The purpose of this study is to determine the impact of hign-flow nasal therapy on the adult with hypercapnia and hpoxemia respiratory faliure in comparison with standard oxygen therapy ang noninvasive ventilation.

Humidified high flow nasal oxygen therapy decreases dilution of the inhaled oxygen and, by matching patient's peak flow, allows accurate delivery of the set FiO2 throughout the whole inspiratory phase.The purpose of this study is to determine the impact of hign-flow nasal therapy on adult hypercapnic respiratory failure by comparing with nasal continuous positive airway pressure

Inclusion Criteria:

• Hypoxemic and no hypercapnic acute respiratory failure :
• severe dyspnea at rest with a respiratory rate >25 breaths/min
• PaO2/FiO2 <300
• PaCO2 <45 mmHg,

Exclusion Criteria:

• age <18 years
• NPPV contraindications
• past history of respiratory chronic disease (COPD, cystic fibrosis…)
• cardiac pulmonary edema
• Pre-defined intubation
• other than respiratory organ failure : systolic pressure <90 mmHg,current treatment with epinephrine or norepinephrine, decreased level of consciousness ( Glasgow score ≤ 12)
• profound aplasia (white cells count <1000/mm 3)