INCIDENCE AND MECHANISMS OF INJURY

Injury to the tracheobronchial tree is an uncommon but wellrecognized complication of both penetrating and blunt chest trauma. Many victims die before emergency care from associated injuries to vital structures, hemorrhage, tension pneumothorax, or respiratory insufficiency. Thus, a substantial number of diagnoses are established only after death. At other times, the diagnosis is not readily apparent and is not made until late symptoms indicating tracheobronchial injury have developed. Thus, the true incidence of injury to the tracheobronchial tree is difficult to discern. In a review of autopsies of 1178 persons dying from blunt trauma to the chest, Bertelsen and Howitz found that tracheobronchial disruptions occurred in only 33 patients, for an incidence of 2.8%; 27 of these died immediately. In a review of survivors and non survivors, Campbell reported on 15,136 patients diagnosed with blunt chest trauma. Forty-nine (0.3%) had a tracheobronchial injury. This series showed an extremely high mortality (67%) but did not describe the severity of associated injuries. In a review of the literature, Asensio described the incidence in penetrating neck trauma with 331 of 4193 patients (8%) presenting with laryngotracheal injuries. More than 80% of blunt tracheobronchial ruptures occur within 2.5 cm of the carina. Mainstem bronchi are injured in 86% of patients and distal bronchi in only 9.3%, while complex injuries are seen in 8%.

Penetrating injury is a straightforward mechanism and consists basically of the hole created by the path of a knife or bullet. Knife wounds occur almost exclusively in the cervical trachea, whereas gunshot wounds occur at any point along the tracheobronchial tree. Intrathoracic injury to the tracheobronchial tree occurs more commonly from blunt trauma but may also result from bullet wounds. These injuries occur at a higher incidence when the projectile crosses the mediastinum. Associated injuries to other mediastinal structures, including the heart, great vessels, and esophagus, are common and contribute significantly to the morbidity and mortality.

There are several mechanisms by which blunt trauma may injure the trachea and bronchus, including direct blows, sheer stress, and burst injury. A direct blow to the neck may produce a “clothesline”-type injury, crushing the cervical trachea against the vertebral bodies and transecting the tracheal rings or cricoid cartilage. Shear forces on the trachea create damage at its relatively fixed points, the cricoid and the carina. A common factor in burst injury along the tracheobronchial tree is rapid anteroposterior compression of the thorax. This compression causes a simultaneous expansion in the lateral thoracic diameter, and the negative intrapleural pressure stretches the lungs laterally along with the chest wall, thereby placing traction on the carina. When the plasticity of the tracheobronchial tree is exceeded, the lungs are pulled apart and the bronchi avulsed. Closure of the glottis before impact may convert the trachea into a rigid tube with increased intratracheal pressure, which may cause a linear tear or blowout of the membranous portion of the trachea or cause a complex disruption of the trachea and bronchi. As predicted by the Law of LaPlace, this type of burst injury occurs where the airway diameter is greatest, usually within 2.5 cm of the carina, but may occur anywhere along the airway. A combination of these mechanisms is probably responsible for producing most injuries. Given the protected nature of these structures, a significant amount of high-energy transfer is usually required to create these injuries.

DIAGNOSIS

Presentation

A variety of clinical presentations result after injury to the tracheobronchial tree, with most depending on the severity and the location of the injury. Patients with cervical tracheal injuries may present with stridor and severe respiratory distress or with hoarseness, hemoptysis, or cervical subcutaneous emphysema. The presentation of thoracic tracheobronchial injury depends on whether the injury is confined to the mediastinum or communicates with the pleural space. Thoracic tracheobronchial injuries confined to the mediastinum usually present with massive pneumomediastinum. Pneumopericardium is occasionally described. Injuries that perforate into the pleural space usually create an ipsilateral pneumothorax that may or may not be under tension. A pneumothorax that persists despite adequate placement of a thoracostomy tube and has a continuous air leak is suggestive of tracheobronchial injury and bronchopleural fistula. Dyspnea may actually worsen after insertion of the chest tube due to the loss of total volume via the tube. In 1969, Oh and colleagues described a highly specific finding of bronchial rupture, which they termed the “fallen lung” sign. Its characteristic radiographic features show the lung falling away from the hilum, laterally and posteriorly, in contrast to the usual simple pneumothorax, which collapses toward the hilum. This sign results from the disruption of the normal central anchoring attachments of the lung and, although pathognomonic, it is rarely seen. Other radiographic clues to possible airway injury are seen with endotracheal intubation and show abnormal migration of the tube tip or overdistension of the endotracheal tube balloon outside the confines of the normal tracheal diameter.

