessment of respiratory and ventilatory function at 24 hours after ARDS diagnosis under standardized ventilatory conditions. This study is the confirmatory phase of the study NCT02288949.

In 2012, an update of 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 mmHg), and mild (>200 - ≤300 mmHg) on positive end-expiratory pressure (PEEP) ≥5 cmH2O. However, despite that there is sufficient evidence about the interactions between PEEP and FiO2, these cut-off values did not mandate the assessment of hypoxemia under standardized 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, is still a matter of debate whether the assessment of hypoxemia must be performed at 24 hours instead of at ARDS onset, and whether the assessment of hypoxemia under standardized ventilator settings is the most appropriate tool for stratifying lung severity in patients with ARDS.

The investigators will examine whether the values of relevant variables (including age, plateau pressure, driving pressure, compliance, PaO2/FiO2) at 24 after ARDS diagnosis under standardized ventilator settings have an impact on the stratification and prediction of death in ARDS patients. This study is an extension of the study NCT02288949.

Inclusion Criteria:

• Patients meeting the Berlin criteria for moderate and severe ARDS.

Exclusion Criteria:

• None

All consecutive patients with moderate and severe ARDS admitted into participating intensive care units, and requiring endotracheal intubation and mechanical ventilation.