CHAPTER 72 Endovascular Therapy for Thoracic Aortic Aneurysms and Dissections

Penetrating Atherosclerotic Ulcer with Intramural Hematoma

The surgical management of thoracic aortic aneurysms began in the 1950s with the introduction of cardiopulmonary bypass technology.¹^–⁴ Since then, advances in surgical technique and perioperative management have resulted in improved outcome in the management of thoracic aortic disease. Despite these advancements, the morbidity and mortality associated with open thoracic aortic repair remain significant. Furthermore, our population of patients has become increasingly older with more significant comorbidities. This has led to the development of thoracic endovascular aortic repair (TEVAR) in an effort to improve perioperative outcome and potential long-term survival.

Endovascular treatment of aortic disease was introduced in the early 1990s and has dramatically revolutionized the field of cardiovascular surgery. Since Parodi’s first description of an intraluminal stent graft device for the treatment of abdominal aortic aneurysms,⁵ stent graft device technology has been expanded to treat the multiple pathologic processes in the thoracic aorta. In 1994, Dake first reported the initial Stanford experience with 13 patients undergoing endovascular therapy for descending thoracic aortic aneurysms.⁶ Since then, indications involving off-label use have expanded to include the treatment of aortic dissections, traumatic transections, penetrating atherosclerotic ulcers, and intramural hematoma. Currently, there are three stent graft devices approved by the Food and Drug Administration (FDA) for the treatment of descending thoracic aortic aneurysms. However, concerns and questions remain about the appropriate timing, indications for intervention, and durability of this fast-evolving technology. Furthermore, morbidity and mortality compared with the standard open repair remain unclear. This chapter reviews this fast-growing field that has gained worldwide acceptance.

THORACIC AORTIC ANEURYSMS

Natural History and Indications for Intervention

Thoracic aortic aneurysms are abnormal dilations of the aorta characterized by elastin fragmentation and fibrosis, resulting in medial degeneration of the aortic wall.⁷ These age-related changes are likely to result in the reduction of aortic integrity and strength. As our population is ever increasing in age, the incidence of thoracic aortic aneurysms also appears to be increasing. In a Swedish study examining the national health care registry from 1987 to 2002, the incidence of thoracic aortic disease rose by 52% in men and by 28% in women, reaching 16.3 per 100,000 per year and 9.1 per 100,000 per year, respectively. In the same study, the annual incidence of operations performed on the thoracic aorta in men increased from 0.8 per 100,000 per year in 1987 to 5.6 per 100,000 per year in 2002 for an overall 7-fold increase; in women, the increase was 15-fold, from 0.2 per 100,000 per year in 1987 to 3.0 per 100,000 per year in 2002.⁸

The natural history of thoracoabdominal aortic aneurysms has been examined in large single-institution series. Characterized by slow growth over time, aortic aneurysmal degeneration is an indolent pathologic process of aortic dilation leading to potential rupture or dissection. The human aorta grows generally at a rate of about 0.07 cm per year in the ascending aorta and 0.19 cm per year in the descending thoracoabdominal aorta.⁹ If dissection is present, the thoracoabdominal aorta may grow at a slightly faster rate of 0.28 cm per year.^10,¹¹

The major risk in thoracic aortic aneurysms pertains to the catastrophic events of rupture and dissection. The risk of rupture or dissection as a function of maximum aortic diameter has been examined in multiple studies. Clouse and coworkers ¹² examined the Olmstead County database and demonstrated that in patients with a maximum aortic diameter between 4.0 and 5.9 cm, the risk of rupture was 16% during a period of 5 years. With a maximum aortic diameter greater than 6.0 cm, the risk of rupture exceeded 30% during the same period of 5 years. The Yale group has also identified “hinge points” of maximum aortic diameter that represent significant increases in the risk for rupture. In the ascending aorta, this hinge point appears to be at 6 cm, with a risk of rupture or dissection of 34% during the lifetime of the patients. In the descending thoracic aorta, the hinge point appears to be at 7 cm, with a 43% risk of rupture or dissection.¹⁰ The annual risk of rupture or dissection as a function of maximum aortic diameter has also been examined. Davies and coauthors⁹ demonstrated that in patients with a maximum aortic diameter of less than 6.0 cm, the yearly rate of rupture, dissection, or death is less than 8%. However, the annual risk of rupture, dissection, or death dramatically increases to 15.6% when the maximum aortic diameter is 6.0 cm or greater.

