In open pneumo-thorax, the first and most frequent form, air passes from the lung into the pleura at each inspiration, and flows back from the pleural cavity towards the bronchus at each expiration. The intra-pleural pressure is then approximately equal to the intra-bronchial pressure, and undergoes similar oscillations. (It should be noted that the aperture in the lung is seldom sufficiently large to establish an absolute equality of pressure between the bronchus and the pleural cavity. Respiration, therefore, though very seriously impeded, generally continues in a modified form.)
In a second variety, termed “valvular pneumo-thorax,” air passes freely from the lung into the pleural cavity, but is unable to return from that cavity towards the lung, because a flap of tissue acts as a valve and closes the orifice at the commencement of expiration. As soon as intra-pleural pressure rises above that of the inspiratory effort, the valve remains permanently closed.
In the third variety, called “closed pneumo-thorax,” the orifice of communication is obstructed by some mechanism, and the pleural sac only contains a film of air.
In practice, valvular pneumo-thorax is recognised by the movement of the thoracic wall (which in open and closed pneumo-thorax remains depressed), as well as by extreme intensity of the dyspnœa and attacks of threatened suffocation. Closed pneumo-thorax, which is only a termination and a stage in the cure of open pneumo-thorax and of valvular pneumo-thorax, is suggested by progressive improvement in the symptoms. Scientifically it is very easy to make this diagnosis by putting a manometric apparatus in communication with the pleural cavity by means of a simple hollow needle provided with a thick-walled rubber tube.
In open pneumo-thorax the liquid column in the manometer undergoes rhythmic oscillations corresponding to the respiratory movements; in valvular pneumo-thorax the intra-pleural pressure increases progressively until it becomes higher than the external pressure; and finally, in closed pneumo-thorax, the column of the manometer assumes a certain level at which it rests.
Prognosis. The prognosis is very variable, according to the primary cause of the accident. Animals might recover, but economically there is little advantage in preserving them when the diagnosis is assured, except in cases of animals of great value, and when the primary disease admits of it.
Causation. Pneumo-thorax may be produced by various causes. The most frequent cause in large animals is pulmonary echinococcosis, during the course of which a peripulmonary vesicle, after having injured several lobules, one of the air passages or even a bronchiole, may break through the pleura, thereby setting up direct communication between the bronchi and the pleural cavity.
To pulmonary tuberculosis, with peripheral softened tubercles, perforating simultaneously into an alveolus or a small bronchus and into the pleura, must be assigned the second place.
Vesicular and interstitial subpleural pulmonary emphysema is also a frequent cause of pneumo-thorax, the pleura being ruptured over the emphysematous points.
Finally, and exceptionally, an abscess of the lung may open into the pleura and form sinuses, which may establish a communication between the digestive reservoirs and pleural sacs; but such accidents produce pyo-pneumo-thorax and septic pleurisy of a rapidly fatal character.