 In immunocompromised patients, Acute Respiratory Failure ARF is associated with a high case-fatality, particularly when invasive Mechanical Ventilation MV is required. In the most hypoxemic patients, oxygen administration through High Flow Nasal Cannula HFNC has been reported as an alternative to the venturi mask. The aim of this study is to compare HFNC and venturi mask on early respiratory deterioration and patients comfort in that setting. The investigators planned a prospective randomized study in 4 Intensive Care Units ICUs. As respiratory deterioration occurs early after ICU admission, patients are randomized to receive two hours of oxygen therapy either through HFNC or venturi mask. The primary endpoint is defined as the need for invasive or noninvasive MV in the 2-hour period. Secondary endpoints include comfort, dyspnea and thirst. 