 The second important issue to be solved regards better patient selection. By collecting the baseline and follow-up data of the COPD patients initiated on NIV, the investigators aim to find predictors of a favourable response to NIV. It is known, from clinical practice and from studies, that not all patients respond favourably. Until now, data have shown that patients with CHRF benefit. Furthermore, benefits at least in terms of improvement in gas exchange, seem to be more prominent in patients with severe stable hypercapnia. In contrast, our group has shown that patients that remain hypercapnic after an exacerbation do not uniformly benefit. An individual COPD phenotype benefiting most from this demanding therapy has not been identified. 