Blunt Trauma to the Great Vessels of the Chest

INCIDENCE Approximately 80 to 90 percent of patients with blunt trauma to thoracic great vessels, particularly the aorta, die at the scene, and up to 50 percent of the remaining patients die within 24 h if not promptly treated. The frequency of these injuries appears to be increasing and is primarily related to the use of high-speed automobiles. Each year at least 5000 to 8000 individuals in the United States suffer traumatic rupture of the thoracic aorta or one of the other great arteries in the chest. Over 80 percent of the cases are due to automobile accidents.

MECHANISM OF INJURY The mechanical factors responsible for traumatic rupture of the thoracic aorta and its major branches are probably somewhat different for each anatomic area. For the descending aorta at the level of the isthmus, three mechanical factors thought to contribute to rupture are shearing stress, bending stress, and torsion stress. The difference in deceleration between the mobile aortic arch and the relatively immobile descending aorta puts the aortic isthmus under tension, and the resultant shearing stress can lead to rupture or tears opposite the site of fixation. Bending stress is produced as the heart exerts downward traction on the aortic arch, resulting in the hyperflexion of the blood-filled aortic arch on a transverse fulcrum created by the hilar structures of the left lung. Torsion stress occurs when anteroposterior compression of the chest with resultant displacement of the heart to the left is combined with an intravascular pressure wave transmitted to the aorta. These three forces can combine to produce maximum stress to the inner surface of the aorta at the ligamentum arteriosum, which is its point of greatest fixation.

The aortic injury tends to progress from the intima out toward the adventitia. The adventitia, which has the lowest elastic limit, seems to withstand these stresses better than the intima or media.

Rupture of the innominate or left subclavian artery at their origins probably results primarily from the interaction of two forces. One is a compression force that displaces the heart into the left chest and places the brachiocephalic vessels under tension at their attachment to the aortic arch. The other force occurs when hyperextension of the neck with rotation of the head to one side places the contralateral subclavian arteries under tension. Subclavian artery injuries can also occur just over the first rib, and injuries at that site are usually caused by direct trauma and/or excessive stretching.

PATHOLOGIC CHANGES Blunt aortic tears usually extend partially or completely around the vessel in a transverse or spiral direction. Preexisting disease, such as atherosclerosis or medial necrosis, does not appear to predispose to traumatic rupture. When the aortic tear involves all layers of the aortic wall, death by exsanguination is usually almost instantaneous. If the aortic tear does not involve the adventitia, and the parietal pleura and the surrounding mediastinal tissues remain intact, a false aneurysm often forms. The false aneurysm tends to expand, particularly if the patient is hypertensive, and about 50 percent of these, if untreated, will rupture within 24 h. However, some posttraumatic aortic false aneurysm remain intact and may not be detected for 20 years or longer. Although a lacerated subclavian artery occasionally forms a false aneurysm, it usually just occludes and does not require surgery.

It should be emphasized that the small hemothorax that is often present with blunt trauma to the aorta does not result from the aortic injury itself but rather from tears to adjacent small mediastinal vessels or other structures. In the same manner, although the widened mediastinum may be partly due to the aortic pseudoaneurysm, much of it is actually caused by bleeding from small mediastinal vessels.

NATURAL HISTORY Of the patients who reach a hospital and survive for 1 h, about half die within 24 h, and three-quarters die within 7 days. Of the remainder, most die within the next 1 to 3 months.

Many of the early deaths are caused by associated injuries, but even when the aortic injury is an isolated problem, diagnosis and repair should usually be accomplished on an urgent basis. All too often the patient dies of exsanguination before a definitive repair can be effected. Keeping the systolic blood pressure <120 mmHg and prevention of Valsalva maneuvers may avoid many of the early deaths.

LOCATION At least 90 percent of blunt aortic injuries in patients who reach the hospital alive occur in the isthmus of the aorta, between the left subclavian artery and the ligamentum arteriosum. The next most common sites involved are the innominate or left subclavian artery at their origin or a subclavian artery over the first rib. Patients do not usually survive injury to the ascending aorta, but this injury may be seen in up to one-third of the individuals who die at the scene of an accident, especially with vertical deceleration from falls from great heights or plane crashes. Tears in the lower aorta below the ligamentum arteriosum are quite uncommon but tend to occur adjacent to severely comminuted fractures of vertebral bodies.

DIAGNOSIS History The single most important factor in establishing the diagnosis of acute traumatic rupture of the aorta (TRA) is a high index of suspicion because of the nature of the trauma (IĀ§bie...251:5). Even if there is no external evidence of chest injury, one should still be acutely aware of the possibility of this injury in anyone who has sustained an accident characterized by sudden severe deceleration or a high-speed impact from the side.

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