The intervention under investigation will be High flow nasal therapy. Investigators will deliver HFNT to enrolled patients using any available device able to produce it. The flow will be initially set at 60 liters per minute and temperature at 37° C. The target will be an oxygen saturation (SpO2) of 88-92%. In case of patient not tolerating these settings, flow and temperature will be titrated to the maximum tolerated level. The intervention will last until one of the following conditions: 1) physician in charge decide to interrupt HFNT and start NIV because the patient shows signs of persisting worsening respiratory failure; 2) physician in charge decide to interrupt HFNT and perform endotracheal intubation and invasive mechanical ventilation because patient shows one or more of these conditions: respiratory arrest, respiratory pauses with loss of consciousness, psychomotor agitation making nursing care impossible, heart rate < 50 with loss of alertness, hemodynamic instability with systolic arterial blood pressure <70 mmHg, development of conditions requiring intubation either to protect the airway or to manage copious tracheal secretions, inability to tolerate the devices; 3) clinical improvement defined as all these conditions: normal mental status, hemodynamic stability, respiratory rate below 25 per minute, absence of activation of accessory respiratory muscles and paradoxical abdominal motion, no dyspnea arterial pH > 7.35, Arterial partial pressure of carbon dioxide (PaCO2) < 70 mmHg and Arterial partial pressure of oxygen (PaO2) > 55 mmHg with an inspired oxygen fraction (FiO2) < 0.35.