In this condition necrosis of a definite piece of lung tissue occurs. Necrosis of the lung is of frequent occurrence without gangrene, the latter implying that putrid decomposition has taken place and that the necrosed tissue forms a slough. The occurrence of putrescence will depend on the local conditions, and chiefly on the presence of the microbes of putrefaction in the dead piece of lung. As such microbes are mostly present in the bronchi, putrescence will occur unless, as in tuberculosis, the preceding lesion fills up and obliterates the bronchi, or the conditions otherwise are such as to interfere with the microbes. The gangrene may itself arise by the action of decomposing material. If a foreign substance, such as a piece of solid food, gets into a bronchus it may induce a bronchitis with putrescence of the secretion, and the irritation of the putrid juices may induce gangrene of the lung. Similarly, putrid juices inspired from ulcers and wounds, of the mouth and air-passages, or perforation of abscesses or ulcers into the trachea or bronchi may set it up. Again, the juices in cavities, especially in those arising by dilatation of bronchi, may stagnate and decompose, and lead to gangrene. Wounds and contusions may cause necrosis directly. Sometimes the lung tissue dies in severe cases of typhoid fever or other zymotic diseases. We have also seen that gangrene may occasionally follow the haemorrhagic infarction or acute pneumonia, and that it is a constant feature of the metastatic abscess. Lastly, there are some cases in which the cause of the gangrene is obscure, but these cases, as well as those with a more definite cause, are somewhat common in debilitated persons and those given to alcoholic excess.

It is customary to divide gangrene of the lung into a circumscribed and a diffuse form. In both the lung tissue dies and decomposes, ultimately becoming separated, if the patient survive, as a shreddy slough, which occupies the cavity formed by the loss of tissue. In the diffuse form there are gangrenous patches throughout the lung, or a considerable portion of it, and there is little probability of the effects becoming limited by reactive inflammation in the neighbourhood. The diffuse form not infrequently develops from the circumscribed, the decomposing juices from the slough causing still further necrosis.

The various changes which occur around a gangrenous piece of lung, and in more distant parts of the organ, are related to the irritating character of the* slough. These changes are mainly inflammatory. The immediately neighbouring lung tissue is acutely inflamed, and there is thus a zone of condensation around having the usual features of acute pneumonia, often with a specially haemorrhagic character. In this inflammatory zone the gangrene may advance. On the other hand the slough may be detached by the inflammatory process, and through time, a more chronic inflammation having occurred, the slough may be separated from the lung tissue by a layer of granulation tissue which produces pus abundantly into the interior of the cavity. At the same time there may occur in a considerable tract of lung around a chronic inflammation causing thickening of the alveolar wall and condensation of the lung similar to that of chronic pneumonia. If the slough be small enough, the cavity may, after the discharge of the slough, ultimately contract and form, a cicatrix, but in the case of larger sloughs a suppurating cavity may long remain.

The effect on the bronchi is of importance. The decomposing juices from the slough and from the inflamed lung tissue find their way into the bronchial tubes, where they set up an acute inflammation of a highly suppurative character. A rich secretion of putrid pus is the result. This secretion carried to the bronchi in other parts of the lung may set up a lobular pneumonia (see ante), or there may be as a result gangrene in numerous small isolated patches, and in this way multiple small abscesses may occur. If the gangrene be near the surface an acute pleurisy is the result, with fibrinous exudation. Sometimes the cavity opens into the pleura, and we have a suppurative pleurisy, perhaps with pneumothorax.

An occasional complication of gangrene is Haemorrhage. As the slough separates the more resistant tissues retain their connection longest. The bronchi and larger vessels sometimes remain as rigid trabecule in the midst of the soft slough. The arteries remain longest in connection, but they are usually filled with thrombi and obliterated. Occasionally, however, the gangrene advances around an artery which is still pervious, and in that case haemorrhage of a serious or even fatal character may result.

Sometimes the gangrene leads to a definite septicaemia, or metastatic inflammations result, having their seats especially in the brain. In these cases the decomposing material gets into the pulmonary veins, having first caused thrombosis of them.

A peculiar feature in gangrene of the lung is the very abundant and highly Putrid sputum. The decomposing juices from the slough set up, wherever they are carried, acute suppurative inflammations, and the abundant inflammatory products also undergo decomposition. The bronchial tubes being weakened by the severe inflammation often undergo bronchiectasis, and the material stagnates in them all the more, and decomposes. So it happens that in the cavity itself and in the dilated bronchi there are usually large quantities of putrid secretion. This is expectorated at intervals, and sometimes so abundantly that it pours out of nose and mouth. The sputum is extraordinarily foetid, and, if allowed to stand, deposits triple phosphates, crystals of margarine, etc. It also contains abundant pus corpuscles, many of them broken down by decomposition, pieces of lung tissue, and bacteria isolated and in colonies. Sometimes the sputum contains also spirilla.

Literature

LAennec, Traite d'auscult.; Leyden, Volkmann's Sammlung, No. 26, 1871; Hertz, in Ziemssen's Handb., v., 514, 1877; Hanot, Progres med., 1876, No. 14.