Non-invasive ventilation (NIV) in severe hypercapnic Chronic Obstructive Pulmonary Diseases (COPD) may be associated - during sleep - with recurrent episodes of patient ventilatory asynchrony, which in turn may affect quality of sleep, efficacy of ventilation and comfort of nocturnal NIV.Polysomnography (PSG) under NIV is necessary to detect these events.

Adjusting ventilator settings according to respiratory events detected by PSG with NIV may improve quality of sleep, efficacy of ventilation and comfort of nocturnal NIV.

Patients under NIV for hypercapnic COPD have several reasons to develop patient-ventilatory asynchrony: delayed cycling, and insufficient expiratory time may induce progressive dynamic hyperinflation, and increase intrinsic positive end-expiratory pressure (PEEPi); too high levels of pressure support may also contribute to dynamic hyperinflation. Increase in PEEPi is associated with two respiratory events: unrewarded inspiratory efforts, and auto-triggering.

Our hypotheses are: 1/that these events occur frequently in COPD under NIV and that they are not detected by medical history or usual monitoring tools (SpO2; PtcCO2); 2/ that they can be easily detected by polysomnography; 3/ that simple adjustments of ventilator parameters aiming to reduce dynamic hyperinflation and unrewarded inspiratory efforts may improve efficacy of ventilation, quality of sleep and comfort of treatment.

The present study compares the results of two consecutive sleep studies: 1.PSG under NIV in severe stable COPD under "usual ventilator settings" with 2.PSG under NIV after adapting ventilator settings to results of initial PSG.

Inclusion Criteria:

• COPD, with chronic hypercapnic respiratory failure, treated by NIV, in stable clinical condition, aged above 18 years

Exclusion Criteria:

• Unstable clinical condition