Acute hypoxemic respiratory failure may require invasive mechanical ventilation. However, invasive mechanical ventilation is associated with a variety of complications. Non-invasive ventilation has been presented as an alternative treatment but controversy remains. The investigators hypothesize that high-flow nasal cannula system is effective enough to prevent intubation in acute hypoxemic respiratory failure and not inferior to non-invasive ventilation.

Inclusion Criteria:

• Age above 18
• patients with acute hypoxemic respiratory failure

Exclusion Criteria:

• age < 18 years
• hypercapnia (arterial carbon dioxide tension (PaCO2) >45mmHg) at admission
• need for emergency intubation, including cardiopulmonary resuscitation
• recent esophageal, facial or cranial trauma or surgery
• severely decreased consciousness (Glasgow coma score <11)
• cardiogenic shock or severe hemodynamic instability
• systolic blood pressure <90 mmHg associated with decreased urinary output(<20 mL.h-1) despite fluid repletion and use of vasoactive agents
• lack of co-operation
• altered mental status with decreased consciousness and/or evidence of inability to understand or lack of willingness to co-operate with the procedures
• tracheotomy or other upper airway disorders
• severe ventricular arrhythmia or active myocardial ischemia
• active upper gastrointestinal bleeding
• inability to clear respiratory secretions
• more than one severe organ dysfunction in addition to respiratory failure