- Then, the patients were randomized into two groups: control and intervention group. Intervention group used NIV with a bi-level positive airway pressure (an inspiratory pressure to generate a tidal volume of 6ml/kg and an expiratory pressure of 7cmH2O) with a FIO2 0.4. Control group used oxygen by Venturi mask(FIO2 0.4) in order to keep a good oxygenation.