One of the main reasons that many unnecessary aortograms are preformed is a technically poor chest x-ray. The upper mediastinum tends to appear wider than
normal if the chest x-ray is taken (1) anteroposterior^ (AP) rather than posteroanteriorly (PA), (2) with the patient less than 3 4 ft (100 cm) from the origin of the x-ray beam, (3) with the patient lying flat, or (4) with poor inspiration. The optimal chest x-ray is an upright PA chest x-ray taken at a distance of 6 ft (about 2 m) with the patient leaning forward about 10 to 15°.
The most accurate radiographic sign of TRA is usually deviation of the esophagus more than 1 to 2 cm to the right of the spinous process of T4 ( Fig 2.5.1.-7). Patients in whom the esophagus is deviated less than 1.0 cm from the midline are unlikely to have a TRA. A nasogastric tube in a normal position virtually excludes TRA.
FIG. 251-7. Deviation of the esophagus (nasogastric tube) to the right is generally a very accurate sign of traumatic rupture of the aorta. If the distance from the nasogastric tube to the spinous process of the fourth thoracic vertebra is greater than 2.0 cm, it is almost 100 percent indicative of a torn descending thoracic aorta.
Blurring or obscuration of the aortic knob or descending aorta is almost as accurate an indication of TRA. Studies have shown that patients with a normal aortic contour and no evidence of deviation of the trachea or nasogastric tube to the right on the chest x-ray do not have TRA.
Other chest x-ray signs include displacement of the left main stem bronchus more than 40° below the horizontal, obliteration of the usual clear space between the aortic knob and the left pulmonary artery (apical cap), widening of the right paratracheal stripe, and displacement of the right paraspinous interface ( Fig 2.5.1-8).
FIG. 251-8. Mediastinal hematomas, indicated by the blackened areas, may widen and displace the paratracheal stripe separating the right side of the tracheal air column from the medial border of the right lung by more than 5 mm. Mediastinal hematomas may also displace the right and left paraspinal lines in the lower thorax rather widely from the lateral edges of the thoracic spine. The paraspinal lines are not readily seen on most x-rays because of overlying structures.
The paratracheal stripe is a linear structure just to the right of the tracheal air column ( F.i.g:...251-8). It extends from the thoracic inlet to the proximal right bronchus and normally measures less than 5 mm in thickness at a level 2 cm above the azygos vein. If the paratracheal stripe is more than 5 mm wide and/or is deviated to the right, this may be another sign of mediastinal hemorrhage.
The paraspinal lines lie between the pleura and the lung, projected away from the lateral margin of the thoracic spine. The right paraspinal line is usually not visible on routine chest x-rays, but if it is seen and if it is displaced to the right in the absence of spinal or sternal fractures, it may be of some diagnostic value.
The left paraspinal line may be distinguished from the image of the descending aorta by the fact that it is not continuous with the aortic knob. When displaced more than one-half the distance from the spine to the left margin of the descending aorta without spinal or sternal fractures, it is highly specific.
It often takes great force to fracture the first or second ribs or sternum, especially in young patients. Consequently, such fractures tend to be associated with an increased incidence of major intrathoracic injuries; however, it is now very controversial whether fractures of the first or second ribs are associated with a significantly increased incidence of TRA.
One should not assume that a TRA has been ruled out if the initial chest x-ray is normal. In up to one-third of patients with TRA, widening of the mediastinum and other characteristic changes may not be apparent on the chest x-ray until several hours after the injury. Two-thirds of patients above 65 years of age with TRA may not show mediastinal widening. Consequently, serial chest films should be taken in any patient with severe chest trauma at 6- to 12-h intervals during the first day and then daily for at least the next 3 days. Indeed, the circumstances of the trauma in such individuals should be the main indication for ordering an aortogram.
