Thoracic Aortic Stent-Grafting and Management of Traumatic Thoracic Aortic Lesions



Thoracic Aortic Stent-Grafting and Management of Traumatic Thoracic Aortic Lesions


Nina M. Bowens, Edward Y. Woo, Mark A. Farber and Ronald M. Fairman


Treatment of descending aortic aneurysms has been the most common application for thoracic endovascular aortic repair (TEVAR) (Fig. 83-1). However, thoracic stent-grafts are preferentially used for repair of traumatic injuries to the aorta, acute type B dissections complicated by malperfusion, rupture, and failure of medical therapy, and virtually all thoracic aortic pathology when anatomically feasible. This is not surprising given that open surgery mandates a thoracotomy, single-lung ventilation, aortic cross-clamping, typically partial cardiopulmonary bypass, and for very proximal thoracic aortic lesions, circulatory arrest (Fig. 83-2). In the United States, three commercially available devices, including the Gore (Flagstaff, Ariz.) TAG and more recently the conformable TAG (CTAG), the Cook Medical (Bloomington, Ind.) TX2, and the Medtronic (Minneapolis, Minn.) Talent and Valiant, have U.S. Food and Drug Administration (FDA) approval and labeling for treatment of thoracic aortic aneurysms (TAAs). The pivotal trials evaluating these devices clearly demonstrated improved 1-year outcomes compared to open surgery (Table 83-1), and the 5-year TEVAR data are now available and continue to indicate freedom from aneurysm-related mortality.18





There has been widespread acceptance and utilization of endovascular therapy whereby it has become the preferred approach for thoracic aortic transections and complicated type B dissections at most centers of excellence. Furthermore, it is remarkable that even in the absence of FDA approval, physicians have recognized the broader applications of an endovascular approach and embraced off-label use of TEVAR to treat various thoracic aortic pathologies.9-22



Indications


Thoracic aortic stent-grafting evolved from abdominal aortic stent-grafting. Initial reports of TEVAR involved using “homemade devices” or abdominal components in an off-label manner. Dedicated thoracic components have been commercially available in the United States since 2005, and device iteration continues to progress beyond the first generation.23-25 The historical indication for operative intervention has been an aneurysm size of 6.5 cm, based on studies demonstrating significantly increased rupture rates.9 However, with the widespread availability of thoracic endografts, this rigorous anatomic selection criteria may be evolving in real-world practice to earlier intervention for smaller aneurysms. Fusiform and saccular aneurysms have been the most commonly treated pathology, but worldwide, thoracic stent-grafts have been increasingly used for aortic dissection (acute and chronic), penetrating ulcers, aortic (airway and gastrointestinal) fistulas, aortic rupture, and blunt traumatic injury, which has recently received FDA approval in the United States.1926


Management of traumatic injury to the thoracic aorta or great vessels is often complex. Arterial injury is confined to the thoracic aorta in 81% of cases, affects the aortic branch vessels alone in 16%, and involves both the thoracic aorta and great vessels in 3%.27,28 Penetrating injuries involving these structures frequently results in rapid exsanguination and death. Common causes of penetrating thoracic trauma include stab wounds and medium-velocity bullet wounds. The anatomic location and extent of intrathoracic vascular injury secondary to penetrating trauma is difficult to anticipate, owing to variant depth and trajectory of the penetrating object. Blunt thoracic trauma, although less common, may result in injury to the ascending aorta, aortic arch, descending thoracic aorta, or great vessels.


Establishing the diagnosis of injury to the thoracic aorta or great vessels usually begins with considering the mechanism of injury. Frequently, especially in the setting of blunt trauma, multisystemic injury and depressed levels of consciousness limit acquisition of historical data. In these circumstances, algorithmic protocols such as the Advanced Trauma Life Support (ATLS) guidelines allow coordinated assessment, diagnostic evaluation, and resuscitation. As in the elective setting, computed tomographic angiography (CTA) has become the diagnostic study of choice, given its widespread availability and rapid image acquisition time (Fig. 83-3).29,30




Contraindications


An absolute contraindication to thoracic stent-grafting is anatomic ineligibility. Absence of proximal or distal landing zones prevents treatment with a standard thoracic stent-graft. Successful exclusion of the pathology depends on achieving a satisfactory proximal and distal seal. Current commercially available thoracic stent-graft devices have a maximal postdeployment diameter of 46 mm, thereby prohibiting successful achievement of seal zones in vessels with diameters greater than 42 mm. At the other end of the spectrum, the smallest current-generation thoracic devices have a postdeployment diameter of 21 mm, prohibiting successful seal zone achievement in vessels with diameters less than 16 mm. More complex endovascular options, including branched grafts and hybrid reconstructions with aortic debranching, can be performed in anatomically complex cases, but these options will not be discussed in this chapter; such procedures play a limited role in emergency management of thoracic catastrophes.


