- The primary cause of impaired expiration in patients with COPD is loss of alveolar elasticity.
- In COPD, especially emphysema, the destruction of the alveolar walls leads to a reduction in the elastic fibers that normally help maintain airway patency during expiration.
- Normally, the elastic recoil of the lungs helps to push air out efficiently during expiration. When this elasticity is lost, the small airways tend to collapse prematurely, causing airflow obstruction and trapping air in the lungs.
- While increased mucus production and bronchial smooth muscle hypertrophy can contribute to airway obstruction, they are secondary mechanisms and less significant in impairing expiration compared to the loss of elastic recoil.
- Thickening of the alveolar basement membrane is more relevant in other lung diseases such as interstitial fibrosis and does not primarily affect expiration in COPD.

Therefore, the hallmark feature leading to impaired expiration in COPD is the loss of alveolar elasticity, resulting in airflow limitation and difficulty in expelling air.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 16: Chronic Obstructive Pulmonary Disease