46 DISEASES OF THE LUNGS 



CHRONIC ALVEOLAR EMPHYSEMA. 



Definition. — Chronic alveolar emphysema is a permanent 

 overdistention of the alveoli with an increased amount of 

 air present in the lung. The walls of the alveoli become 

 anemic and atrophic. 



Occurrence. — Most common in horses, especially old, 

 hard-worked individuals. It is a common cause of so-called 

 "heaves." 



Etiology. — (a) Continued severe exercise (hard pulling, 

 fast driving, high jumping). This causes repeated physio- 

 logical dyspnea inducing overdistention at inspiration ands 

 causes the expiration to become forced. These two factors 

 in time lead to weakening and finally atrophy of the alveolar 

 walls. The alveoli can be ten. times their normal diameter, 

 the walls very thin and anemic. Later rupture of the 

 interalveolar septa occurs, permitting direct communication 

 between the distended air cells. The lung thus loses its 

 power of contraction at expiration, which causes this act to 

 become forced so that the air from the emphysematous 

 part of the lung is expelled, (b) Severe dyspnea due to 

 diseases of the larynx or bronchi is a rare cause, (c) Severe 

 coughing, especially in chronic bronchitis, (d) Occasionally 

 chronic alveolar emphysema results from the acute form. 

 (e) The feeding of bulky forage, particularly clover or 

 dusty timothy hay is no doubt an important predisposing 

 cause of pulmonary emphysema, the dilatation of the 

 stomach and bowels which such foods produce interfering 

 with respiration, (f) As some strains of horses seem more 

 subject to emphysema than others, an inherited predisposi- 

 tion (lacking resistance in the septa) has been assumed. 

 Of this, however, there is no substantial proof. Use and 

 feeding methods may explain the assumption. 



Symptoms. — Dyspnea which increases on exercise. It is 

 always most pronounced at expiration and is often 

 accompanied by a double movement of the flanks and 

 the interrupted protrusion of the anus. The first part 

 of the expiratory act is passive, but after a very brief 

 pause the movement becomes active, /the abdominal muscles 



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