Relative contraindications to endovascular repair of the thoracic aorta also exist. Long-term durability of TEVAR has not yet been established, so its applicability in the young patient remains controversial. Historically, open repair was used for unstable patients who were unable to undergo preoperative imaging. However, the Vascular Group at Albany established the safety of stent-grafting in traumatic rupture of the thoracic aorta, and practice guidelines developed by the Society for Vascular Surgery continue to support the use of TEVAR for traumatic thoracic aortic injury.31,32 The concern of contrast-induced nephropathy in patients with concomitant thoracic aortic pathology and renal insufficiency can be successfully circumvented by using intravascular ultrasound (IVUS), allowing TEVAR to be performed with minimal use of contrast material.



Preoperative Evaluation


Preoperative imaging is critical for successful endovascular repair of the thoracic aorta. Cross-sectional imaging, preferably CTA with thin slices focused on the aorta, is required. Three-dimensional reconstructions, such as the MMS (Medical Metrix Solutions Inc., West Lebanon, N.H.) or TeraRecon system (DuPont, Wilmington, Del.), are then important for determining accurate centerline measurements. Contrast-enhanced angiograms are rarely indicated before the procedure.


For many years, arch aortography with visualization of the great vessels has been the gold standard for diagnosing arterial injuries of the thoracic aorta and great vessels. Liberal application of multidetector CTA in the setting of thoracic trauma has increased the ability to rapidly diagnose injuries of the thoracic aorta and great vessels and provided adequate information on which to base therapeutic treatment algorithms. Although somewhat controversial, CTA in many circumstances obviates the mandatory requirement for diagnostic arteriography, even in the setting of traumatic injury. Many physicians now opt for selective utilization of traditional arteriography, reserving this option for indeterminate findings on CTA, or more commonly using arteriography as an adjunct to percutaneous therapeutic intervention.


Successful exclusion of an arterial defect—whether aneurysm, dissection, or injury—with a stent-graft fundamentally relies on achieving a seal between the device and the vessel wall both proximal and distal to the site of pathology. Achieving a satisfactory seal in these locations depends on several factors. Of critical importance is correct device sizing. An undersized device results in inadequate apposition between the stent-graft and vessel wall, resulting in incomplete exclusion. Slight oversizing (≈10%) allows circumferential transmission of postdeployment radial forces from the device to the aortic wall, thereby facilitating device/vessel apposition and seal zone achievement. Unfortunately, zealous oversizing may result in crimping of graft material, predisposing to type I endoleak. Exaggerated oversizing may precipitate potentially life-threatening intimal dissection or arterial injury induced by the stent-graft itself.


Achieving a proximal seal is frequently more challenging than obtaining a distal seal. This is due to several factors, including angulation of the distal aortic arch and proximity of the great vessels. In most patients, the likelihood of achieving satisfactory proximal and distal seal zones can be determined by carefully evaluating preprocedural CT images, correlating this information with intraprocedural angiographic data, and having a low threshold for performing partial (or complete) over-stenting of the left subclavian artery. Coverage of the left subclavian artery remains a controversial subject because the literature is devoid of randomized prospective data. Most would support coverage of the left subclavian in the emergency setting, with selective revascularization based on vertebral anatomy following TEVAR. However, guidelines in the elective setting remain in evolution.32,33



Anatomy and Approach


Preprocedural abdominal and pelvic CTA (performed simultaneously with chest CTA) allows accurate evaluation of the anatomic features of the abdominal aorta and iliac arteries. Primary considerations include identifying stenotic or occlusive lesions and determining the degree of arterial tortuosity. The minimal iliac arterial diameter required to allow retrograde passage of the device varies with the size and type of stent-graft selected. Current commercially available devices make use of large-caliber introducer sheaths (20F to 24F) that generally require an iliac arterial diameter of at least 7 to 8 mm. Further consideration is also given to the patient’s surgical history and physical examination findings in an effort to avoid unnecessary dissection through scar tissue from prior surgical procedures. This information is then used to formulate an appropriate target strategy for surgical access.



Equipment


The process of optimal device selection can be complicated. In addition to the basic considerations of device diameter and length, the importance of device conformability, trackability, deployment accuracy, availability, and operator familiarity should not be underestimated.


