Aorta Including Abdominal Aorta

Motor vehicle trauma is a major cause of aortic trauma (273-275). Traumatic aortic rupture is seen in up to one-fourth of deceased motor vehicle occupants (273, 274,276-282). The large majority of traumatic aortic rupture victims are dead at the scene (273,276,283,284). There is usually other associated severe trauma, and aortic injury is rarely an isolated finding (273,275,283,285-287).

The isthmus—i.e., the area of the descending thoracic aorta below the left subclavian artery ostium—is the most commonly injured site (Fig. 33; refs. 273-275 and 283). The isthmus is a "fixation" point at the junction between the mobile proximal aorta and the descending thoracic segment bound to the spinal column (275,283). It is an inherently

Fig. 32. Commotio cordis. Young child playing baseball was hit in the chest by a hard plastic ball. Cardiac arrest at scene. Victim pronounced dead after 40 min of resuscitation.
Fig. 33. Transection of descending thoracic aorta (isthmus). Ostium of left subclavian artery (arrow).

weak part of the aorta because of the ligamentum arteriosum, the scarred remnant of the fetal ductus arteriosus (275). Increasing age and disease (e.g., atherosclerosis) further weaken the isthmus (288). Aortic tears are unusual in the young (275,289,290).

Fig. 34. Multiple traumatic intimal tears (arrow) of descending thoracic aorta below a partial tear of the aortic isthmus. Multiple superficial tears have been considered a sign of increased aortic pressure at the time of impact. (Courtesy of Dr. E. Tweedie, London Health Sciences Centre, London, Ontario, Canada.)

Fig. 34. Multiple traumatic intimal tears (arrow) of descending thoracic aorta below a partial tear of the aortic isthmus. Multiple superficial tears have been considered a sign of increased aortic pressure at the time of impact. (Courtesy of Dr. E. Tweedie, London Health Sciences Centre, London, Ontario, Canada.)

Various theories have been proposed to explain why this relatively protected area of the aorta is predisposed to traumatic rupture (274,275,283,291). These explanations have included chest crush-compression, deceleration, increased intravascular pressure, and other hemodynamic forces (Fig. 34). The likely common pathway is longitudinal stress on the aortic isthmus leading to typical transverse tearing, ranging from intimal laceration to rupture (275). At autopsy, an isthmic tear is a cause of a left hemothorax. Following its quantitation, inspection of the course of the descending thoracic aorta along the thoracic spine, before removal of organs, reveals a pleural tear with an underlying isthmic rupture. Multiple rupture sites are possible (273,275). Rupture sites can heal (292). Rib fractures are not always present (140,275,283). In one study in adults, aortic rupture was associated with a minimum of five to seven rib fractures (see Subheading 6.1. and ref. 275).

The ascending aorta, another fixation point, is the second most frequently injured site (Fig. 35; refs. 274 and 275). Other fixed sites are the descending thoracic aorta at the diaphragmatic opening, the aortic arch, and the abdominal aorta above and below its bifurcation (273,275).

Rupture of the abdominal aorta is uncommon (5% of the frequency of thoracic aortic injuries [293-295]). This part of the aorta lies in a protected retroperitoneal location and is attached to the spine. A fractured vertebra can directly lacerate the aorta. Abdominal wall compression raises aortic intravascular pressure. Decelerative forces

Fig. 35. Tear of ascending aorta. Great vessels of arch (arrow).

acting on points of major arterial attachment (e.g., inferior mesenteric artery) and the iliac bifurcation can initiate tearing (293). Pre-existent atherosclerosis may be a factor predisposing to rupture. Instead of immediate retroperitoneal bleeding, a false aneurysm can form. Existing aneurysms can rupture (295).

Frontal and near-side motor vehicle collisions are associated with an increased risk of aortic rupture (274,296). The minimum equivalent barrier speed for isthmic and descending aorta tears in one study was 54 km/h (34 mph) in frontal collisions and 31 km/h (19 mph) for side collisions (275). In another study of aortic injury, the mean crash severity, expressed as 8 V (change in velocity) was 43 km/h or 27 mph (range 21-68 km/h, 13-42 mph) in near-side collisions and 57 km/h or 35 mph (range

21-160 km/h, 13-100 mph) in frontal collisions (284). In another recent study, the mean 8V was 48.0 km/h (30 mph) in frontal impacts and 36 km/h (24 mph) in side collisions (283). Significant intrusion into the occupant compartment is observed in many cases (275,296). Occupants in near-side impacts have a higher risk of aortic trauma than those involved in frontal and far-side impacts (284). Seat belts and air bags reduce but do not eliminate the risk of aortic injury (276,283,284).

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