Although many studies have investigated the clinical benefits of nasal high flow during acute hypoxemic respiratory failure, there is no data (and even less so recommendations) on how to best conduct this technique, including its initiation and its weaning periods. Investigators will assess in a multicenter, observational study, the way clinicians use nasal high flow therapy in patients with acute respiratory failure in order to try identify one or more strategies that may be then compared in an interventional study.