Precisely the way in which the congestion or hemorrhage was occasioned has not been elucidated in a similar manner by all. Virchow years ago (1856) recognized that one or other was due to vascular stasis and reflux of venous blood from neighboring vessels; in other words, the explanation here given was the same as for infarctus of the kidney or spleen. Jürgensen regards infarctus as being similar in structure to lobular pneumonia. It has been also affirmed that owing to incomplete obstruction tissue supplied by the artery was at first anæmiated, and later, by reason of excess of backward pressure from venous trunks, it became congested or hemorrhage was effected. Duguet states that the arterial walls beyond the embolic plug become inflamed, and thus act as a cause of hemorrhage. The first effect, then, of an embolic clot being arrested in the lung is that of anæmia. Soon this state is followed by hemorrhage occasioned in the way I have mentioned. In the lung the hemorrhage means of necessity rupture of a vessel; in the spleen and brain this is not so invariable. Whilst the smaller bronchi are sometimes congested, they are rarely infiltrated with blood. For this reason gangrene is not a frequent sequela of pulmonary infarctus. It is not admissible that hemorrhage should occur without rupture of the vessel in many instances, for the reason that the sanguineous effusion is not always limited to the area supplied by a given vascular division obstructed, nor is it in the centre of the lung conoid in shape. The catarrhal changes in the lungs are very constant, although usually superficial in character and only affecting the epithelium. As Cohnheim58 has pointed out, there is a proneness to degeneration rather than to inflammatory action.
58 Untersuchungen über die Embolischen Processe, Berlin, 1872.
Due consideration being given to the changes of tissue effected by an arrested embolus, we can more fully understand the clinical phenomena connected with them. True it is, however, that the troubles of innervation and respiration thus brought on may pass unperceived, and for the simple reason that the pathological lesion follows, as a rule, only the transport of an embolus into a small arterial division. In a similar way the intensity of the venous reflux is in direct relationship with the functions of the heart and lungs, and if either the diseased hemorrhagic effusion is rendered more certain.
It is probable that a simple embolus cannot be followed by a gangrenous focus in the lung. This result is recognized frequently when the embolus originates in a purulent deposit, whether it be the consequence of an abscess, of puerperal fever,59 of a compound fracture, etc. The gangrenous cavity finally softens, its contents are expectorated, and the pulmonary tissue becomes indurated and cicatrizes around the excavation.
59 Dublin Journ. of Med. Science, May, 1875.
Pulmonary embolism may at times be the occasion of a pneumonic consolidation limited to the area of distribution of an obstructed pulmonary division. Sometimes the consolidation extends beyond this limit, and is seemingly the immediate effect of neighboring irritation. When the consolidation exists near the surface of the lung, it may extend to the pleura, producing considerable effusion and pseudo-membranous deposit upon the visceral layer. Both sides of the chest may occasionally be thus affected.
Capillary emboli of simple nature have long been described. Unless they obstruct a great many vessels simultaneously, they rarely cause death (Feltz). They do not, moreover, produce hemorrhages or infarctus, inasmuch as a collateral circulation is so easily established. The principal sources of these emboli exist outside of the vascular system, and in this variety we find emboli of air, fat, of the débris of new growths, etc.
Since 1866, the period at which Zenker first directed attention to fatty emboli in the pulmonary capillaries as a complication of an accident in which a patient was crushed between two wagons, many observers have noted accidents due to these obstructing bodies. Fatty emboli may follow numerous causes (contusions, suppurations, osteomyelitis, etc.), but are more frequent and fatal after comminuted fractures of the limbs than from any other single cause (Flournoy).
Occasionally the patient will have recovered from the shock following the fracture, when he is suddenly attacked with intense dyspnoea and expires within a few hours. The only effectual remedy would seem to be immediate amputation of the limb above the seat of the fracture. When the vessels of the lungs have been examined in these instances, they have been found to contain elongated masses, several millimeters in length, possessing a particular brilliancy, "disappearing under the action of ether, and becoming a deep, black color with osmic acid."60
60 Déjerine, Le Progrès médical; Med. Record, Jan. 15, 1879.