The aim of the study is to determine if increased retrograde shear would further deteriorate the already impaired vascular function in severe COPD patients.

Inclusion Criteria:

• COPD
• chronic hypoxemia

Exclusion Criteria:

• an acute exacerbation, active respiratory infection or infection of other localisation 6 weeks prior to the visit
• relevant coexisting lung disease such as interstitial lung disease, cancer, renal failure, thromboembolic disease or major cardiovascular event during the previous year
• informed consent not provided or inability to comply with the study protocol