In the present investigation, in 50 COPD with persistent hypercapnia after an acute exacerbation, the investigators want to assess the acceptability of HFNC and its effectiveness in further reducing the level of PaCO2, and to eventually verify the hypothesis, based on previous physiological studies, that the response to HFNC is dependent on the level of baseline hypercapnia and eventually on the presence of overlap syndrome. This latter parameters to eventually calculate the sample size and the target population to perform future definitive randomized long term trials vs NIV.

Fifthy COPD patients inpatients recovering from an acute exacerbation of their disease and persisting hypercapnia, despite having reached a stabilization in pH (i.e. pH>7,35 and PaCO2>45 mmHg on 3 consecutive measurements) will be enrolled in the study after having signed a written informed consent OSA/COPD overlap syndrome was not considered and exclusion criteria and is defined as the presence of 15 or more obstructive respiratory events per hour of sleep, when a previous full night polysomnography (PSG) was available or from a positive Epworth questionnaire and a Body Mass Index>25 Cardiac decompensation, restrictive thoracic disorders, renal insufficiency, cancer, and neurological disease were considered exclusion criteria.

On day the patients will undergo a preliminary trial with HFNC to set the optimal flow, using the AIRVO2 (Fisher & Paykel Healthcare, Auckland, New Zealand). To this aim the patients will be asked to breathe keeping their mouth closed, as much as they could, at different levels of flow, starting from 20 L/min up to 40 if tolerated for a minimum of 15 minutes for each trial. At the end of this test the maximum level tolerated will be chosen as the flow to be set for the experimental procedure. Temperature will be set according to the patient's tolerance starting from 34C, up to 37C, while FiO2 will be kept constant to maintain an SaO2 between 92 and 94%.

From 9 am of day 2 to 9 am of day 5 (72 hours period), the patients will undergo HFNC for at least 8 hours/day and during the nightime. The nurse on shift will be in charge of supervising the adherence to this schedule and to report any violation of the protocol on a dedicated sheet. Every morning at 10 am (1 hour after HFNC suspension) Arterial Blood Gases (ABGs), effective hours of HFT and tolerance to the treatment as reported by the patient, using a dedicated scale where: )1. Very bad 2. Bad 3. Moderate 4. Good 5.very good) will be recorded

Inclusion Criteria:

• inpatients recovering from an acute exacerbation of their disease
• persisting hypercapnia, despite having reached a stabilization in pH (i.e. pH>7,35 and PaCO2>45 mmHg on 3 consecutive measurements)

Exclusion Criteria:

• Cardiac decompensation,
• restrictive thoracic disorders,
• renal insufficiency,
• cancer,
• neurological disease

cannot release data prior to the termination of the study