Inclusion Criteria:  COPD III/ IV 35mmHg < PCO2 <= 55mmHg (under resting conditions, breathing room air) Hypoxemia (PaO2<60mmHg) under room air conditions (rest or during exercise) or SpO2 <88% during exercise established oxygen therapy or given indication for oxygen therapy written informed consent  Exclusion Criteria:  acute exacerbation of COPD exercise limiting cardiac or orthopedic comorbidites PaO2 <50mmHg at rest, breathing room air