A general recommendation for surgical intervention has been made on the basis of these large institutional population studies. The decision to intervene surgically must be based on a balance between the benefit of surgery or intervention (in the case of TEVAR) and the risk of rupture or dissection with medical management. The consensus for conventional open repair is to intervene surgically at a diameter of 5.5 cm for the ascending aorta and a diameter of 6.5 cm for the descending thoracic aorta. Certainly, patients with significant family history of aortic disease or connective tissue disorder such as Marfan syndrome may warrant intervention at a lower threshold of aortic diameter. Furthermore, for patients with symptomatic aneurysmal disease, urgent surgical intervention is recommended regardless of size, for symptoms are an early indication of impending rupture.

In the era of TEVAR, the perceived lower rates of morbidity and mortality have urged the question, Should the threshold for intervention in the descending thoracic aorta be lowered? With a mortality of less than 5% with TEVAR in most centers of excellence,¹³^–¹⁶ most patients with a descending thoracic aortic diameter greater than 5.5 cm may be considered for endovascular repair (Fig. 72-1). However, the final decision to intervene must be based on the previously established surgical dictum: the benefit of surgery must outweigh the risk of rupture, regardless of approach.

Figure 72–1 Angiogram demonstrating endovascular treatment of thoracic aortic aneurysm.

Device Development

Since the early devices were first described by the Stanford group in 1994,⁶ TEVAR devices have undergone multiple modifications and clinical trials. Three thoracic endoprostheses are currently approved for the treatment of descending thoracic aortic aneurysms. Second- and third-generation devices as well as disease-specific devices are currently being investigated.

Gore TAG

The Gore TAG thoracic endoprosthesis (W. L. Gore & Associates, Flagstaff, AZ) was approved by the FDA in March 2005 for the treatment of descending thoracic aortic aneurysms. The first device to be approved, the Gore TAG is a self-expanding expanded polytetrafluoroethylene endograft composed of nitinol support. Concern about fracture of the longitudinal support wire led to the revision of the original Gore device to its current Gore TAG endoprosthesis. Unique to this device system is the introducer sheath, which enables the delivery of multiple devices and minimizes traumatic injury to the femoral access vessels. The introducer sheath ranges from an inner diameter of 20 Fr to 24 Fr depending on the size of the Gore TAG devices required for treatment. The Gore TAG endoprosthesis ranges from a minimal diameter of 26 mm to a maximum diameter of 40 mm, with lengths ranging from a minimum of 10 cm to a maximum of 20 cm (Fig. 72-2). The mechanism of deployment involves release of the endograft from a center to peripheral fashion. Thus, precise deployment of the proximal and distal landing zones may be challenging.

Figure 72–2 The Gore TAG thoracic endoprosthesis.

Medtronic Talent and Valiant Thoracic Endoprostheses

The Talent Device

Approved in June 2008 by the FDA for the treatment of descending thoracic aortic aneurysms, the Talent device is a preloaded stent graft incorporated into a CoilTrac delivery system. It is a stent graft composed of a polyester graft (Dacron) sewn to a self-expanding nitinol wire frame skeleton. Radiopaque markers are sewn to the graft material to aid in visualization during fluoroscopy. The CoilTrac delivery system is a sheathless, push-rod delivery system. Preloaded onto an inner catheter, the Talent device is deployed by pulling back an outer catheter, allowing the device to self-expand and to contour to the aorta. A balloon may be used to ensure proper apposition of the graft to the aneurysmal aorta after deployment.

The Talent device is designed as a modular system; 47 different configurations ranging from a diameter of 22 to 46 mm and cover lengths from 112 to 116 mm are available. To accommodate the size differences often found between the proximal and distal portions of the aorta in thoracic aneurysms, tapered grafts are available for better aneurysmal conformability and prevention of junctional endoleaks. Four configuration categories are available: proximal main, proximal extension, distal main, and distal extension (Fig. 72-3). The proximal configurations and the distal extension are offered with a bare spring design (Free-Flo design). The bare spring design allows placement of the device crossing the arch vessels proximally and the celiac artery distally for suprasubclavian and infraceliac fixation, respectively.

Figure 72–3 The Medtronic Talent thoracic endoprosthesis (Medtronic Vascular, Santa Clara, CA).

The Valiant Device

The Valiant device is designed on the basis of experience with the Talent device and is currently under investigation as part of the VALOR II trial. Similar to the Talent device, the Valiant device is also a preloaded stent graft made of the same polyester graft built onto a self-expanding nitinol skeleton. Modification has been made to improve trackability, conformability, and deployment of the Valiant device. First, device lengths have been increased to a maximum of 230 mm (130 mm for Talent). Because the device is a sheathless system, each piece requires an individual deployment through the access vessel, resulting in repeated exchange in the artery. Longer lengths have been designed to minimize device exchange during deployment. Second, the connecting bar has been removed in the Valiant device for improved conformability, especially in the arch. Third, the number of bare springs at the proximal and distal ends of the device has been increased from five to eight in the Valiant device to improve circumferential force distribution and fixation along the aorta wall. Finally, the Valiant device is introduced in a new delivery system, the Xcelerant delivery system. First available to physicians in the United States for the AneuRx AAA device, the Xcelerant delivery system has been modified for the Valiant device to provide a more comfortable deployment mechanism, especially in tortuous anatomy of the distal arch and thoracic aorta. As opposed to a simple pullback unsheathing mechanism, the deployment of the Xcelerant delivery system includes a gearing, ratchet-like mechanism in the handle to allow easy deployment. The amount of force required to deploy the device is reduced significantly without compromising the precision of the deployment.

Cook Zenith TX2 Thoracic Endoprosthesis

Approved in 2008 by the FDA, the Cook Zenith TX2 thoracic endoprosthesis (Cook Inc., Bloomington, IN) is designed as a modular system with a specific proximal and distal configuration (Fig. 72-4). The grafts consist of stainless steel Z-stents with full-thickness polyester fabric. Similar to the Medtronic delivery system, the Zenith TX2 system does not require a delivery sheath, and it is introduced as a preloaded catheter with triggers. The device sheath has a hydrophilic coating, and sizes range from 18 Fr to 22 Fr, depending on the diameter of the endoprosthesis. The diameter of the endoprosthesis ranges from 22 to 42 mm, and lengths range from 120 mm to 207 mm. The two components are designed to be deployed from a proximal to distal direction, and tapered devices are available.

Figure 72–4 The Cook Zenith TX2 thoracic endoprosthesis (Cook Inc., Bloomington, IN).

Deployment of the TX2 is achieved through a trigger system to ensure a controlled deployment. Flushed at the proximal end with barbs to prevent migration and endoleak, the device is deployed by an unsheathing mechanism. To minimize the “windsock” effect during deployment (thus distal migration), the proximal barb component of the device is not released until the graft is deployed and the trigger released. The distal component is deployed by a similar mechanism. However, in addition to barbs, the distal component also has bare springs at its caudal portion. Proximal and distal extensions are available if additional coverage is necessary.

Anatomic and Technical Considerations

Anatomic requirements and key technical considerations for successful TEVAR revolve around answering the question, What makes a patient a suitable anatomic candidate for a thoracic aortic stent graft? Initial assessment of a stent graft candidate begins with an extensive preoperative workup and evaluation. Key points of the history and physical examination include detailed neurologic and cardiovascular examinations. Distal vascular pulses and preoperative neurologic deficiencies must be documented. Previous abdominal, pelvic, and inguinal surgeries are noted because these procedures may demand an alternative access route.

Access

Safe vascular access for thoracic aortic device deployment is the key to thoracic aortic stent grafting. The majority of the morbidity and mortality is a direct result of arterial access complications.^13,^14,¹⁶ Extensive preoperative planning with appropriate imaging is mandatory. The “gold standard” for preoperative evaluation is computed tomographic (CT) angiography, which includes the thorax, abdomen, pelvis, and femoral arteries. Fine-cut helical CT scanning with a minimum of 3-mm slices is ideal (Fig. 72-5). In those patients who cannot undergo CT scanning with the administration of contrast material, magnetic resonance angiography is an acceptable substitution.

Figure 72–5 Computed tomographic angiogram of descending thoracic aortic aneurysm.

All of the thoracic endovascular devices are long to reach the descending aorta, of large caliber to contain the thoracic aortic endoluminal graft, and relatively stiff to allow “pushability” through the iliofemoral access points and through the abdominal aorta. The management of the delivery of the thoracic aortic stent graft is often the most challenging aspect of the case. Delivery systems of the three currently FDA-approved thoracic endoprostheses will require arterial access size of a minimum of 7.5 to 8.0 mm. Creation of a conduit to the femoral or iliac artery may be necessary to achieve adequate access. Not only the size of the arterial vessels but also the anatomy of the iliofemoral and abdominal aorta must be considered in planning the access route. Excessive tortuosity and atherosclerosis with occlusive disease may provide barriers to safe delivery of the endograft. In approximately 20% of patients, a retroperitoneal access to the common iliac arteries will be required because of issues of femoral or external iliac artery size and tortuosity.¹⁷

Careful review of preoperative studies will indicate which patients will have difficult access. Patients with atherosclerotic occlusive iliac disease may be treated with standard endovascular techniques of balloon angioplasty to reduce the obstruction. Iliac stents should be avoided because of the potential interference of these stents with the thoracic aortic access devices. These procedures on the access vessels should be carried out at least 6 weeks before thoracic aortic stent grafting to allow healing of the iliacs after angioplasty and manipulation. At the completion of the thoracic aortic endografting, iliac stents may be placed if appropriate.

