DEFINITION.—The term atelectasis is derived from [Greek: atelês], incomplete, and [Greek: echtasis], expansion, and designates a condition in which the lung has failed to expand or has returned in part or throughout its whole extent to the state of non-expansion which is normal in foetal life. In the former case the state is one of congenital atelectasis, and is of course met with only in the new-born; in the latter it is acquired atelectasis, or collapse of the lung, a portion or portions of the organ which have once been expanded having the air excluded from their alveoli, so that these collapse and return to the pre-natal state. To this condition of acquired atelectasis the term apneumatosis, from [Greek: a] negative, and [Greek: pneumatôsis], filling with air, was applied by Fuchs in 1849, and it has since been adopted by Graily Hewitt.
HISTORY.—For a long time this affection was regarded as a peculiar form of pneumonia, for the reason that at post-mortem examinations patches of collapsed lung-tissue were found which appeared to have undergone solidification. Inasmuch as the condition was most frequently met with in young children, and the supposed solidification was often limited to certain lobules of the lung with intervening healthier spaces, it was described as the lobular pneumonia of children.
The secondary nature of the affection, and the fact that it is very generally preceded by bronchitis, and sometimes by catarrhal pneumonia, were pointed out by Barthez and Rilliet in 1838. Some other important distinctions between this affection and general or lobar pneumonia had been referred to by various writers, but it was not until 1844 that its true nature was satisfactorily elucidated by Bailly and Legendre, who showed, by blowing air into the lungs after death, that the lobules supposed to be hepatized were not really solidified by exudation, but had simply collapsed for want of air.
ETIOLOGY.—The congenital atelectasis of new-born children may be due to original feebleness, to protraction of labor interfering with the blood-supply through the cord, or to obstruction of the air-passages by mucus or other substances. In any case, it is the result of non-expansion of the chest, so that the lungs are not unfolded. This constitutes atelectasis in the strict sense.
Acquired atelectasis, apneumatosis or collapse of the lung, is an affection most frequent in early infancy, though not limited to that period of life, since bronchitis with defective innervation and great impairment of strength, the essential factors in the production of the disease, may occur at any period of life.
It is probably in almost every case secondary to bronchitis, and due to the occlusion of the smaller bronchi by the presence of mucus allowing the egress, but impeding the ingress, of air, so that the lobules to which they lead are gradually evacuated of air, and thus finally collapse.
Obstruction of a bronchial tube by a foreign body or by the pressure of a morbid growth within the lung may produce collapse of the lobules to which such tube leads, a smaller or larger part of the lung being involved in proportion to the size of the obstructed bronchus. Such cases are, however, very rare, and they more closely resemble the condition brought about by the pressure of a pleural effusion giving rise to the state of carnification, which is, in effect, an atelectasis involving the greater part or the whole of a lung, and not limited to certain lobules nor taking place lobule by lobule.
The principal cause of lobular collapse is no doubt bronchial catarrh, the action of which is aided by impairment of the general strength and of muscular respiratory power; for the natural elasticity of the lung-tissue would favor the exit and oppose the entrance of air unless it were counterbalanced by muscular action in inspiration. If, then, this inspiratory action is lessened, the requisite amount of air will not enter the alveoli, and that which they already contain will be in part driven out, and perhaps in part absorbed into the blood, by the pressure to which it is subjected. Deficient innervation and lower vital power are thus important elements in determining collapse, which is most common in very young infants or in those who, though somewhat older, have had their nutrition impaired by malhygienic influences or by other diseases.
The mechanism of the production of lobular collapse by the presence of mucus in the bronchial tubes has been well explained by the classical observations and experiments of Gairdner and of Hutchinson. They showed that the physical result of collapse is in part due to the force of expiration being greater than that of inspiration, and in part to the anatomical formation of the bronchial tree. As to the former of these causes, it was shown by the experiments of Hutchinson, already alluded to in the article on [EMPHYSEMA], that the force of expiration capable of being applied for the overcoming of obstruction in the bronchial tubes is greater than that of inspiration—in opposition to the teaching of Laennec, who regarded the inspiratory as the greater force. Repeated efforts to clear the bronchial tubes of accumulated secretion by the forced expiration of coughing must therefore remove air from the alveoli in greater amount than it can be returned to them by inspiration, and so they must ultimately be evacuated of their contents and consequently collapse.