Split-night polysomnography: Obstructive sleep apnoea syndrome refractoriness with continuous positive airway pressure and bilevel positive airway pressure, nasal mask. (Evaluation at 4 months). The first part of the night confirmed the severity of obstructive sleep apnoea syndrome (apnoea and hypopnoea index of 64.9/h with minimum O2 saturation of 29%). The second part allowed a gradual increase in positive pressure, first in continuous positive airway pressure for up to 16 cmH2O, then in bilevel positive airway pressure with a maximum inspiratory pressure of 24 cmH2O and a maximum expiratory pressure of 20 cmH2O. There were persistent obstructive events with marked desaturation (minimum O2 saturation of 45% in continuous mode and of 82% in bilevel mode).