Brief Summary People with severe chronic obstructive pulmonary disease (COPD) often have too much air trapped in their lungs (pulmonary hyperinflation). This makes it hard to breathe and reduces quality of life. This study tests whether a single session with a medical device called Simeox® can reduce the amount of air trapped in the lungs. Simeox® works by applying gentle intermittent negative pressure during exhalation to help air move out of the lungs more easily. Patients with severe or very severe COPD and documented hyperinflation will undergo lung function measurements before and immediately after a 20-minute Simeox® session. The main measurement is the change in residual volume (RV), which is the amount of air left in the lungs after a full exhalation. We will also measure changes in other lung volumes, breathlessness, and any side effects. This is a single-arm pilot study enrolling 23 patients at one center in Italy (ASST Lodi). The study is non-profit and has been approved by the Ethics Committee Comitato Etico Territoriale Lombardia 1 (CET Lombardia 1).