It is a combination of permanent dilatation of air spaces distal to the terminal bronchioles & destruction of the wall of dilated air spaces.
-The commonest form of COPD is the combination of chronic bronchitis & pulmonary emphysema. The 2 common etiologic factors are tobacco smoke & air pollutants.
-Bronchial changes related to a deficiency of serum α1- antitrypsin, also called as an α1 protease inhibitor, is a glycoprotein it is normally synthesized in the liver & distributed in circulating blood, tissues & macrophages. The protease is derived from neutrophils & which has the capability of digesting lung parenchyma but is inhibited from α1 antitrypsin (protease inhibitors)(α1-AT)(α1-Pi).
-The α1 AT deficiency develops in adults & causes pulmonary emphysema. The mechanism of alveolar wall destruction in emphysema by elastolytic action is based on the imbalance between proteases & antiproteases.
A lobule is composed of about 5 acini distal to terminal bronchioles that an acinus consists of 3-5 respiratory bronchioles, a variable number of alveolar ducts & alveolar sacs. It is classified according to the portion of acinus involved:- centriacinar, panacinar, para-septal, irregular & mixed.
- Centriacinar(Centrilobular):- It is characterized by initial involvement of respiratory bronchioles that is central & proximal part of the acinus, the lesion is more common & severe in the upper lobe of lungs. It shows distended air spaces in the center of lobules surrounded by a rim of normal lung parenchyma in the same lobule.
- Panacinar(Panlobular) Emphysema:- In this type, all portion of acinus are affected, but not of the entire lung. It involves the lower zone of lungs more frequently & more severely than the upper zone. The involvement may be confined to a few lobules & may affect the lobe or part of a lobe of the lung. Lungs are enlarged & over inflamed.
- Para septal or distal:- Part of acinus involve is distal while a proximal part is normal. It localized along the pleura & along perilobular septa. Involvement is seen adjacent to the area of fibrosis & affects the upper part of the lung.
- Irregular(Para-cicatricial):- seen surrounding sears from any cause. Involvement is irregular as regards the portion of acinus involved as well as within the lungs as a whole.
- Mixed(Unclassified):- Same lung may show more than one type. It is usually due to more severe involvement resulting in loss of clear cut distinction between one type & other.
Grossly- The lungs are voluminous, pale & the edges of the lungs are rounded. Mild cases show dilatation of air spaces visible with a hand lens. Advanced cases show subpleural bullae & blebs bulging outwards from the surface of the lungs. Bullae are air-filled, bubble-like structure, larger than 1cm In diameter. They are formed by the rupture of adjacent air spaces where blebs are the result of the rupture of alveoli directly into subpleural interstitial tissues.
Microscopically- Depending upon the type there is dilation of air spaces & destruction of septal walls of part of acinus involved, that is respiratory bronchioles alveolar ducts & alveolar sacs. Bullae & blebs when present show fibrosis & chronic inflammation of walls.