Thoracic aortic injury

Epidemiology and incidence

Most cardiothoracic vascular injury (90%) is due to penetrating trauma, with 28% of penetrating aortic injuries in the thoracic portion. Based on a classic study evaluating outcomes of nearly 6,000 vascular injuries, Mattox et al noted 14% of all injuries involved the thoracic aorta, of which 86% were due to penetrating mechanisms. The most common location for injury is the descending thoracic aorta (47%-75%), followed by intrapericardial/ascending (8%-27%), with the aortic arch representing the most infrequent (8%). Most patients who die from thoracic aortic injury are men (71%). The most common risk factor is alcohol or illicit drug use, which is seen in approximately 40% of cases.

For those who die following penetrating trauma to the chest, most will have evidence on autopsy of cardiac injury (50%), followed by aortic arch branch vessel injury (40%) and aortic injury (10%). Of those dying following aortic injury, greater than 60% will involve the descending thoracic aorta. This may reflect the increased propensity to freely bleed into the hemithorax following this injury, resulting in rapid exsanguination.

While BTAI is less common than penetrating injury, it is more frequently encountered in victims who survive transport to a hospital. Nonetheless, it is still a highly lethal condition and often results in rapid demise. Based on autopsy studies, nearly 70% of BTAIs occur at the isthmus, which is just distal to the left subclavian artery. Approximately 10% of victims will sustain isolated arch injuries, with nearly 20% of victims having sustained multiple injuries. Commonly associated injuries include blunt cardiac injury, rib fractures, hemopneumothorax, pelvic/long bone fractures, and abdominal solid organ/hollow viscus injuries.

For those patients who die following a blunt injury, the presence of aortic injury is strongly associated with early death, with approximately 70% to 80% having died at the scene, in contrast to only 60% dying on scene in the absence of BTAI. A full one third of patients who die from blunt mechanisms will have evidence of BTAI. For those who survive to undergo evaluation at a hospital, approximately 25% will die over the next 24 hours. This not only reflects the lethality of aortic injury but also underscores to overall severity of injury in this population.

Mechanism

The most common penetrating mechanisms resulting in thoracic aortic injury involve missiles (handguns or rifles) and bladed weapons. These are overwhelming the result of violence, in comparison with blunt mechanisms that are largely caused by accidental injury. Owing to their anterior location within the chest, the arch of the aorta, innominate artery, and the carotid arteries are particularly susceptible to penetrating injury.

With respect to blunt aortic injury, the most common mechanisms include motor vehicle accidents, pedestrian struck, falls from height, and motorcycle crashes. BTAI is also commonly identified in approximately 30% of airplane crash victims. The underlying mechanism of this injury pattern is typically rapid deceleration. Because the descending thoracic aorta is fixed to the chest wall and the heart is fairly mobile by comparison, sudden deceleration results in anterior displacement and/or rotation of the heart with resulting torsional and shear stress on the aorta due to tethering at the ligamentum arteriosum, the remnant of the fetal ductus arteriosus. Additional theories involve osseous compression of the aorta from anterior/posterior compressive forces, stretch or shear from axial forces, as well as “water hammer” effects from abrupt increases in aortic pressure due to aortic compression. Another proposed mechanism involves rotation of the heart about its axis, resulting in torsion and tearing at the aortic root, which may result in death from pericardial tamponade, exsanguination, or sudden cardiac arrest. Finally, descending thoracic aortic injury has been observed more distally, where the aorta passes through the diaphragm and is subject to shear forces.

Classification

BTAI represents a constellation of pathologic or radiographic findings, with varying degrees of clinical significance. Common findings that have been used to help injury classification include the presence and degree of intimal injury/disruption, intramural hematoma, the presence and extent of pseudoaneurysm formation, evidence of contrast extravasation or pathologic evidence of free rupture, and evidence of aortic contour abnormalities. In general, the severity of injury increases with increasing grade of the injury, with most authors utilizing a system involving 4 grades of injury. Yet, there has been a heterogenous approach to the classifying BTAI over the past 20 years. The most common classification systems include the Vancouver simplified, Simeone et al, and the Society for Vascular Surgery, which all classify aortic injury according to four distinct grades. Gavant et al additionally proposed a system that denoted aortic injury as either minimal or severe. In practice, most clinicians utilize a blend of these systems as follows: grade I—intimal tear, grade II—intramural hematoma or large intimal flap, grade III—pseudoaneurysm, grade IV—rupture or contrast extravasation ( Figure 1 ).

Concern has been raised around the utility of the common four tier grading schemes, as there is heterogeneity in the classification of low-grade injuries based on the specific criteria that is applied. Moreover, this approach has been problematic from a prognostic standpoint and may not be useful in determining the need for repair of grade I/II injuries. Others have proposed an alternative approach, focusing on emphasizing morphologic features that more strongly predict the need for repair. The University of Washington group has placed significance emphasis on external aortic contour changes as a determinant of injury severity, with the decision for intervention being based on this factor. Recently, the University of Washington group has proposed a three-tier system as a method for determining the need for, and urgency of, intervention: minimal (no external contour abnormality or intimal tear < 10 mm)—no repair; moderate (external contour abnormality or intimal tear > 10 mm)—semielective repair; and severe (active extravasation)—immediate repair.