Mechanism
The structures in the tracheobronchial tree are well protected, so it normally takes a large amount of force to injure them. In blunt trauma, TBI is usually the result of violent compression of the chest. Rapid hyperextension of the neck, usually resulting from vehicle crashes, can also injure the trachea, and trauma to the neck can crush the trachea against the vertebrae. A crush injury of the larynx or cervical trachea can occur in head-on collisions when the neck is hyperextended and strikes the steering wheel or dashboard; this has been called a "dashboard injury". The larynx and cervical trachea may also be injured in front-on collisions by the seat belt.
Although the mechanism is not well understood, TBI due to blunt trauma is widely thought to be caused by any combination of three possible mechanisms: an increase in pressure within the airways, shearing, and pulling apart. The first type of injury, sometimes called an "explosive rupture", may occur when the chest is violently compressed, for example when a driver strikes the steering wheel in a vehicle accident or when the chest is crushed. The pressure in the airways, especially the larger airways (the trachea and bronchi), quickly rises as a result of the compression, because the glottis reflexively closes off the airways. When this pressure exceeds the elasticity of the tissues, they burst; thus the membranous part of the trachea is more commonly affected by this mechanism of injury than cartilaginous portions.
The second mechanism may occur when the chest is suddenly decelerated, as occurs in vehicle accidents, producing a shearing force. The lungs are mobile in the chest cavity but their movement is more restricted near the hilum. Areas near the cricoid cartilage and carina are fixed to the thyroid cartilage and the pericardium respectively; thus if the airways move, they can tear at these points of fixation.
The third mechanism occurs when the chest is compressed from front to back, causing it to widen from side to side. The lungs adhere to the chest wall because of the negative pressure between them and the pleural membranes lining the inside of the chest cavity; thus when the chest widens, they are pulled apart. This creates tension at the carina; the airway tears if this tensile force exceeds its elasticity. This mechanism may be the cause of injury when the chest is crushed. Most TBI are probably due to a combination of these three mechanisms.
When airways are damaged, air can escape from them and be trapped in the surrounding tissues in the neck (subcutaneous emphysema) and mediastinum (pneumomediastinum); if it builds up to high enough pressures there, it can compress the airways. Massive air leaks from a ruptured airway can also compromise the circulation by preventing blood from returning to the heart from the head and lower body; this causes a potentially deadly reduction in the amount of blood the heart is able to pump out. Blood and other fluids can build up in the airways, and the injury can interfere with the patency of the airway and interfere with its continuity. However, even if the trachea is completely transected, the tissues surrounding it may hold it together enough for adequate air exchange to occur, at least at first.
Read more about this topic: Tracheobronchial Injury
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