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The term Emphysema is also used to describe other forms of dilated air space.
Generalized emphysema is characterized by dilatation of air spaces and destruction of alveolar walls without scarring.
Emphysema can be defined as 'a permanent dilatation of any part of the respiratory acinus (air spaces distal to the terminal bronchiole), with destruction of tissue in the absence of scarring'.

In practical terms, there is loss of elastic recoil in lungs as respiratory tissue is destroyed and the area available for gas exchange is reduced. Individuals with severe emphysema have reduced oxygen uptake despite increase in ventilation. Although they manage to maintain blood oxygenation by a rapid respiratory rate, they feel breathless on the slightest exertion and become hypoxic (type 1 respiratory failure). In pure emphysema, cyanosis, hypercapnia and cor pulmonale develop only late in the disease,after progressive decline in respiratory function.

There are two main forms of generalized emphysema, defined by the location of damage in the respiratory acinus.

Centriacinar emphysema (centrilobular emphysema)} is the most common form and is associated with cigarette smoking.
chronic bronchitis and inflammation of distal airways It is probable that chronic bronchiolitis
seen in early cigarette smokers progresses to this form of emphysema.
It is most often seen in the upper lobes.
It is likely that the pathogenesis of this type of emphysema is related to secretion
of extracellular proteases by local inflammatory cells.
Cigarette smoke may also inhibit the effect of protease inhibitor a-1 antitrypsin,
thereby potentiating tissue destruction.

Panacinar emphysema involves the whole respiratory acinus and is commonly associated with smoking.
As with centriacinar emphysema, its pathogenesis is related to excessive activity of
extracellular proteases secreted by inflammatory cells. Individuals with congenital deficiency
in a-1 antitrypsin also develop this pattern of emphysema at an early age.
Generalized emphysema.
The normal lung acinus distal to the terminal bronchiole consists of respiratory bronchioles,
alveolar ducts and terminal acini.
In centriacinar emphysema, there is dilatation of the respiratory bronchioles at the centre of the acinus.
This is seen in a very high power macroscopic photograph.
In panacinar emphysema there is dilatation of the terminal alveoli and alveolar ducts,
which later affects respiratory bronchioles, thereby affecting the whole acinus.
Macroscopically lungs are voluminous and, on cut surface, show large dilated air spaces. More than one
pattern may exist in the same lung.
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