Aortic Injury

Incidence

Aortic trauma is most common in men between the ages of 20 and 40 years. In 1989, Mattox and colleagues reported that 90% of cardiovascular trauma was caused by penetrating mechanisms and only 7% was caused by blunt injury. The descending thoracic aorta is the one anatomic location where blunt mechanism is nearly as common (44%) as penetrating trauma (56%). The higher incidence of injury in the descending thoracic aorta is as a result of its relatively fixed nature at the ligamentum arteriosum, which, as the remnant of the ductus arteriosus, connects the pulmonary artery to the inferior surface of the aortic arch. In cases of thoracic trauma with significant deceleration, this anatomic relationship predisposes the proximal portion of the descending thoracic aorta to tears in the wall of the vessel ( Figure 1 ).

Overall, blunt aortic trauma only accounts for 0.3% of all trauma. Automobile accidents account for the majority of those causes, followed by pedestrian–versus–motor vehicle accidents. In the abdominal aorta, the leading cause of injury is penetrating mechanism, which represents 88% to 95% of the injury pattern. In the conflicts in Iraq and Afghanistan, aortic trauma accounted for 2.9% of all vascular injuries, and the primary mechanism was explosions or gunshot wounds.

Pathophysiology

The majority of blunt aortic injuries occur at the aortic isthmus, which is an area of slight constriction immediately distal to the left subclavian artery origin at the point of attachment of the ligamentum arteriosum (see Figure 1 ). In a classic 1958 report, Parmley described the direct, indirect, deceleration, compression, and blast forces that produce aortic rupture. Direct forces include displacement of the vertebrae that cause rupture of the aorta by shearing forces. Indirect forces act through a sudden and significant increase of pressure within the aorta.

Experiments have been performed, and the pressure to burst the aorta by this mechanism has been shown to be as high as 2500 mm Hg. Deceleration forces such as those often sustained in automobile crashes are the most common cause of blunt aortic rupture as a result of the stress and strain placed on the aorta at the ligamentum arteriosum. At this location, it is likely a combination of these forces that causes first aortic wall tear and then rupture. Of victims who suffer aortic rupture, approximately 70% have been shown to be involved in a broadside or head-on collision, suggesting that rupture involves lateral oblique compression of the chest, causing numerous different force vectors.

Presentation and Diagnosis

The majority of those who suffer blunt aortic injury die at the scene; approximately 60% die within 30 minutes and only 6% to 8% live longer than 4 hours. In studies of this injury pattern, of patients who survived to arrival at the hospital, 76% lived past the triage phase of care. Patients who suffer abdominal aortic injuries also have a high mortality rate. To improve survival following this injury pattern, consistent and rapid prehospital care and retrieval from point of injury is required, as are early injury detection methods.

Patients who have sustained penetrating torso trauma and who are hemodynamically unstable should receive measured hypotensive resuscitation and undergo immediate surgical control of the bleeding. Such patients need to be recognized early and treated in an expeditious fashion to maximize chance of survival. For patients who arrive to the emergency department hemodynamically normal, the classic treatment algorithm includes radiographic evaluation. After primary and secondary survey, evaluation of trauma patients with significant mechanism of injury should include a chest radiograph. Signs indicating the need for further aortic imaging include a widened mediastinum (>8 cm); fractured sternum, first rib, or multiple ribs; obscured aortic knob; deviation of the left main stem bronchus or nasogastric tube; and opacification of the aortopulmonary window and widened paraspinal line. If an upright chest x-ray is negative, it has a negative predictive value of 95%.

In cases of significant trauma, patients who remain hemodynamically normal undergo contrast-enhanced computed tomography (CT) of the aorta. In most studies, aortic CT has been shown to have near 100% sensitivity and negative predictive value ( Figure 2 ). If the CT scan raises question of an aortic injury, traditional contrast aortography is indicated (see Figure 1 ).

Recognizing that not all blunt aortic injuries are the same, Starnes and colleagues have performed multiple studies to advance understanding of the natural history based on CT imaging. Specifically, this group created a novel classification system based on the presence or absence of aortic external contour abnormality, defined as an alteration in the symmetric, round shape of the aorta ( Box 1 ). The grades of aortic injury are intimal tear without an external contour abnormality and an intimal defect and or thrombus of less than 10 mm of length or width; large intimal flap without an external contour abnormality and intimal defect and/or thrombus of greater than 10 mm of length or width; pseudoaneurysm with an external contour abnormality and contained rupture; and rupture with an aortic external contour abnormality and contrast extravasation. This grading allows one to characterize mortality and make recommendations regarding the timing of appropriate management strategies for patients with blunt aortic injury.