Inclusion Criteria:  Diagnosis of COPD ((post-bronchodilation forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) <0.7) Optimal medical treatment according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD)) at least 3 months before screening  Exclusion Criteria:  abnormalities of the thorax or the lung other than COPD, acute exacerbation within one month obesity with a body-mass index (BMI) ≥35 kg/m², malignant co-morbidities, severe heart failure (New York Heart Association stage IV), unstable angina, severe arrhythmias, neuromuscular diseases and/or impairment of basal brain nerve function with disturbed swallowing and choking and impaired general condition that could preclude regular follow-up visits.