The maintenance of life of an individual with an open double pneumothorax under differential pressure depends essentially upon the normal position of the lower lobes of the lungs and their close approximation to the diaphragm, especially of the posterior parts of the lobes. A complete dislodgment of both lower lobes leads invariably to the death of the individual, which may occur in a very short time, or after fifteen to twenty-five minutes. In all cases the respiration is affected first; it slows almost at once and stops invariably before the heart. The result is the same whether the vagi are intact or both nerves are cut. Exceptionally, respiration may continue even after the separation of the lungs from the diaphragm, but only by having all the lobes well approximated to the thoracic walls. When by dislodgment of the lower lobes the respiration is stopped and the heart is feeble and slow, or stopped completely, it is rarely possible to restore life by artificial respiration or by other appropriate means.
The extent of the exchange of gases occurring in normal respiration, with closed thoracic cavity, exceeds greatly the need for the maintenance of life, since normal respiration is provided with an abundance of factors of safety. Under differential pressure, however, life is carried on with an exchange of gases which amounts to a small fraction only of the extent of the exchange that takes place in normal respiration; respiration under differential pressure is, therefore, deprived of all factors of safety and is incapable of resisting the dangers of exceptional incidents. Deaths occurring in connection with the differential pressure have their cause essentially in this unguarded state of the function of respiration.
Under tracheal insufflation, the function of respiration is surrounded with effective safeguards, at least as much as is normal respiration. Dislodgment of the lungs has no detrimental effect.
After complete collapse of the lungs, capillary adhesions within the alveoli and the small bronchi become an additional obstacle to the redistension and respiration. Differential pressure holds this obstacle in abeyance.
When the lungs become collapsed during an open pneumothorax, it should be kept in mind that the force which is required for redistension is greater than that which is sufficient to keep the lungs continually distended. At the beginning of a redistension, therefore, a higher pressure should be employed for a short time.