Main hypothesis is that Neurally Adjusted Ventilatory Assist (NAVA) allows asynchrony reduction in non invasive ventilation (NIV) in comparison with reference Pressure Support (PSV) mode.

The purpose of the protocol is to compare asynchrony rate between periods of pressure support and periods of NAVA.

After written information and consent, patients are randomised in two groups: NAVA first or PSV first.

They receive a NAVA catheter, which has exactly the same appearance as a standard naso-gastric catheter.

A standardised set of 1 hour of non-invasive ventilation is started. Depending on the randomisation group, patients start with a 30 minutes set of NIV with NAVA settings or with a 30 minutes set of NIV with PSV settings. A 30-minute watch-out period is then delivered with no mechanical ventilation.

Then a second 30-minute set of NIV in the other settings (NAVA or PSV depending on the randomisation) is delivered.

During each set of NIV, flow, volume, pressure and EADi are recorded during 10 minutes, after a 10-minute period of equilibration. Recordings will be processed by two blind investigators in order to count the number of asynchronies. In case of disagreement between investigators, a third one will be interviewed.

Inclusion Criteria:

• - Patient older then 18 years
• Patient requiring first non invasive ventilation set because of acute respiratory failure

Exclusion Criteria:

• Patients protected by the law
• Pregnant women