BREATH, SHORTNESS OF.—Under normal conditions the adult male draws 16 to iS breaths per minute ; the adult woman, iS to 20. New-born children breathe 40 times and more per minute ; young children, 25 times per minute. Disturbances of the frequency of breathing and of the depth of the individual respirations occur in so many morbid conditions of the air-passages, and also of other organs, that it is only exceptionally that they may serve to indicate the nature of the existing disease.
The respirations increase in number after every mental, and especially after every bodily, exertion ; in short. after conditions which increase the activity of the heart. The increase may be said to be morbid if painfulness exists in any part of the body which is put in motion by respiration (as in pleurisy, fracture of a rib, rheumatism of the muscles of the chest, inflam mation of the abdomen, etc.). Increased respiration is brought about by any febrile condition, and most particularly by affections which interfere with the exchange of gases in the lungs. This is true whether the passages through which the air enters the lungs are narrowed (as in bronchitis or asthma) or whether the pulmonary vesicles in which the exchange of gases takes place are impervious (as in pneumonia). Narrowing of the air passages may occur in any individual part of the respiratory canal, from the nose to the finest branches of the bronchi, being caused by such diseases as tumour of the nose, swelling of the tonsils, goitre, croup, diphtheria, severe affections of the larynx, and swelling of the mucous membrane of the bron chi. Impervious conditions of the pulmonary vesicle are brought about by pressure upon the lungs (in consequence of fluid, air, or tumours in the pleural cavity), by filling of the pulmonary vesicles with fluid or semi-solid masses (in inflammation and dropsy of the lungs ; and in cheesy disintegra tion of the pulmonary tissue), or by loss of the elasticity of the pulmonary tissue, which causes it to be less expansible as well as less contractile (as in emphysema of the lungs).
The greater the diminution of the respiratory surface, and the more rapidly this diminution occurs, the more marked becomes generally the shortness of breath. The increase of the obstacle to respiration causes the blood to become poorer in oxygen and richer in carbon dioxide gas ; and the overcharging of the blood with the latter poisonous element irritates the respiratory centre in the medulla oblongata (that is, the collection of nerve-cells governing the function of respiration).
Shortness of breath may result from certain affections of the heart, which cause an over-abundance of blood to be conveyed to the lungs. It may be present also in diseases of the abdominal organs if such organs become enlarged sufficiently to force the diaphragm upward, or to impair its ability to contract. Obstructions to respiration usually cause an increase, rather than a decrease, in the frequency of breathing. A decrease occurs when the larynx and the windpipe are narrowed (as in croup or diphtheria), or when an affection of the brain develops in the course of diseases of the lungs. Shortness of breath shows a peculiar character in asthma and in spasm of the glottis.