Endovascular Therapy for Thoracic Aortic Aneurysms: State of the Art in 2012
Conventional surgery for thoracic aortic pathology involves replacing the affected segment of aorta with an interposition graft and often requires the use of extracorporeal circulatory support with or without deep hypothermic circulatory arrest. Although operative results have improved consistently over 60 years, patients with extensive aneurysms face a considerable risk with conventional surgery, particularly when burdened with multiple comorbidities. Thoracic endovascular aortic repair (TEVAR) was first performed in 1994 and has become a well-established alternative therapy for many thoracic aortic pathologies. TEVAR is most frequently performed through a small groin incision to access the common femoral artery. Wires and catheters are used to deliver and deploy the stent graft in the thoracic aorta under fluoroscopic control. Occasionally, TEVAR is performed as part of a complex hybrid procedure including one stage of conventional open surgery that may utilize a thoracic incision and cardiopulmonary bypass support. The less invasive nature of TEVAR offers the potential for lower mortality and peri-procedural morbidity. Although long-term results of TEVAR are still being gathered, mid-term results are excellent and most late vascular complications can be treated with additional transcatheter procedures. Recent development of fenestrated and branched stent grafts is expanding the application of endovascular therapies to complex aortic pathologies involving the thoracoabdominal aorta and aortic arch. Although conventional techniques continue to be the gold standard for treatment of ascending aortic pathology, recent reports have proven TEVAR to be a viable alternative in specific situations. Design improvements continue to expand the indications for TEVAR, and technological advancements in the field of imaging facilitate safer and more accurate planning, delivery, and assessment of patients with thoracic aortic aneurysms. Hybrid operating rooms provide the optimal environment with state of the art imaging technology for the cardiovascular team to perform TEVAR or alternative hybrid procedures.
KeywordsThoracic aorta / aorticTEVAR (thoracic endovascular aortic repair)Thoracic aortic dissectionThoracic aneurysms
The incidence of thoracic aortic aneurysms is around 10.4 per 100,000 person-years and is estimated to be increasing, in part as a result of improved diagnostic imaging of asymptomatic patients with incidental findings . Aortic aneurysm is defined as enlargement of the aorta to a size greater than 1.5 times its normal diameter . The natural history of untreated thoracic aneurysms has been reported to be quite poor, with estimated 5-year survival between 10% and 20% [3, 4]. Reports of patients with large thoracic aneurysms (>6 cm in diameter) show an annual risk of rupture that varies from 10% to 15%, and over 90% of these patients do not survive if the aneurysm ruptures [5, 6]. Most patients with a thoracic aortic aneurysm are asymptomatic and diagnosed incidentally by chest radiograph or CT scan obtained for other reasons, and about 30% to 40% of all thoracic aortic aneurysms involve the descending thoracic aorta.
The most frequent pathology associated with thoracic aortic aneurysms is medial degeneration (also known as cystic medial necrosis), characterized by disruption and loss of elastic fibers and increased deposition of proteoglycans. Atherosclerotic changes are typically superimposed on medial degenerative disease [7, 8]. Other pathologies associated with thoracic aortic aneurysms include chronic dissection, inflammatory disorders (giant cell aortitis, Takayasu’s disease), infection, and connective tissue disorders (including Marfan syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome, Turner syndrome, bicuspid valve aortopathy, and others).
When present, symptoms are most commonly due to chest pain or discomfort, particularly in patients presenting with large or complicated thoracic aneurysms. Rarely, an aneurysm may cause compressive symptoms on adjacent structures including hoarseness, stridor, dyspnea, dysphagia, or facial plethora and edema. Embolization of atherosclerotic debris with end-organ symptoms may occur. History of fever may be related to inflammatory disease or mycotic aneurysms [9••]. Most physical findings are not specific for thoracic aortic disease, and a low threshold for screening CT or MR is required for diagnosing the aortic pathology.
Epidemiologic studies from the early 1990s reported that only 13% of patients with diagnosis of thoracic aneurysms underwent elective surgical treatment in the United States, with a mortality of 23.7%, whereas an additional 19% of patients undergoing surgery for ruptured thoracic aneurysms had an in-hospital mortality of 52% . Substantial improvements have been made since then, and conventional operative results have consistently improved. Contemporary series from centers of excellence have reported operative mortality of 3% to 10% after elective open thoracic aortic repair . Thoracic endovascular aortic repair (TEVAR) has shown potential to improve outcomes even further and extend treatment to patients with added comorbidities that might have not been considered candidates for conventional surgery.
Early diagnosis, adequate control of risk factors, and close follow-up with timely surgical intervention are essential to prevent development of life-threatening complications. Additional pathologies amenable to TEVAR include acute type B aortic dissection, traumatic transection, intramural hematoma, penetrating aortic ulcer, pseudoaneurysm, Kommerel’s diverticulum, and aortic coarctation.
Medical treatment of patients with thoracic aortic diseases
Patients who are not candidates for operative intervention, including those whose aneurysm does not meet the criteria for surgical intervention as well as those patients who are considered inoperable because of coexisting disease, should receive optimal medical management consisting of blood pressure control, lipid profile optimization, smoking cessation, and other atherosclerosis risk-reduction measures. Stringent control of risk factors may slow the rate of growth and preclude development of complications such as aneurysm rupture or dissection.
Lifestyle and diet modifications leading to weight loss and aerobic exercise are standard approaches to treat hypertension, but pharmacologic therapy is usually required for patients with thoracic aortic disease . It is reasonable to reduce blood pressure with beta-blockers and angiotensin-converting enzyme inhibitors or angiotensin receptor blockers to the lowest values patients can tolerate without adverse effects [13–16].
Treatment with a statin to achieve a target low-density lipoprotein (LDL) cholesterol of <100 mg/dL is reasonable, as several experimental studies have demonstrated a delayed development of atherosclerosis, but to date there is no definitive evidence that doing so slows aneurysm expansion [17, 18].
Consistent follow-up of medically treated patients every 6 months to 1 year with complimentary diagnostic imaging is recommended, with frequency dependent upon aneurysm size.
Indications for treatment of thoracic aortic aneurysms
The most frequent criteria used to recommend elective operation in asymptomatic patients with thoracic aortic aneurysm is based on the diameter of the aorta, based on observations that the risk of adverse events (rupture, dissection, or death) exceeds the risk of elective operation when the maximum diameter exceeds 5.5 cm [19–22].
Patients with Marfan syndrome or other connective tissue disorders present complications with smaller aortas, and surgery is indicated with diameters of 4 to 4.5 cm [23, 24]. To adjust for body habitus variation, the use of aortic diameter indexed to height has been reported to better indicate surgical timing than might be recommended from aortic diameter alone for an otherwise asymptomatic patient with Marfan syndrome or bicuspid aortic valve . Due to unproven durability in patients with connective tissue disorders, it is relatively contraindicated to use stentgrafts in patients with Marfan's syndrome or similar disorders.
Asymptomatic patients presenting expansion rates over 0.5 cm per year should also be considered for surgery .
Presence of symptoms determines indication for surgery, as they are associated with impingement of the aneurysm on adjacent structures. Chest or back pain is a predictor of aortic rupture .
Specific surgical procedures
- For patients presenting with aneurysms involving the descending aorta, endovascular stent grafting is quickly becoming the preferred choice of treatment (Table 1). For patients presenting with aneurysms of the ascending aorta, aortic root, and/or aortic arch, conventional open surgery with interposition grafting continues to be the only proven durable therapy, and there are no commercially available stent grafts for aortic aneurysm in these specific locations. Several instances of successful off-label device deployment in selected patients using modified techniques have been described. Experience is growing with the use of fenestrated or branched graft devices for patients with thoracoabdominal aneurysms, and the majority of these have been performed as part of a device exemption trial.Table 1
Endovascular treatment options for thoracic aortic disease in 2012
Aortic diseased segment
Abdominal and thoracic SG
Modified Zenith SG
Modified Relay SG
ASD closure device
Any thoracic SG + debranching
Thoracic as FET
Branched Cook Zenith
Branched Bolton Relay
Gore TAG & Gore CTAG
Approved in US
Approved in US
Approved in US
Cook D (dissections)
Any thoracic SG + debranching
Branched Cook Zenith
Descending thoracic endovascular aortic repair (TEVAR)
TEVAR is performed by deploying a tubular fabric and metallic device (stent graft) within the pathologic segment of aorta. Doing so conducts blood flow from the proximal to the distal normal segments of aorta, diverting pressure away from the diseased wall of that segment of aorta. This therapy has been shown to interrupt progression and reverse the natural history of the disease with subsequent shrinkage of the aneurysm sac around the interposed stent graft device. The main objectives of therapy are to decrease the risk of aortic-associated death due to rupture or ischemic complications.
A small groin incision is most frequently used to access the common femoral artery, and a series of maneuvers with catheters are performed to deliver increasingly stiffer wires into position across the pathologic segment of aorta. Once this delivery “rail” is in position, the aortic stent graft is delivered into position and deployed within the aorta. The entire procedure is currently performed under fluoroscopic guidance, and occasionally using adjunctive intravascular ultrasound (IVUS) or transesophageal echocardiography (TEE). These are usually performed under general anesthesia but may be safely performed using regional anesthesia.
Currently available devices come in delivery sheath sizes ranging from 20 to 25 Fr that require the iliofemoral arteries be greater than 7 to 8 mm in diameter without circumferential calcification or severe tortuosity. In up to 15% of patients, the femoral route of delivery is inadequate. In such situations, the device can be delivered via a retroperitoneal approach with the creation of a prosthetic graft conduit by directly sewing a vascular graft to the more proximal vascular tree. The need for creation of an access conduit does not affect mortality but does increase the risk of bleeding and prolongs recovery .
The most critical requirement for technically successful and durable TEVAR is assuring that the aortic landing zones for both the proximal and distal ends of the stent grafts are adequate. A proper landing zone for stent graft fixation and seal is a relatively straight segment of normal caliber aorta (18–42 mm) measuring at least 15 mm in length.
Additional considerations in planning TEVAR involve the length of aorta to be treated. Although it has been suggested that TEVAR may have a lower incidence of spinal cord ischemia than open repair, there is no evidence to support this claim. In a study comparing open and endovascular approaches to distal aortic repair, there was no difference in risk for spinal cord injury between the methods of repair when they were controlled for the overall length of aorta treated. The most important predictor of spinal cord injury in patients undergoing thoracic aortic repair is the amount of the collateral vascular network that is sacrificed in the repair. In patients requiring more extensive repair, cerebrospinal fluid drainage is typically utilized as an adjunctive measure to optimize spinal cord perfusion, and the mean arterial pressure is maintained above 80 mm Hg during the perioperative phase.
Left subclavian artery coverage
A significant proportion of patients with descending thoracic aortic disease have extension of the pathology into the distal aortic arch. Consequently, aortic coverage extends proximal to the left subclavian artery in up to 40% of patients undergoing TEVAR. The left subclavian artery is the primary artery to the left arm and a major source of blood flow to the anterior spinal artery. Brain and spinal cord perfusion is particularly relevant when collateral pathways are compromised, and 60% of patients have a dominant left vertebral artery . Covering the origin of the left subclavian artery with a stent graft has been associated with ischemic complications of the arm (6%), spinal cord (4%), vertebrobasilar circulation (2%), and anterior circulation stroke (5%) [30, 31]. Additional risk for coronary ischemia occurs in patients with previous cardiac surgery in whom bypass grafts were constructed with the left internal mammary artery.
The Society for Vascular Surgery has recommended preoperative left subclavian artery revascularization in all patients undergoing TEVAR requiring coverage of the left subclavian artery . This is usually performed by an extra-anatomic bypass with a prosthetic graft from the left common carotid artery to the left subclavian artery but may also be performed by transposing the left subclavian artery directly onto the left common carotid artery.
Patients undergoing TEVAR obtain immediate benefit from a less invasive surgical approach by avoiding the large thoracic incision required in an open repair. In a recent meta-analysis including 5888 patients undergoing either open or endovascular descending repair, TEVAR demonstrated reduced early mortality and reduced postoperative complications when compared to conventional surgery. Respectively, these outcomes included paraplegia (1.4% vs 4.9%), renal insufficiency (5.9% vs 15.7%), reoperation for bleeding (0.01% vs 6.5%), and pneumonia (15.9% vs 28.7%) . Postoperative recovery times are usually shorter after TEVAR in comparison with open descending thoracic aortic repair. Sustained benefits on survival have not been proven because only intermediate-term results for TEVAR have been described to date and many of these patients do require additional endovascular procedures [34–36].
TEVAR has also been proposed as an alternative treatment for patients presenting as an emergency with ruptured thoracic aneurysm. Significantly lower 30-day mortality and lower incidence of early postoperative complications have been demonstrated as compared to open surgery in these very high-risk patients. Despite improved early results, a considerable number of aneurysm-related deaths occurred during follow-up in this subgroup, so vigilant postoperative surveillance at 1 month and 6 months after emergency TEVAR is recommended .
Although the currently available versions of thoracic stent grafts have not been designed specifically for the treatment of aortic dissection, TEVAR has an expanding role in the treatment for this pathology. Particularly compelling data for patients presenting with acute complications, including visceral or distal malperfusion, have been recently published with mortality rates around 2% to 3%, significantly less than the mortality of open surgery for these patients, which has historically been in the range of 20% to 40% [38, 39]. For now, uncomplicated type B dissection patients are preferentially treated with medical therapy, as earlier stent grafting has not proven to be beneficial with regard to mortality after 1-year follow-up in the randomized controlled Investigation Of Stent Grafts in Patients with type B Aortic Dissection (INSTEAD) trial, despite improvements in aortic remodeling [40••]. Several reports have explored the high-risk features of aortic dissection that predispose to late complications or the need for intervention. These have included a persistently patent or partially thrombosed false lumen, maximum aortic diameter of greater than 4 cm, and false lumen diameter of greater than 22 mm [41•, 42, 43]. An increasing number of patients are surviving the acute phase after acute aortic dissection, putting them at late risk for aneurysmal degeneration secondary to chronic aortic dissection. Open repair remains the standard therapy for this late disease, but in high-risk patients without other treatment options, remodeling of the chronically dissected aorta with stent grafting may be a reasonable option [44••]. This is especially appropriate for patients in whom the diseased segment is limited to the descending aorta versus those with more extensive dissection extending into the abdominal aorta . Successful remodeling after stent grafting of aortic dissection is more predictable when performed in the acute phase than in the chronic phase due to false lumen filling from distal fenestrations. Although the role for TEVAR in aortic dissection is controversial, accumulating experience continues to provide further understanding of the potential for endovascular treatment of patients with this complex thoracic aortic disease.
Hybrid procedures for aneurysms with involvement of branched segments of the aorta
For patients presenting with disease extending proximally into the aortic arch or distally into the visceral segment, current commercially available stent grafts require implantation in combination with conventional surgical procedures to ensure the perfusion of the branch vessels (ie, hybrid procedures). Rapidly evolving investigational devices custom designed with fenestrations or branches to accommodate the branch vessels are available at a few centers and are described below in the section on evolving strategies.
Essentially, two types of hybrid procedures have been described: 1) the debranching approach, where origins of the branch vessels are re-routed to another position via bypass or transposition before that segment of aorta is covered with a stent graft device; and 2) the direct fixation approach, where the stent graft device is delivered and deployed into a good landing zone on one end and into a poor landing zone on the other end where it is directly sutured and fixed in place. These approaches have been safely performed in patients with both arch and thoracoabdominal disease.
Arch debranching and TEVAR
In patients with aneurysms involving the aortic arch who are not candidates for conventional open surgery, experience is accumulating with procedures that involve translocation of the supraaortic arteries from the aortic arch using bypasses, and covering the arch with an endovascular stent graft deployed into the distal ascending aorta, the entire arch, and a segment of the adjacent descending aorta [46, 47]. After revascularization of the aortic branch vessels is completed, the stent graft may either be delivered in the usual retrograde fashion from the iliofemoral arteries or in an antegrade fashion directly from one of the side branches off the ascending aorta . Debranching of the supra-aortic vessels can either be partial with cervical-only reconstruction or more complete with total debranching from the more proximal ascending aorta. A recent meta-analysis suggested that patients have better technical outcomes when a more complete debranching is performed . This technique requires that the patient have a normal ascending aorta for placement of the stent graft, but many patients with both arch and descending disease also have ascending aortic enlargement. An exception is the subgroup of patients who have undergone prior ascending replacement. Coronary bypass grafts originating from the ascending aorta may also prohibit the safe performance of this procedure, but a major advantage of this technique is that it does not require circulatory arrest or the use of cardiopulmonary bypass. Although direct comparisons with conventional repair have been limited, early results have been similar to historic controls .
Endovascular elephant trunk completion and the frozen elephant trunk technique
Similar to what has been described above for debranching of the aortic arch, debranching procedures have also been performed for patients with complex thoracoabdominal disease. The initial debranching portion of the operation is usually performed through a mid-line laparotomy with the iliac arteries serving as the new source of inflow to the visceral and renal vessels . Less commonly, the aorta, including the ascending aorta, may be used as the trunk from which the debranching occurs . Several reports have demonstrated the feasibility of this approach, but the open revascularization is still a very large operation with significant morbidity and mortality demonstrated [57, 58]. Although this approach is a reasonable choice for select high-risk patients, it is unlikely to replace conventional open surgery. Less invasive endovascular alternatives are currently under development and may hold future promise.
Evolving treatment strategies
Several approaches have been proposed to repair segments of the aorta that involve the origin of critical arterial branches at the level of the arch or the descending thoracoabdominal aorta. Although the hybrid procedures described above represent an initial solution following the basic concepts of vascular surgical reconstruction, alternative creative transcatheter alternatives have been proposed by several groups and applied more extensively to reconstruct visceral arteries.
Fenestrated and branched graft devices
Although the experience with endovascular repair of abdominal aortic aneurysms evolved quickly since the early 1990s, patients with inadequate proximal landing zones (ie, short infrarenal aortic necks) posed a serious technical challenge. Stent graft fenestrations were first suggested by Brown et al.  in the late 1990s as a method for extending the aortic seal zone into the renal and visceral segment. By creating stent grafts with openings to accommodate the branch vessels, the sealing and fixation of the stent graft device could be extended into normal aorta without compromising flow into end organs . These devices are now commercially available in Europe and Australia, with ongoing phase I multicenter clinical trials in the United States .
The devices are conceptually simple, with holes (fenestrations) that are reinforced by nitinol wires and customized in their location to match the geometric relationship of the target vessels. After the device is inserted, oriented, and the sheath is withdrawn, the graft material remains partially constrained using a tethering trigger wire providing the ability to fine-tune rotational and longitudinal device position. The aortic graft is then cannulated from an access sheath in the contralateral iliofemoral system and the branched vessels are cannulated through the fenestrations. Balloon expandable stents are then deployed to join the aortic component with the target branch vessel. Distal and proximal aortic components may then be added to complete the repair [61••].
Branched thoracoabdominal stent grafts
As experience was gained with fenestrated devices, the technology evolved into branches. By mating the fenestrations in the stent graft material with covered stents, a water tight seal could be created that allowed for more extensive thoracoabdominal aneurysm reconstruction with the multi-component branched stent graft system spanning across aneurysmal segments of the thoracoabdominal aorta and several aortic branches. As these devices have evolved over the past several years, they are now available with short side arms that may be directed axially (ie, at a right angle) or as a helical limbs (ie, at an oblique angle). One or more preloaded catheters and wires are introduced through sheaths in the contralateral common femoral or the axillary artery to access the side branches and cannulate the ostia of the target visceral vessels. The procedure is completed by deploying a self-expanding stent graft that connects the lumen of the aortic stent graft to the lumen of the visceral vessel. Several hundred of these procedures have now been performed with outstanding results in a high-risk population of patients with all variations of thoracoabdominal aneurysms, included select patients with chronic aortic dissection . Ongoing advances are focused on designing off-the-shelf versions of these devices with simplified delivery systems and to train more physicians in the practice of safe deployment and delivery. As these devices improve, they may become the predominant approach to treating patients with thoracoabdominal aortic aneurysms.
