Nasal high flow oxygen therapy is increasingly used as a first-line treatment for hypoxemic acute respiratory failure, because of its remarkable tolerance (in comparison with NIV) and its physiological effects (nasopharyngeal dead space washout, positive end-expiratory pressure effect with possible alveolar recruitment, better matching with the patient's inspiratory flow, more reliable and adjustable FiO2); that together contribute to a reduction in respiratory workload and better oxygenation.