Causes

Causes

Cigarette smoke accounts for over 80% of all cases of chronic obstructive lung disease. It contains irritants that inflame the air passages, setting off a cascade of biochemical events that damage cells in the lung, increasing the risk both for COLD and lung cancer. Different effects of smoking can lead to emphysema or chronic bronchitis, but smokers generally have signs of both conditions. The diagnosis of a specific type of COLD depends on which disease process predominates.

Causes of Emphysema

Smoking is the major cause of emphysema. In some rare inherited disorders, emphysema can develop even in nonsmokers.

The Disease Process Leading to Emphysema. The key process leading to emphysema is destruction of a protein in the lung called elastin, which is essential for the "springy" quality of many tissues in the body. This protein is specifically important in the lungs for maintaining flexibility in the alveoli -- the tiny sacs at the end of the airways.

An imbalance in the following chemicals may be important in this process:

• Proteases, particularly those known as elastase and trypsin. Proteases are enzymes released by white blood cells called neutrophils. Under normal circumstances, these enzymes are important for fighting infection and injury. In excess, however, these enzymes can degrade or destroy elastin.
• Alpha 1-antitrypsin (AAT). The AAT protein neutralizes proteases and therefore protects elastin from destruction.

An excess of protease coupled with impaired or deficient AAT can lead to emphysema. Eventually, the imbalance in these factors produces the inelastic walls of the alveoli and the pockets of dead air characteristic of emphysema. Any condition that causes an imbalance in any of these substances may trigger emphysema. Smoking is the major culprit, but genetic factors can also cause this imbalance.

Smoking and Biologic Factors Leading to Emphysema. Emphysema caused by smoking most often occurs in the upper lobes of the lungs. Some experts believe that smoking causes an imbalance between AAT and proteases in the following way: