• Quality of sleep [ Time Frame: One night ]
  
• Efficacy of ventilation [ Time Frame: One night ]
  

• Patient comfort [ Time Frame: One night ]
  

This study aims to compare, in a randomized order, two consecutive nights with a bi-level positive pressure ventilator (Synchrony, Respironics, USA), with and without average volume assured pressure support (AVAPS), in patients with chronic respiratory failure, treated on a long term basis by home nocturnal non-invasive ventilation.

Specific endpoints are :

• quality of sleep assessed by polysomnography scored by an independent investigator (sleep efficiency, distribution of sleep stages, arousals and sleep stage changes) and subjective scores (St Mary's questionnaire),
• perception of comfort of ventilation (clinical score),
• efficacy of ventilation (pulsoximetry, transcutaneous capnography (Tina TCM4 Radiometer, Copenhagen), effective ventilation and tidal volumes, leaks as measured by ventilator, Stardust software, Respironics).

Patients: Fourteen patients with obesity-hypoventilation and/or COPD, treated and stable for at least 3 months, will be included and undergo complete sleep studies in Geneva University Hospital's Sleep Laboratory. Patients included must be compliant to treatment (at least 4 hours of use of ventilator/night).

Methods: Patients are blinded as to whether or not AVAPS is implemented. Target tidal volume is set at 7-8 ml/kg. EPAP (Expiratory positive airway pressure) level is unchanged. Minimal IPAP (Inspiratory positive airway pressure) level is set at usual IPAP - 3 cm H2O. Maximal IPAP is set between 25 and 30 cm H2O. Back up frequency is unchanged. All patients must provide written informed consent. Protocol has been accepted by the ethics committee on clinical research of Geneva University Hospital.

Results of scores, and relevant parameters of polysomnography, oxymetry, and capnography, will be compared.