The Gore TAG was the first FDA-approved device for the treatment of thoracic aneurysms. Design revisions in 2004 (spine removal) simplified its use and significantly decreased late graft-related complications. Five-year results comparing endovascular treatment using the Gore TAG with open surgical repair demonstrated significantly reduced mortality and major adverse events (MAEs) in patents undergoing TEVAR.8 Medtronic completed its pivotal VALOR trial of the Talent thoracic stent-graft, demonstrating TEVAR superiority to open surgical repair.1 Trials evaluating the Valiant, which involves modifications and enhancements to the Talent graft, similarly published favorable early and midterm results for stent use in a wide variety of thoracic aortic pathologies.2,3 The FDA approved the Medtronic Valiant device in 2011. The Cook Medical Zenith TX2 trial also succeeded in demonstrating non-inferiority of TEVAR over open surgical repair.4,5 Moreover, it is important to recognize that following initial FDA approval, these devices undergo further iteration over time as industry responds to physician feedback (Table 83-2).



Common aspects among these devices that separate them from abdominal grafts include a long shaft to deliver the endograft to the proximal thoracic aorta from the groin, larger diameters and lengths to accommodate the thoracic aorta, and a tubular nonbifurcated design. Increased flexibility and conformability to account for the often tortuous thoracic aorta, as well as the aortic arch, is likewise important.


The various grafts have design features that make them unique. Each has a different release mechanism during endograft deployment. The Valiant (Medtronic) offers a proximal self-expanding polished nitinol wire frame for fixation and is available in larger diameters that allow treatment of aortas up to 42 mm. This graft has been modified to include the addition of an eight-peak spring (compared to a five-peak spring in the Talent system) to allow for improved proximal fixation and elimination of the connecting bar for increased flexibility. The proximal bare spring remains constrained following deployment of the stent-graft and is released subsequently. The Zenith (Cook) device uses a completely covered proximal stent and an uncovered stent distally, with barbs both proximally and distally for improved fixation; in the regular graft sizes, it also has a tapered graft. This graft may be used as a one- or two-component system. Two-year data examining use of the TX2 Zenith graft suggest that the two-component system is employed in more complex patients and thus associated with increased perioperative morbidity.5 The recently improved conformable TAG (Gore CTAG) device incorporates a short uncovered stent proximally for better wall apposition and an unflared straight distal configuration. The enhanced device has improved compression resistance and is manufactured in smaller diameters and tapered designs. In addition, a novel trilobed balloon to allow continuous aortic flow during balloon inflation is also available as an accessory. Moreover, the choice of endograft should certainly be tailored to individual patient specifications such as aneurysm anatomy and access availability.


Despite the tubular design, thoracic endografting can often be more challenging than abdominal endografting, so it is critical to have the necessary equipment, facilities, and personnel during procedures. Standard wires and catheters are necessary for obtaining access. Stiff wires, such as the Lunderquist (Cook), are important for device tracking. The devices of all manufacturers can be quite stiff and difficult to track through a tortuous aorta or around the aortic arch. All wires must be at least 260 cm for access from the groin. Marker pigtail catheters can be helpful for intraprocedural length measurements. Balloons used for graft effacement are generally device specific. At times, a noncompliant aortic balloon may be necessary (Table 83-3).



Ideally, these procedures should be performed in an endosuite within a fully functional operating room. Certainly, a mobile system can suffice, but the resolution with a fixed platform is superior. Operating room capabilities are important because most of these procedures require at least one femoral exposure, and retroperitoneal access to the iliac artery is often needed.7,34 Notably, gender analysis of the Talent system revealed that iliac conduits are required significantly more often in women than men.35 In addition to vascular access concerns, emergency situations requiring an open operative procedure can arise, necessitating operating room capabilities.


Assembly of an appropriate operative team is paramount for successful aortic repair and patient outcomes. Technicians familiar with the devices, equipment, and procedural steps are helpful, and anesthetic support is critical. Patients often need invasive monitoring for strict hemodynamic control, especially when working around the arch. Moreover, spinal drains may be necessary to decrease intraspinal pressure and help prevent paraplegia. Neurologic support is also indicated and may be in the form of monitoring motor or somatosensory evoked potentials. Studies have clearly shown that monitoring of such potentials can help manage the incidence of paraplegia, at least during open repair.36-39

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Dec 23, 2015 | Posted by in INTERVENTIONAL RADIOLOGY | Comments Off on Thoracic Aortic Stent-Grafting and Management of Traumatic Thoracic Aortic Lesions
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