this study evaluates high flow oxygen therapy in addition to non invasive ventilation (NIV) to treat hypercapnic respiratory failure. Between sessions of NIV, half of participants will have high flow nasal cannula while the others will have standard low flow oxygen therapy.

High Flow Nasal Cannula (HFNC) is a new way of oxygen therapy that has gained interest in the management of patients with acute respiratory failure. It allows reaching a high flow air up to 60 liters / min via a nasal cannula with a humidification and warming of the air administered. It has a number of physiological effects such as wash out of anatomical dead space, generation of a small PEEP and high inspired fraction of oxygen which enhances compliance and reduces inspiratory efforts.

NIV is the corner stone in the treatment of severe COPD exacerbation. Nevertheless, prolonged application of the facial mask expose to local complications and intolerance which can be a cause of failure, so reducing the duration of exposure to this procedure is important.

The role of HFNC in supplementing NIV effect during hypercapnic respiratory failure has not been assessed. Much of the data available on HFNC are about hypoxemic respiratory failure.

Because of its physiological effects, it can be hypothesized that HFNC in addition to NIV can shorten its duration by facilitating carbon dioxide clearance.

Inclusion Criteria:

• Patients aged more than 18 years with a diagnosis of acute-on-chronic respiratory failure and respiratory acidosis requiring NIV.

Exclusion Criteria:

• Patient included in another study
• Patients intubated at ICU admission or within 12 hours
• Contraindication to NIV including: pneumothorax, Hemodynamic instability, GCS less than 12, facial malformation
• Asthma
• A do not intubate order
• Neuromuscular disease