The pathobiology and epidemiology of human emphysema.

Emphysema is defined in anatomical terms as enlargement of the gas-exchanging part of the lung (the acinus) accompanied by destruction of respiratory tissue. Emphysema is classified by the way that the acinus is dominantly involved. In proximal acinar emphysema, the proximal part of the acinus-respiratory bronchioles-is dominantly involved and two forms of proximal acinar emphysema are usually recognized: centrilobular emphysema and simple pneumoconiosis of coalworkers. The acinus is more or less uniformly involved in panacinar emphysema, and several clinical associations have been described with this lesion. In distal acinar emphysema, alveolar ducts and sacs are particularly involved, and spontaneous pneumothorax of young adults is associated with this form of emphysema. Scarring is usually associated with irregular involvement of the acinus (irregular emphysema) and is usually asymptomatic. No uniform agreement exists as to the application of this classification and there is widespread discrepancy of classification of emphysematous lungs between experts, especially when emphysema is severe. The precise definition of destruction of respiratory tissue in emphysema has not been agreed on, and this had led to wide variations in the assessment of prevalence of emphysema in autopsy series. Tobacco smoking is the most important cause of emphysema and is thought to bring it about by imbalance between the protease-antiproteinase mechanisms in the lung. Increasing severity of emphysema is accompanied by increasing frequency of symptoms, but a substantial proportion of subjects with severe emphysema will be apparently free from symptoms. The major functional characteristics of severe emphysema are reduction in expiratory flow, increase in lung volumes, and diminished diffusing capacity. Diminished expiratory flow in emphysema is determined in part by loss of elastic recoil and in part by associated airway disease. Loss of recoil in emphysematous lungs may be brought about by functional changes in the apparently normal intervening lung between the emphysematous spaces.