Aortic Transection

Etiology

Blunt force trauma is the primary etiology of aortic transection. Penetrating trauma to the chest may result in aortic injury, though less commonly. In blunt thoracic injury, the rapid deceleration of the torso upon the impact of the encountered object causes shearing stress on the aortic wall. The aorta is fixed to the mediastinum at the ligamentum arteriosum or isthmus. Distal to this, which is just beyond the take-off of the left subclavian artery, the aorta is more mobile. This anatomy creates a focus of shear stress during the deceleration mechanism, resulting in varying degrees of wall disruption. A mild disruption may cause a tear in the intima, while more severe mechanisms may cause a complete transection of the aortic wall. 

Epidemiology

Aortic transection occurs most commonly due to blunt thoracic trauma. The common mechanisms that result in the injury pattern are motor vehicle collision, pedestrian struck by a vehicle, and fall, in decreasing order. In an autopsy study of patients that experienced blunt thoracic trauma, the average age of the patient was 43 years, and they were 71% male. Close to 40% of the victims were intoxicated at the time of injury.[2]

History and Physical

The presentation of the patient with aortic transection exists on a spectrum. As previously mentioned, most patients with this injury pattern die at the scene. The cohort that survives transport and evaluation represents a more hemodynamically stable group with a less severe grade of injury. Blunt trauma often results in multiple affected organ systems, including central neurologic injuries. This may impede the provider’s ability to obtain a history. In the awake and conversant patient, he or she may be completely asymptomatic or may complain of chest pain radiating to the neck, back, or shoulder. These symptoms are nonspecific, and the injury requires an increased suspicion based on the mechanism to obtain a complete workup. On physical examination, the patient may have normal blood pressure or be profoundly hypotensive and in hemorrhagic shock. This often reflects the degree of wall disruption. A pulse examination can reveal asymmetry, which may indicate a transection involving the arch. 

Evaluation

The workup of aortic transection begins most often with a plain film chest x-ray. This is included in the standard trauma workup following the primary survey of the patient. On chest x-ray (CXR), findings that may indicate aortic pathology include a widened mediastinum, loss of the aortic knob contour, inferiorly displaced left bronchus, and left pleural effusion. Any chest x-ray with abnormal findings should be followed by helical imaging when clinically appropriate. The utility of a chest x-ray for aortic transection has been studied. The positive predictive value of widened mediastinum on CXR is only 5%, and the negative predictive value is 99%. Therefore, a widened mediastinum on x-ray may not represent aortic transection; however, its absence nearly confirms the lack of aortic pathology.[3] The sensitivity of the CXR is about 40%, demonstrating its poor suitability as a single screening tool for aortic injury.[4] Computed tomography then has a sensitivity of 95%, the specificity of 40%, and a negative predictive value of 99%.[3] Aortagraphy is considered the gold standard imaging technique for aortic injuries. However, angiography is an invasive procedure with complications and risks.

Treatment / Management

In grades 2 to 4, the aortic injury requires repair of the defect. The surgical treatment of aortic transection has been radically changed with the introduction of the aortic stent graft. In 2005 the FDA approved the first thoracic stent-graft device. Many products have been placed with “off-label” use, but more products have become available to the surgeon with time. The endovascular repair of the injured thoracic aorta has now nearly replaced the former open repair. Occasionally, patient anatomy may prohibit the use of endovascular technology. Factors include a lack of a proximal landing zone and poor femoral artery access sites. Femoral artery size that is less than 7 mm in size results in increased access site complications. 

Prompt repair of the aortic injury is ideal. Additional injuries may supersede the aortic injury and require more expeditious treatment. As soon as the patient is clinically fit, endovascular or open repair should be planned. Some data suggest the value of delayed repair with a decreased overall mortality compared to early repair, though at the cost of increased complications and ICU and hospital length of stay.[5][6](B2)

The technique for endovascular repair begins with transporting the patient to a hybrid endovascular suite operating room if available. These clinical settings allow the use of a fixed angiography imager with the full capabilities of an operating room, should open repair or other operative exploration become necessary. After positioning the patient in the supine position and administration of anesthesia, the abdomen and bilateral groins are prepped. Access of the bilateral common femoral arteries is gained by the Seldinger technique and confirmed by imaging. A wire and catheter are advanced to the aortic root, and an aortogram is obtained. After the injury is identified, a stent-graft device is selected and advanced into position over a wire. Often, due to the curve of the arch, it is necessary to cover the ostium of the left subclavian artery to achieve adequate apposition at the graft landing zone. This is necessary for approximately 40% of patients.[7] Post-deployment balloon angioplasty is selectively employed. Completion angiography is then used to examine for endoleak. If the seal is complete, the femoral arteriotomies are closed, and the patient is emerged from anesthesia or is transferred for further resuscitation. If the left subclavian artery is covered by the graft, select patients may require delayed carotid subclavian bypass. (B2)

