Inclusion Criteria:  interstitial lung disease, diagnosed by high-resolution CT FEV1/FVC >70% and FVC <85% predicted stable medication for at least 4 weeks Desaturation in SpO2 in 6-minute walk test of ≥ 4% to absolute values ≤ 92% Signed Informed Consent Male and female patients 18 to 80 years of age  Exclusion Criteria:  Severe daytime hypoxemia (PaO2 ≤7.3 or <55 mmHg at rest) Other clinically significant concomitant disease states (e.g., cardiovascular disease, hepatic dysfunction, etc) Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia or confusional state of the subject, neurological or orthopaedic problems with walking disability or inability to ride a bicycle Women who are pregnant or breast feeding