Patients with cystic fibrosis older than 18 years will performance a Bruce test with non invasive ventilation (once) after 30 minutes they perform the same test with oxygen supplementation.

Measurements: Transcutaneous level off carbon dioxide, saturation, puls and dyspnea scale.

measurements before and after the test: transcutaneous level off carbon dioxide, saturation, respiratory rate, dyspnea scale.

Inclusion Criteria:

• Subjects with Cystic Fibrosis at Stockholm Cystic Fibrosis center
• > 18 years of age
• Forced expiratory volume in one second (FEV1) between 20 and 69% of predicted value
• Developed alveolar hypoventilation at the physical performance test
• Use oxygen supplementation during aerobic training

Exclusion Criteria:

• Symptomatic upper airway infection characterized by fever less than 1 month before entering the study
• Cognitive dysfunction
• Cardio-respiratory instability
• Infection with Burkholderia cepacia complex
• Gastroesophageal reflux
• History of pneumothorax
• Massive hemoptysis.