Open repair is achieved via left posterolateral thoracotomy. The patient is positioned in the lateral position and flexed. If possible, single lung ventilation can facilitate improved visualization of the aorta. The aorta and arch are exposed. A clamp is applied distal to the left subclavian and distal to the extent of the injury. A tube graft is elected and sewed into place proximally and distally. Care is taken to incorporate the adventitia into the repair, as this contributes to most of the strength of the aortic wall. Because this technique requires interruption of flow to the distal aorta, often cardiac bypass is necessary. Left heart bypass is achieved by cannulation of the left inferior pulmonary vein and the distal thoracic aorta. This allows for continued perfusion of the aorta, and subsequently, the viscera, spinal cord, and lower extremities. If a proximal clamp cannot be placed, full cardiopulmonary bypass may be necessary. This is achieved by the cannulation of the femoral artery and vein. It is important to consider that bypass requires systemic anticoagulation, which may not be possible in patients with polytrauma. After the repair is complete, the clamps are removed while the patient is mildly hypothermic at 32 to 34 C.

Differential Diagnosis

The differential diagnosis for the patient with blunt thoracic trauma is extensive. Blunt cardiac injury, pulmonary contusion, pneumothorax, hemothorax, rib fractures, great vessel injury, and injury to the aerodigestive tract are all possible. The patient with aortic transection frequently has multiple other thoracic injuries that may require prompt treatment.

• Aerodigestive tract injury
• Blunt cardiac injury
• Great vessel injury
• Hemothorax
• Pneumothorax
• Pulmonary contusion
• Rib fractures

Staging

Aortic transection represents a range of severity in traumatic disruption of the integrity of the aortic wall. The severity is classified by grade, which reflects the degree of disruption present. Grading of the injury is important because treatment is guided by the extent of the injury. A grade 1 injury describes an intimal tear. This injury does not change the contour of the aorta and is unlikely to be visible on plain film X-ray. Grade 1 injuries may be seen on CTA or intravascular ultrasound (IVUS). Angiography may fail to identify an intimal tear. Grade 2 injuries are intramural hematoma or dissection caused by a disrupted media layer. The contour of the aorta may be affected by a grade 2 injury. These are readily visible on CTA, IVUS, and angiography. Grade 3 injuries describe a pseudoaneurysm. Similar to grade 2 injuries, these affect the aortic contour and are seen on all imaging modalities. Grade 4 injuries describe a complete rupture of the aortic wall.

Prognosis

The majority of mortalities due to aortic transection occur before hospital arrival. In-hospital mortality occurs in 18.8% of all patients. This is much higher in the non-operative managed patient (34.4%) versus the patient managed with endovascular repair (8.6%). Mortality related to the aortic injury itself is 6.5% for all patients but only 2.5% after endovascular repair. Patients who die have been found to have high injury severity scores, which indicates severe polytrauma. Risk factors for aortic relate mortality include higher injury severity scores and higher grades of injury.[7] 

Complications

The repair of thoracic aortic injury is associated with known complications. Common to all procedures are the risks of hemorrhage and infection. The most discussed complication following thoracic aortic injury repair is spinal cord ischemia. This occurs due to the interruption of blood flow from the aorta to the anterior spinal cord. Either aortic clamping or covering branches with a covered stent graft can result in spinal cord ischemia. The dreaded result of this ischemia is paraplegia. Considerations for minimizing the risk of spinal cord ischemia include the length of time of aortic clamping, the length of covered stent-graft, level of stent-graft deployment, duration of hypotension, distal aortic pressure, and the number of ligated intercostal branches. Several adjunctive measures have been enlisted to decrease the risk of spinal cord ischemia. Placement of a lumbar CSF drain, corticosteroid administration, and induction of hypothermia have been described to minimize the risk of spinal cord ischemia.[8] A less common neurologic complication is stroke secondary to left subclavian coverage. 

The other primary source of complications is related to the stent-graft device. Preliminary data suggested that 18% of endovascular pairs had a graft complication. The need for a repeat intervention after thoracic endovascular stent deployment is only 1.8% in the trauma setting.[9] With the increasingly universal implementation of the endovascular approach and improved stent-graft devices, this number has decreased significantly. Recent data confirms that the endovascular approach results in less perioperative morbidity and mortality than the open approach.[10] The majority of device-related complications are are endoleaks. Rarely, the graft can migrate from its deployed position. The incidence of endoleak is minimized with proper sizing of the stent graft to the aorta.

The complication profile differs between open and endovascular approaches. Thoracic endovascular aortic repair (TEVAR) has been shown to have less early mortality, paraplegia, renal injury, blood product transfusions, reoperations, cardiac complications, pneumonia, and length of stay compared to an open approach.[11][12]