Intermittent Hypoxemia, Lung Function Decline, Morbidity, and Mortality in COPD (PROSA Study).

Inclusion Criteria: 1. Patients of both sexes with a diagnosis of chronic obstructive lung disease (COPD) stages 2-3, groups B and E according to GOLD 38 as assessed by their primary family doctor or pneumology specialist who are treated with long-acting bronchodilators on a stable dose for at least four months prior to inclusion into the study; 2. Age at inclusion 18 - 80 years; 3. Physical ability and willingness to perform 6-minute walk testing and bicycle ergometry testing; 4. Signed written informed consent form Exclusion Criteria: 1. Participation in another clinical study within the last 3 months before inclusion into the present study; 2. Any somatic or psychic disease that may hamper participation in the study or compliance with the study protocol (at the investigator's discretion); 3. Any somatic disease that puts the patient at increased risk when performing the exercise tests or impairs the patient's ability to properly perform the exercise tests, like advanced coronary artery disease, advanced chronic heart failure, uncontrolled hypertension, orthopaedic, or neurological diseases (non-exclusive list of examples); 4. Any disease or health condition that reduces the patient's life expectancy to less than the expected time frame of this study, other than chronic obstructive lung disease (at the investigator's discretion); 5. Pregnancy or nursing.