Taken recent literature together, there is a sufficient number of trials investigating the effect of different oxygen devices. However, studies comparing oxygen delivery via portable oxygen concentrator (POC) and liquid oxygen device (LOD) with appropriate exercise testing and sufficient power are missing.

Given that walking is the most important activity of daily life to preserve the maintenance and to participate in social life, we aim to investigate the effects of two different oxygen delivery systems during walking in hypoxemic COPD patients (POC vs. LOD).

The endurance shuttle walk test (ESWT) is well validated for measuring endurance walking capacity in COPD patients with good repeatability. The advantage of this test over the 6MWT is that the ESWT is performed at 85% of the individual maximum which is close to the intensity of typical daily activities. Due to the fact that the ESWT enables us to determine the maximum duration of exercise and to compare values at isotime (at the point of time when the shortest of the 3 ESWTs ends), we use the ESWT as exercise test in our trial.

Inclusion Criteria:

• COPD patients (GOLD stage III to IV) with hypoxemia at rest or during exercise (paO2<60mmHg)
• Participation in an inpatient pulmonary rehabilitation program (Schön Klinik BGL)
• Written informed consent

Exclusion Criteria:

• General exclusion criteria for exercise tests, e.g. acute coronary syndrome, acute myo- or pericarditis, acute lung embolism, pulmonary infarction, acute uncontrolled heart insufficiency
• Signs of acute exacerbation
• Any orthopedic or neurological disabilities that prevent patient from walking