Transesophageal Ultrasound Although aortography is the "gold standard" for the diagnosis of traumatic disruption of aorta (TDA), it is positive in only about 10% of patients in whom injuries are suspected based on plain films. Further, it is an invasive procedure that requires 1 to 2 h. Unlike aortography, CT, or magnetic resonance imaging (MRI), transesophageal echocardiography (TEE) can be safely performed in hemodynamically unstable patients. 33
The thoracic aorta is not readily imaged by TTE. However, with the advent of new-generation TEE transducers, the entire thoracic aorta can be imaged. TEE visualizes the aortic isthmus and descending aorta very well and allows assessment of the pericardial cavity (hemopericardium, tamponade), valve function (and potential rupture), pulmonary veins (potential avulsion), and regional wall motion abnormalities.
One study that examined the role of both aortography and TEE found 100 percent sensitivity and specificity, respectively, with higher accuracy for TEE than aortography.34 In this study, there were no complications from TEE, which was performed in less than 30 min on average, as compared with aortography, which required over 75 min on average. Other studies have found the sensitivity and specificity of TEE (in the hands of cardiologists) to be as low as 63 and 84 percent respectively.35 The differences in the various studies are likely attributable to local factors such as patient population, limited study enrollment, and operator experience. TEE in this setting may be performed by cardiologists, surgeons, and radiologists. Because of the variability of findings using this relatively new diagnostic modality, the exact role of TEE for aortic imaging in both blunt and penetrating chest injuries remains to be defined.
The complication rate from TEE is very low, with esophageal perforation (perhaps the most feared complication) being on the order of 2 to 3 per 10,000.
CT Scans CT scans, particularly helical (spiral) CT scans, are increasingly used as screening tools to evaluate patients with blunt chest trauma who are thought to be at risk for aortic and great vessel injury. Reports in the literature are contradictory regarding the sensitivity, specificity, and safety of this technique, which may well reflect the rapidly evolving technology as much as variation in technique and interpretation. The following comments apply to situations where technically adequate images can be obtained using a late-generation helical-capable CT scanner. Primitive CT scanners cannot be recommended for screening for aortic injury.
CT scanning is not a substitute for plain radiography or aortography. When a patient with an appropriate mechanism of injury (rapid deceleration) also has a chest radiograph suspicious for an aortic or great vessel injury, aortography should be the next step. However, there is a group of patients with an equivocal history and an equivocal radiograph. These patients may be screened to exclude an aortic injury using CT scans provided that certain conditions are met. The first and most important condition is that the patient is not thought to be at risk for great vessel injuries, since injuries to the great vessels are commonly missed by axial CT. The second condition is that the patient should be hemodynamically stable and capable of tolerating two dye loads, the first in CT and the second if aortography is required. We generally obtain a "dry" (noncontrast) view of the thoracic aorta first. If there is evidence of mediastinal hematoma on this initial study, we proceed to angiography. If the study appears normal, a CT "angiogram" is obtained using rapid infusion and helical scanning to collect images through the aortic arch and isthmus. If this study is entirely normal, we consider the aorta "cleared." Abnormalities require an aortogram. This special use of CT has found its widest application in our practice with obese patients who cannot tolerate an erect chest x-ray owing to skeletal instability. We reiterate that CT scanning is not a substitute for the proven methods of chest x-ray and aortography, and the use of CT should probably be limited to screening under specific circumstances. Difficulties in demonstrating injuries near or involving the great vessels should allow information about a "high-probability" mechanism of injury to prevail and thus justify proceeding with aortography even in the face of a "negative" CT.
Magnetic Resonance Imaging MRI is a technically seductive method for investigating injuries to the aorta and great vessels. MRI has certainly emerged as the preferred tool for the evaluation of dissecting aneurysms of the thoracic aorta. However, even current-generation MRI instruments require long intervals in quiet patients who lie in an isolated room free of metallic objects to obtain satisfactory images—a requirement that is often difficult to meet in patients who are suspected of harboring major vascular injuries. For these reasons, the role of MRI in the evaluaton of blunt thoracic injuries remains indeterminate. As scan times become faster, the other limitations become less onerous.
Aortography If an aortic rupture is suspected on clinical or radiologic grounds, an aortogram should be performed. While waiting for the aortogram or surgery, it is important to ensure that the systolic blood pressure is kept below 120 mmHg. It is also important to protect the patient from excessive gagging or straining.
