Such settings will be kept unchanged during the whole study period. An oesophageal catheter will be placed and secured to measure oesophageal pressure (Pes) and gastric pressure (Pga) (Nutrivent, Italy): the reliability of the measured pressure will be confirmed with an airway occlusion test during NIV with oronasal mask. Work of breathing will be estimated with the pressure-time product (PTP) of the pleural pressure.