The study will be conducted on 2 consecutive days. 15 patients will be randomized into two different groups: First group will start with POC and will continue the following day with LOD; The second group will start with LOD and continue with POC (cross-over design). All patients will use the same oxygen devices during the study assessments (LOD: Companion 1000 (CE 0050), Chart Industries Inc., Garfiel Heights, OH, USA; POC: Activox™ 4L, Inovalabs Inc., Texas, USA). On both days of the study, patients will be connected to one of the two systems via nasal cannula for a total time period of 40 minutes, while the patient remains in a sitting position without talking. The oxygen flow rate starts at 1liter/min (LOD) or Level 1 (POC) and will be increased every 10 minutes to the next higher level until the maximum of 4l/min (LOD) or level 4 (POC) is reached. Blood gases were taken and breathing frequency will be recorded at the end of each Oxygen Level.