The most common finding on aortogram is a pseudoaneurysm of the isthmus of the aorta. A slight pouching out of the inferior or inner border of isthmus, sometimes referred to as a pseudodiverticulum, is normal but may be confused with a traumatic pseudoaneurysm. Bulging of the aorta laterally is a more reliable indicator of TRA. A linear filling defect caused by torn intima and media is the best evidence that a TRA is present.
A patient who is in shock from a suspected TRA or who has a rapidly expanding mediastinal hematoma should be taken directly to the operating room without undergoing aortography. It should be remembered that occasional false-negatives occur with aortography.
Although it is often thought that there is relatively little risk to angiography, local complications with conventional angiography may occur in up to 23 percent of cases, and systemic complications may occur in up to 9 percent.36 Although the rates of amputation (0.1 percent) or death (0.3 percent) resulting from transfemoral studies is relatively low, if they occur in an individual with a negative study, the indications for the aortogram are apt to be questioned. Death has also occurred in at least two instances when the angiographic catheter was manipulated through the aorta at the level of a tear. If angiography is done in a hospital where relatively few cases are done each year, the incidence of complications can be increased up to 32-fold.
Intraarterial Digital Subtraction Angiography In an effort to improve the speed and accuracy of angiography and reduce the dose of contrast material, intraarterial digital subtraction angiography (IA-DSA) has been investigated. Studies have shown IA-DSA to be up to 100 percent accurate, as indicated by the results of surgery, conventional arteriography, serial chest x-rays, and clinical follow-up. The method is 50 percent faster than conventional aortography, and it can significantly reduce x-ray film costs. The use of smaller-caliber catheters for the intraaortic injection and a decrease in radiographic contrast media requirements also make this method safer than conventional arteriography.
TREATMENT Although it is essential to resuscitate severely injured patients aggressively and to correct hypotension and hypoxemia rapidly the patient with a TRA should not be allowed to develop a systolic blood pressure over 120 mmHg or to perform a Valsalva maneuver. Endotracheal intubation can likewise cause gagging and coughing. Therefore these patients should not be intubated "awake." In addition to the standard paralytic and sedative drugs administered during rapid-sequence intubation, preemptive intravenous administration of lidocaine (1 mg/kg) to attenuate the vascular and bronchial responses prior to laryngoscopy may be advisable in patients with suspected or proven TRA. Fluid administration should be watched carefully, and administration of sedatives, analgesics, vasodilators, or even beta-adrenergic blockers may be required to keep the patient's systolic blood pressure at safe levels.
It is often important to insert a nasogastric tube in patients with multiple injuries, but it is essential that the patient with a suspected TRA not perform a vigorous Valsalva maneuver. Sudden gagging or bearing down can cause intraaortic pressure to rise abruptly to well over 200 mmHg and complete the rupture of a partially torn aorta. Similar precautions must be undertaken when inserting an endotracheal tube.
Since the initial report of a successful repair of an acute traumatic thoracic aortic disruption in 1958, 37 emergency operation has become the accepted standard for treatment. However, in selected cases, delays in surgical intervention may be warranted and safe. Such delay should be considered if (1) the patient is stable but the conditions for surgery are not ideal, or (2) the patient represents an extremely high operative risk because of associated injuries or preexisting medical conditions.
In a few centers with extensive experience with thoracic aortic surgery, the aortic repair may be preferentially performed by a rapid "clamp and sew" technique without an external shunt or cardiopulmonary bypass. Under these circumstances, an intravenous infusion of an alpha and beta blocker may be used to keep the systolic blood pressure in the upper portion of the body less than 150 mmHg so as to diminish the chances of intracerebral hemorrhage or left heart failure while the aorta is clamped. The operation must be rapid and precise, because clamping of the descending aorta for more than 30 min without perfusion of the distal aorta greatly increases the risk of damage to the spinal cord and abdominal viscera.
Because it allows increased time for a meticulous, unhurried repair and reduces the risk of ischemic damage to the spinal cord and abdominal viscera, repair of traumatic rupture of the thoracic aorta is often performed under partial cardiopulmonary bypass. If the patient's condition is stable, transfer to a hospital where cardiopulmonary bypass is available is wise, just in case problems develop during the repair.
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