Some retrospective reports show that up to two thirds of these intrathoracic tracheobronchial tears will go unrecognized longer than 24 hours and up to 10% of tracheobronchial tears will not produce any initial clinical or radiological signs and are recognized months later after stricture occurs. Immediate intubation of patients with multisystem trauma can mask laryngeal or high cervical tracheal injuries and contribute to a delay in the diagnosis. After tracheobronchial transection, the peribronchial connective tissues may remain intact and allow continued ventilation of the distal lung analogous to the way perfusion is maintained after traumatic aortic transection. If unrecognized, this injury heals with scarring and granulation tissue and may possibly create bronchial stenosis or obstruction such as in the duck reported by Winslow. After a latent period, granulation tissue and stricture of the bronchus will develop. Distal to the stricture, pneumonia, bronchiectasis, abscesses, and even empyema can result. Complete obstruction without infection leads to prolonged atelectasis and diminished pulmonary function.

While concomitant injury is the rule rather than the exception, patterns of associated injuries vary widely. Major vascular, cardiac, pulmonary, esophageal, bony thoracic, and neurologic injuries are common and reflect the site, magnitude, and mechanism of the trauma. The mechanisms of trauma may alert one to search for the presence of injury. For example, transcervical and transmediastinal penetrating injuries pose particular danger to the respective traversing structures. Associated injuries may be severe, and in at least one series, were responsible for all of the deaths. It has been suggested that corresponding rib fractures would be seen in all patients over 30 years old who had a rupture of the tracheobronchial tree, but this is not always true. The absence of a chest wall injury does not exclude serious chest trauma, but the presence of such an injury should alert one to investigate further for a major underlying injury. A high index of suspicion must be maintained in order to diagnose and treat an injury promptly and appropriately.

Diagnosis should be suspected based on the clinical history and the constellation of signs and symptoms previously listed. Evaluation of the patient with a suspected injury to the tracheobronchial tree is shown in the algorithm in Figure 1. The advent of spiral computed tomography (CT) has created interest in evaluation of injury with this technique. Three-dimensional reconstruction has been used to elegantly demonstrate the site and extent of injuries in case reports. While tracheobronchial injury may be well demonstrated on CT in some cases, there is no evidence that CT is adequate to exclude an injury and obviate the need for diagnostic bronchoscopy. CT scans suggesting injury should prompt bronchoscopy for definitive diagnosis. In addition to visualization of possible tracheal injury on CT, indications for bronchoscopy include large pneumomediastinum, refractory pneumothorax, large air leak, persistent atelectasis, or, occasionally, marked subcutaneous emphysema. Bronchoscopy, whether rigid or flexible, is the best-studied means of establishing the diagnosis and determining the site, nature, and extent of the tracheobronchial disruption. A potential disadvantage of rigid bronchoscopy is that it requires general anesthesia, as well as a stable ligamentous and bony cervical spine. A rigid scope has the advantage of direct visualization and the ability to provide ventilation. Flexible bronchoscopy may be performed without general anesthesia, and offers the potential for controlled insertion of a nasal or orotracheal tube while maintaining cervical stabilization. The most critical determinant seems to be the experience and comfort level of the endoscopist. It has been shown that, in the hands of an experienced bronchoscopist, either technique can be performed with a high degree of accuracy. Lesions may be missed initially or their severity may be underestimated. These lesions may evolve into more obvious or severe injuries, and for this reason bronchoscopy should be liberally repeated as needed.