Chronic respiratory diseases are a global burden. Treatment options have improved in recent years, pulmonary rehabilitation plays a key role. Oxygen therapy is recommended in patients with a low saturation at rest, but no clear guidance is given for patients who desaturate during exercise. The effect of ambulatory oxygen during exercise is not yet completely understood, especially in those patients with exercise-induced desaturation.

Aim: The goal of this study is to analyse the effect of supplemental oxygen given during a constant work rate exercise test (CWRET) on a cycle ergometer compared to sham air.

Methods: We plan to include 25 Patients respiratory patients undergoing pulmonary rehabilitation (male and female; aged >18 years; stable condition >3 weeks (e.g. no exacerbations); resting oxygen saturation (SpO2) ≥ 88%) with exercise induced hypoxemia defined by a fall in oxygen saturation by ≥ 4% during a 6-minute walking test. Patients will undergo an incremental exercise test with a ramp protocol (for evaluating the maximal workload) and two CWRET (75% of the maximal workload) with ambulatory oxygen or placebo (sham air) via standard nasal canula at a flow rate of 5l/min. Patients and assessors will be blinded. The difference endurance time of the CWRET with oxygen vs. sham air will be the primary outcome of this study.

Data will be summarized by means (SD) and medians (quartiles) for normal and non-normal distributions. Effects of treatment will be evaluated by mean differences with 95% confidence intervals, T-tests or Wilcoxon matched pair tests as appropriate. A p-value threshold of <0.05 or a confidence interval not including zero will be considered as statistically significant. Analyses will be performed according to the intention to treat principle.

Inclusion Criteria:

• We will include male and female lung disease patients undergoing pulmonary rehabilitation: aged ≥ 18 years; stable condition > 3 weeks (e.g. no exacerbations); resting oxygen saturation (SpO2) ≥ 88% and exercise induced hypoxemia defined by a fall in SpO2 by ≥ 4% during a 6-minute walking test (6MWT); informed consent as documented by signature.

Exclusion Criteria:

• Severe daytime resting hypoxemia (SpO2 < 88% or partial pressure of oxygen (PaO2) < 8 kPa); unstable condition requiring adaptation of pharmacologic and other treatment modalities or requirement of intensive care or relevant severe concomitant disease; inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, neurological or orthopedic problems with walking disability or inability to ride a bicycle; women who are pregnant or breast feeding; enrolment in another clinical trial with active treatment.