Upon enrolment, patients will be randomized to receive NIV via one of the two available interfaces. In group H (for "helmet"), the continuous positive pressure is obtained by a high influx of fresh gases (air + oxygen) flowing through a high-compliance reservoir, with a positive end-expiratory pressure valve limiting outflow. In the M ("oral-nasal mask") group the same flow scheme is obtained using a T-tube attached to the mask. Patients' heads will be elevated to about 45° in both groups.