We designed this study to dtermine whether invasive mechanical ventilation (MV) would have an impact on the reclassification of patients with acute hypoxemic respiratory failure (AHRF) -treated previously with non-invasive respiratory support- into categories of severity (mild, moderate, and severe). Our hypothesis is that the assessment of PaO2/FiO2 ratio on PEEP greater or equal to 5 cmH2O after intubation, in patients labeled as mild/moderate/severe AHRF while on non-invasive respiratory support, would identify that a marked proportion of patients would change the degree of severity after a brief period of invasive MV

Current criteria for definition of the acute respiratory distress syndrome (AHRF) are inadequate for inclusion of patients into clinical trials due to: (i) the lack of standardization for measuring the oxygenation defect (as assessed by the PaO2/FiO2 ratio) and (ii) the inclusion of non-intubated patients into the mild category of severity. We questioned whether the PaO2/FiO2 (P/F) ratio calculated before endotracheal intubation in non-intubated patients who otherwise meet AHRF criteria, would still meet the severity threshold once patients are intubated.

We will examine at 24 after intubation and initiation of MV whether adult patients with AHRF remained in the same AHRF category of severity (mild, moderate, severe) or do not meet the PaO2/FiO2 criterion for AHRF. If our hypothesis is supported, it would suggest that stratification of patients labeled based on P/F ratio while treated with non-invasive respiratory support is of limited utility for testing specific therapies for AHRF in non-intubated patients.

This is a high quality screening, longitudinal, defined population, retrospective study.

Inclusion Criteria:

• Patients meeting criteria for AHRF while they are on non-invasive respiratory support.
• We will only include patients who had arterial blood gases (ABG) within a time-frame of the last 5 hours of NIV and the first 5 hours of invasive MV.
• We will only include patients treated with high-flow oxygen nasal cannula (HFNC), or continuous positive airway pressure (CPAP), or bilevel positive pressure (BiPAP).

Exclusion Criteria:

• Patients who never required HFNC, or CPAP, or BiPAP before intubation.
• Patients with no ABG during the last 5 hours of NIV before intubation.
• Patients with no ABG during the first 5 hours after initiation of invasive MV.
• Patients treated with inhaled pulmonary vasodilator only before or only after intubation.

Patients with AHRF (defined as PaO2/FiO2 lower or equal to 300 mmHg and FiO2 greater or equal to 0.3, on non-invasive ventilatory support) prior to intubation.