Chronic obstructive pulmonary disease is a long-term lung condition that causes breathing difficulties. People living with this disease often feel short of breath, especially during physical activity, which can make daily tasks more challenging. This breathlessness is partly caused by mucus accumulation within the airways and air getting trapped in the lungs. Both of these reduce the amount of fresh air that can enter with each breath and increase the effort required to breathe. To manage symptoms, people with chronic obstructive pulmonary disease may use medications, oxygen therapy, or be referred to pulmonary rehabilitation programs. These programs often include exercise training, self-management strategies, and airway clearance techniques. While chest physiotherapists typically guide patients in these techniques, certain medical devices can also help individuals manage their condition independently at home. One such device uses a method called intermittent intrapulmonary deflation, which involves applying gentle pressure changes through a mouthpiece to help loosen mucus and release trapped air from the lungs. Although some individuals already use this kind of device as part of their daily care, more scientific evidence is needed to determine whether the improvements in airway clearance and reduced air trapping actually lead to measurable relief of breathlessness. The goal of this clinical trial is to find out whether this type of medical device designed to remove airway mucus and trapped air in the lungs (using intermittent intrapulmonary deflation) can help reduce breathlessness (dyspnea) after physical activity in people with COPD. The main questions this study aims to answer are: * Does using the device at home for one week reduce the intensity of breathlessness after walking? * Does it also improve walking distance, reduce the feeling of muscle fatigue, or increase satisfaction with the airway clearance method? Researchers will compare the active device to a placebo device that looks and feels identical but does not deliver the active treatment. This comparison will help determine whether the observed effects are due to the device itself or other factors such as expectations or chance. Participants in this study will be adults diagnosed with moderate to very severe chronic obstructive pulmonary disease. All participants must have recently completed a pulmonary rehabilitation program, continue to experience breathlessness while walking, and show signs of lung hyperinflation (when air remains trapped in the lungs). People will not be included if they have other serious health conditions that could make walking difficult or unsafe, or if they have any contraindications to using the device. Participants who agree to participante in the study will: * Attend a visit at the pulmonary rehabilitation center for baseline testing and to learn how to use the device; * Be randomly assigned to use either the active device or the placebo device at home for 7 days; * Return to the center for walking tests, lung function tests, and questionnaires about breathing and fatigue; * After a one-week break, use the device with alternative settings (the one not used in the first phase) for another 7 days; * Complete a final visit at the end of the study to repeat the same tests. Participants will also be invited to take part in an ancillary study during their first clinic visit to examine how a single session with the device affects breathlessness immediately after walking. This research will help determine whether this non-pharmacological, device-based method of clearing mucus and reducing trapped air can bring meaningful improvement in breathlessness for people with chronic obstructive pulmonary disease, especially when used at home. If effective, it may offer a simple and convenient way to help people breathe more comfortably during everyday activities. Participants will receive compensation for their time and travel.