4. Gangrene is said to be a termination of croupous pneumonia in about 2 per cent. of cases, but this estimate is based on too few statistics to be wholly reliable. It is met with in bad constitutions where there is very great vital depression, in chronic alcoholismus, and in cases of intense septic poisoning. Interference with the blood-supply, causing the formation of pulmonary or bronchial thrombi, leads to its development.8 While usually limited to a small area of lung-tissue, it may invade large tracts, and be either circumscribed or diffuse.
8 Huss, Pneumonia; Carswell, Ill. El. Forms of Disease.
The gangrenous portion of the lung is changed to a dark, dirty, pulpy mass, sometimes wanting the fetor of gangrene. When the mass has become diffluent, a sort of cavity is formed, in which are found fetid fluid and shreds of gangrenous lung-tissue. Around the gangrenous mass there is a zone of gray hepatized, friable tissue, which in turn is bounded by a zone of red hepatized tissue. When the above-named zones are not present in diffused gangrene, the cavities are large, and shreds of tissue and vascular bands traverse the cavity, which swarms with bacteria. Such a gangrenous mass may lead to sloughing of the pleuræ. It has been denied that a croupous pneumonia can terminate in gangrene, but modern pathologists all support the opposite view. It may be mentioned that gangrene in croupous pneumonia of the horse is of frequent occurrence.
5. Chronic (interstitial fibroid) pneumonia may result when the process of resolution in croupous pneumonia is delayed and the inflammatory process does not advance beyond the stage of gray hepatization. In such cases the walls of the alveoli, and finally the inter-alveolar tissue, become the site of new connective-tissue developments.
The peculiarly hard and oedematous condition that sometimes marks gray hepatization is, by some observers, regarded as an intermediate stage between croupous and interstitial pneumonia. An abundant cell-production in the second or third stage of croupous pneumonia may be followed by shrivelling of the alveolar contents, and subsequent cheesy changes may bring about one form of phthisis. Whether this can ever occur independent of tubercle is still a mooted question. This is called (by those who believe in such an origin of phthisis) cheesy infiltration, and is to be differentiated from tubercular infiltration.
In childhood croupous pneumonia is not of so frequent occurrence as catarrhal. In its anatomical changes it does not differ from croupous pneumonia in adults. In old age the pneumonic changes usually begin in the upper lobes of the lung, and extend downward—the reverse of what occurs in adult life. In the stage of engorgement crepitation is usually wholly absent, and when the stage of red hepatization is reached the color is found much darker than in adults, sometimes being blue or black; the lung is much more strikingly marbled, and on section the granules, in those cases where they are present, are much larger than in adult life. Frequently in senile pneumonia the granular look is absent. Gangrene is a far more frequent termination of croupous pneumonia in old age than at any other period. The highly-rarefied condition of the lungs at this period seems to favor the development of small abscesses.
Croupous pneumonia involves either the whole lobe or a whole lung. Its most frequent seat is the lower lobe of the right lung. Its next most frequent seat is the lower lobe of the left lung, then the upper lobe of the right, the middle lobe of this lung being least frequently involved. Double pneumonia has been variously estimated as occurring in from 5 to 50 per cent. of cases, but in all probability the percentage rarely, even in epidemics, exceeds 12 or 15 (Huss, Grisolle, Barth, Ziemssen). In old age the difference in point of frequency of attack between the two sides is very slight, and some affirm that sthenic is more frequent on the right and typhoid pneumonia on the left side. Double pneumonia is more frequent in the senile period than during adult life.
The average duration of the different stages is as follows: The stage of engorgement lasts from two to three days; the stage of red hepatization, from three to five days; and the stage of gray hepatization, from two to six days. In old age the stages rapidly merge into each other, and suppuration of the lung may occur within thirty-six or forty-eight hours from the onset of the pneumonia, while it is not at all infrequent for complete red hepatization to occur within the first six or eight hours.
The changes in the pleura over a pneumonic lung are quite characteristic. An uneven, thin, downy-looking layer of plastic lymph is spread over the pleural surface, which presents a fine arborescent vascularity. At times this plastic layer may partially conceal the liver-brown color of the pneumonic lung. As the stage of gray hepatization is reached, pleuritic adhesions are apt to be formed, which subsequently undergo absorption, and thus the pleuritic changes follow, to a certain extent, those which are taking place within the lung. The cell-elements in this fibrinous meshwork are chiefly pus- and large endothelial-cells. The pleura itself is opaque, congested, and ecchymotic, and may be so thickened as to give rise to a dull note on percussion after the pneumonia has undergone resolution. If there are adhesions from previous pleurisies, or pleuritic changes that have occurred prior to the lighting up of a pneumonia, they will modify its course and termination.
The right heart is dilated, and on inspection immediately after death it is not unusual to find both ventricles filled with pale, firm clots that insinuate themselves between the columnæ carneæ and sometimes extend into the vessels. The pulmonary vessels running to the affected portion of the lung may be the seat of thrombosis. Pericarditis is so frequently found at the post-mortem of those who die of pneumonia that its occurrence must be regarded as something more than either accident or complication.