<qa>  What is the pH of the COPD patients?Study population: Seventy-two COPD patients with a NIV indication COPD GOLD stage III or IV; partial arterial carbon dioxide pressure PaCO2 6.0 kPa in stable condition, i.e. no COPD exacerbation for 4 weeks and a pH 7.35, a sufficient social network at home, without severe cardiac comorbidities, will be included. 