Inclusion Criteria:  Phase 1:  severe COPD with FEV1<50%, using home NIV successfully for at least 2 months through the Quebec National Program for Home Ventilatory Assistance (NPHVA).  Phase 2:  severe COPD with FEV1<50%; pCO2>= 52 on arterial or capillary blood gas measured 2-4 weeks after acute NIV use and discontinuation  Exclusion Criteria:  Phase 1  COPD exacerbation requiring treatment (including hospitalization) within the last 6 weeks; surgical procedure or major illness within the last 3 months; kyphoscoliosis, neuromuscular disease, other lung disease (e.g. fibrosis); active cancer; expected survival < 2 months; listed on the transplant list.  Phase 2:  any additional condition potentially predisposing to hypercapnia such as: obesity (BMI> 30kg/m2), kyphoscoliosis, neuromuscular disease, other lung disease (e.g. fibrosis), severe obstructive sleep apnea (AHI>30/h on prior testing if available, but will not test as part of the study); active cancer; expected survival < 2 months; listed on the transplant list at start of study.