 Background : Patient with hypoxemic respiratory failure treated with NIV receive between NIV session oxygen therapy. Gaz exchange disorder with a decrease of the ratio between Partial Pressure of Arterial Oxygen PaO2 and Inspired Fraction of Oxygen FIO2 are noticed when NIV is stopped at the end of the session du to alveolar derecruitment Optiflow is a high flow oxygen delivery system used a heated humidifier and heated breathing circuit. In observational studies, Optiflow increase oxygenation of patients with hypoxemic respiratory failure. Oxygenation is better than under high-concentration mask and work of breathing is reduced. 