The form of collapse of the lung which occurs in infants whose lungs have been fully expanded at birth is a very common lesion. It occurs almost invariably in the course of a pulmonary catarrh, and is one of the accidents which render this form of derangement so fatal in weakly or rickety children.
Causation.—The immediate cause of collapse of the lung is the presence in the bronchial tubes of mucus which the child is unable to expel by reason of feebleness of the respiratory apparatus. Dr. Gairdner, of Glasgow, iu his treatise explains very clearly the mechanism by which exhaustion of the lobules is effected. In the act of inspiration a plug of mucus is carried inwards along a tube the calibre of which is constantly diminishing. When the narrowness of the tube prevents further advance, the mucus forms a plug which completely obstructs the channel. During expiration the plug is slightly dislodged so as to permit of the escape of some of the air con tained in the lobule ; but at each inspiration it is again drawn backwards so as to close the tube completely against any air entering to replace that which has just escaped. In this manner after a time the lobules beyond the point of obstruction are completely exhausted and the tissue becomes shrunken and condensed. Even if the plug of mucus be completely im pacted in the tube so that it cannot be dislodged during expiration, col lapse may still occur, for the pent-up air in the alveoli is exposed to such pressure by the elasticity and contractility of the alveolar parieties that it is absorbed.
The retention of mucus in the tubes is the consequence of inability to cough it away, and any cause which diminishes the energy of the inspiratory act increases the difficulty of drawing in air past the impediment in the bronchus. New-born infants do not know how to cough, for the act of coughing is only partly involuntary. It is in part an effort of volition to remove an obstacle to the free passage of air in the tubes. An infant who has not acquired a knowledge of the means by which the impediment may be expelled, suffers the obstruction to remain without employing the nec essary force to effect its removal. Even if the child knows how to cough, he may not have the power to carry out the act with sufficient energy to make it effectual. In the act of coughing a full inspiration is first taken.
The glottis is then closed, and pressure is brought to bear upon the lungs by the muscles of expiration. While this pressure is at its height the glottis is relaxed, and the rush of air passing out carries with it the mucus. which was obstructing the tubes. If, however, the lungs cannot be suffi ciently filled, or if, owing to weakness of the patient, the force of the expi ratory muscles is insufficient to bring adequate pressure to bear upon the lungs, the cough is ineffectual in freeing the tubes of their contents.
Weakness of the inspiratory act is a powerful agent in preventing the entrance of a sufficient supply of air. In ordinary respiration the elastic ity and contractility of the lung have to be overcome by the muscles of in spiration. If these muscles are feeble, as they are in a weakly infant, the obstacle to efficient inflation of the lungs is already great. - If, however, in ad-. dition, the respiratory muscles are opposed by reflex contraction of the bron chial muscles, owing to the irritation of the catarrhal process, and also by mucus in the tubes, they may prove quite unequal to the task. Therefore. any cause which increases the child's general weakness predisposes to pul monary collapse. Thus vomiting, diarrhoea, insanitary conditions, proper feeding, and all the exhausting forms of illness may have this result.
Besides the causes which have been enumerated, the force of the in spiratory act may be weakened by mechanical means. Interference with the action of the diaphragm may have important consequences in this re spect. This influence is especially seen in the case of young infants. For some time after birth respiration is principally diaphragmatic on account of the circular shape of the chest, which allows of little lateral expansion. Therefore any resistance to the descent of the diaphragm, such as would be produced by ascites, or great increase in size of the abdominal organs, or flatulent distention, may so weaken the force of the inspiratory act that a very trifling catarrh determines wide-spread and fatal collapse of the lung.