Branched aortic arch grafts
Applying lessons learned from the branched thoracoabdominal aneurysm repair experience to the aortic arch has led to the development of completely endovascular options for this segment of the aorta as well . The arch branched graft devices began with designs directed at maintaining patency of the left subclavian artery. Several companies have developed devices to accommodate a single arch branch. Cook Medical has a device that has been used in tens of patients worldwide with branches to both the brachiocephalic and left common carotid artery. These patients undergo a left common carotid to left subclavian artery bypass first. The main aortic component has two internal branches that are then mated to stent graft devices delivered from each axillary artery to complete the seal and deployment. One requirement for technical success of this approach is that the patient has a normal ascending aorta or previously placed ascending graft free of important coronary bypass grafts to provide an adequate landing zone for the stent graft device.
Chimneys, periscopes, and snorkels
Currently, the branched grafts for the arch or thoracoabdominal aorta are only available to a limited number of clinical centers involved in device trials. Frequently used off-label alternative procedures involve perfusion of one or multiple target branch arteries by deploying smaller caliber covered stents alongside larger aortic stent grafts within the treated segment. These approaches essentially create an internal bypass in the space between the larger thoracic stent graft and the native aortic wall. The safety and durability of these techniques have not been validated by any large volume or long-term experiences .
TEVAR for ascending aortic pathology
There are currently no commercially available stent grafts specifically designed for use in the ascending aorta. Nonetheless, several patients with focal disease, such as aortic pseudoaneurysm in the ascending aorta, have been treated with endovascular solutions both as part of an investigational device exemption and with the off-label use of commercially available devices (personal experience of the author). Anatomic limitations for the development of a standard ascending aortic stent graft include the lack of an adequate proximal seal zone in most ascending pathology due to the proximity of the coronary ostia and aortic valve, as well as lack of an adequate distal seal zone due to the origin of the arch vessels. With the advent of transcatheter aortic valve replacement (ie, TAVI or TAVR) for aortic stenosis, the prospect of combining this technology with conventional TEVAR devices is currently being explored for the treatment of ascending aortic disease in multiple laboratories.
Advances in imaging modalities have made all of the endovascular treatment options for aortic disease possible. TEVAR requires accurate preoperative imaging to determine the indication for surgery and to plan the conduct of the procedure.
Contrast-enhanced computed tomography (CT) scanning with a multi-slice scanner provides detailed information that can be displayed and analyzed in three dimensions. Software tools, such as centerline of flow processing, allow for detailed measurements of anatomic relationships (eg, of branches from the aorta) that are critical for the precise planning of complex endograft solutions.
CT analyses are also paramount for the postoperative follow-up of patients, as it provides accurate assessment images of the aorta’s response to treatment, position and integrity of the stent graft, and the presence of endoleaks .
One of the criticisms of endovascular therapy is the need for repeated CT and the cumulative radiation exposure during long-term follow-up. Dual-source CT scanning (DS-CT) represents one of the most recent developments in the field, featuring two X-ray tubes independently transmitting at different energy levels (ie, 80 kV and 140 kV) . This technology allows the differentiation of various tissue types during one gantry rotation. Current follow-up regimens often require a combination of non-enhanced CT, contrast-enhanced CT, and delayed phase CT. Post-processing of DS-CT contrast-enhanced images provides virtual non-enhanced CT images, which would reduce the radiation received by the patient, and might provide all necessary data for endoleak detection [64, 65].
Another development in the field of imaging is fusion imaging. For example, preoperative CT scan images can be synchronized with live fluoroscopy in the operating room, creating a three-dimensional roadmap that facilitates aortic navigation, improves procedural accuracy, and thus decreases radiation exposure. Other fusion imaging modalities under investigation combine fluoroscopy with ultrasound technology to further improve patient safety.
Dynamin contrast-enhanced magnetic resonance angiography (CE-MRA) using gadolinium as the intravenous contrast agent also provides accurate images of the aorta and its branches . MRA can provide dynamic information about the flow through an endoleak and details about aneurysm wall pulsatility that may have implications for determining the treatment approach to endoleaks [67–69].
The advances in the field of medical imaging will continue to facilitate the diagnosis and treatment of vascular pathologies, and physicians treating patients with aortic disease need to be facile with the use of specific features of different imaging modalities. Imaging plays a major role in all aspects of contemporary aortic disease management.
N.A. Brozzi: none; E.E. Roselli is a speaker for Vascutek, a speaker and co-investigator for Cook Medical, and consultant and co-investigator for Medtronic.