The heart is suspended in the pericardial sac by the aorta, pulmonary artery, and superior vena cava. Any force that violently compresses the anterior chest and forces the heart downward may exert sufficient traction on the aorta to tear it transversely. The superior vena cava and the pulmonary artery are rarely torn. Aortic lacerations are most often seen in automobile accidents, less commonly in falls. In automobile accidents, aortic lacerations occur in both head-on and side-impact crashes.
Virtually all lacerations of the thoracic aorta involve the descending portion, immediately distal to the origin of the left subclavian artery (Figure 5.2). The arch of the aorta is anchored by the great vessels arising from the aortic arch, that is, the right innominate, left common carotid, and subclavian arteries, and the ligamentum arteriosum (which connects the left pulmonary artery to the arch of the aorta). Partial or complete lacerations of the descending aorta occur at almost precisely the same location — just distal to the origin of the left subclavian artery, at the junction of the aortic arch and the descending aorta (figure 5.2-5.3).
The precise mechanism of this injury is not known. The relatively constant location of aortic lacerations, the relative fixation of the descending aorta just below the aortic isthmus, the relative fixation of the aortic arch by the vessels, and the constant association of the aortic laceration with deceleration injuries, such as automobile collisions, suggest that the abrupt deceleration of the body and resulting forceful compression of the anterior chest and underlying mediastinal structures cause the heart and great vessels to be jerked away from the posterior chest wall to which the thoracic aorta is attached. This traction on the ligament ductus arteriosus and descending aorta at its point of fixation is sufficient to lacerate the aorta immediately below the origin of the left subclavian artery.
Rarely, a periaortic hematoma due to an aortic laceration may evolve into a false aneurysm. The blood at the periphery of the hematoma, which is contained by the periaortic and mediastinal soft tissue undergoes organization until a restraining fibrous connective tissue wall is formed — the false aneurysm. This outer wall becomes adherent to the surrounding mediastinal structures, e.g., the tracheobronchial tree and esophagus, incorporating them
into the wall. Ultimately, the lining of the aneurysmal sac becomes continuous with the endothelium lining the aortic lumen. Because the false aneurysmal wall is composed of fibrous tissue without elastic tissue, continued aneurysmal enlargement is inevitable.
Bursting rupture of the ascending portion and arch of the aorta occur when a violent force compresses the heart and intrapericardial portion of the ascending aorta, producing a sudden rise in intracardiac and intraluminal pressure that results in a transverse tear of the aorta immediately above the cusps of the aortic valve (Figure 5.4).10,11 This usually involves only a portion of the aorta's circumference. Death rapidly ensues from severe hemorrhage. These injuries are associated with fractures of the upper ribs and sternum.
While transmural rupture of the aorta due to trauma is common, traumatic dissection of the aorta is relatively rare.12 Even rarer is a traumatic dissecting aneurysm of the ascending and arch of the aorta.13,14 With regard to traumatic dissecting aortic aneurysms, Papadopoulos et al. reported the case of a 50-year-old hypertensive male who had sustained a steering wheel injury 4 years prior to admission to the hospital for sudden onset of pain in the lower back and left lower extremity.15 The pain subsided rapidly, but the patient developed severe intermittent claudication. A translumbar aortogram revealed a dilated aorta with complete occlusion of the left common iliac artery. At surgery, a huge dissecting aneurysm of the descending thoracic aorta was found, with the intimal tear starting just distal to the left subclavian
artery. The true lumen of the distal thoracic aorta was very narrow. A review of the literature up to October 1975 by these authors revealed 138 cases of chronic traumatic aneurysmal lesions of the thoracic aorta.15
Traumatic rupture of the ascending aorta must not be confused with spontaneous ruptures associated with cystic medial necrosis, which frequently occur in this area. Grossly, the lesions might resemble each other. However, absence of injuries and the microscopic characteristics of cystic medial necrosis will differentiate the nontraumatic from the traumatic rupture. In cases of suspected traumatic laceration of the aorta, all natural diseases that might cause spontaneous rupture or aneurysmal formation, e.g., atherosclerosis, syphilis, or cystic medical necrosis, must be sought. It must be realized, however, that even if these conditions do exist, rupture could still be due to trauma.
Heggtveit et al. reported two cases of innominate artery aneurysms occurring after blunt trauma following a motor vehicle accident.16 In such a case, the victim sustains injury to the chest just to the right of the sternum. This impact may produce nonpenetrating injury to the innominate artery and development of an aneurysm.
Traumatic rupture of the abdominal aorta, either partial or complete, is relatively uncommon, if not rare. Most but not all are caused by automobile accidents.17 Rupture is commonly associated with fracture-dislocation of the underlying vertebrae.
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