Access to the retroperitoneum allows several options for safe device deployment. The common iliac artery may be used for device deployment. An open surgical conduit can be constructed to allow an end-to-end anastomosis or a side-to-side anastomosis. A 10-mm conduit of synthetic Dacron is commonly used and is of ample size for insertion of all necessary devices. The conduit may be brought through a separate counterincision in the groin for better angulation of the relatively long and stiff deployment devices. At the conclusion of the procedure, these conduits may be used to revascularize distal obstructions if needed.

Alternatively, the retroperitoneal iliac vessels or even the distal aorta may be accessed by direct sheath insertion. A double pursestring of 4-0 Tycron is used to secure the vessel and to provide hemostasis with the application of two sets of tourniquets. Direct needle puncture of the artery is followed by dilation and insertion of the device. At completion, the device is removed, and the pursestring sutures are tied down. Excessive tortuosity of the iliofemoral arteries requires adaptive strategies. External manual manipulation provides a simple method of straightening some of the tortuosity of the aorta and iliac arteries. During fluoroscopy, the operator hand can provide gentle force to the tortuous arterial segment to allow straightening and subsequent endovascular access.

In cases of iliac artery tortuosity, advanced endovascular techniques may aid in straightening these segments. The use of stiff wires or buddy wire techniques can provide some degree of straightening of the diseased arteries. In severe cases of tortuosity, brachiofemoral access may be required to perform “body flossing” with an appropriately stiff wire. Typically, a 5-Fr sheath with a long catheter is placed into the aortic arch and then into the descending aorta. A long stiff wire, such as a 450-cm SS guide wire (Boston Scientific), is guided from the brachial and retrieved through the femoral artery. Gentle traction on both the brachial and femoral sites will straighten out the tortuosity. Of note, a long catheter must be placed through the brachial artery into the aorta to prevent excessive trauma by the stiff wire along the arch and innominate vessels.

At the completion of the deployment and ballooning of the thoracic stent graft, the entire route of access must be carefully examined to ensure that there has been no injury. The stiff guide wire that was used to position the thoracic endograft should be left in place as the sheaths and remaining endovascular materials are removed. A smaller sheath should be reinserted, and diagnostic aortoiliac arteriography should be performed to evaluate for thrombus, dissection, or complete avulsion.

The removal of the large sheath, especially when it is inserted with some force and manipulation, is a particularly dangerous period when injury may occur. There have been numerous reports of successful thoracic endografting with a sheath removed with a complete iliac artery avulsed and attached (Fig. 72-6). At the time of recognition, a stiff wire through this injured artery may be lifesaving and allow insertion of an occluding balloon proximally for control of a potentially life-threatening bleed. In addition, both the blood pressure and heart rate should be carefully monitored during removal and completion of the endovascular procedure for signs of an occult injury.

Figure 72–6 Iliac artery avulsion after removal of endovascular delivery system.

Landing Zones

The proximal aorta is divided into landing zones as illustrated in Figure 72-7. Unless revascularization is performed, proximal landing in zone 0 and zone 1 is unacceptable because of the necessary occlusion of the left common carotid artery in zone 1 and the innominate artery in zone 0. Proximal landing in zone 2 is commonly used with either partial or total occlusion of the left subclavian artery. Zone 3 landing is dependent on the exact anatomic neck at the arch. Proximal landing in zone 3 can lead to angulation of the graft, which provides inadequate sealing of the proximal graft along the lesser arch and “bird beaking” or “stove piping” graft placement, resulting in a high incidence of type II endoleaks. Zone 4 landing is usually straightforward because of the lack of angulation and distance from the arch vessels.

Figure 72–7 Classification of landing zones in thoracic aortic endovascular repair.

(From Mitchell RS. First International Summit on Thoracic Aortic Endografting. J Endovasc Ther 2002;9:II98-105.)

The proximal and distal landing zones must be of an appropriate diameter to allow stent grafting with available devices. In general, devices should be oversized between 15% and 20% above the diameter of the landing zone, depending on the presenting aortic disease. As discussed, the proximal landing zone of the aorta must be carefully examined on preoperative imaging workup. The goal is to create a good “seal” of 15 to 20 mm between the graft and the aortic wall on a disease-free, nontapered, nonangulated portion of the aorta. There should be adequate length of the proximal landing zone, minimal angulation, minimal tortuosity, and minimal calcification. Angulation of the aortic arch is acceptable if the inner radius is greater than 35 mm and the outer radius is greater than 70 mm. These parameters allow adequate conformation of the aortic stent graft to the arch.

Because zone 2 is often the site of best proximal landing to avoid excessive angulation and tortuosity, the management of the left subclavian artery requires preoperative planning. Some of the possible complications of covering the left subclavian artery include vertebrobasilar artery insufficiency or stroke, left arm ischemia, and ischemia of the heart in patients with a previous left internal mammary artery to left anterior descending coronary artery bypass graft. An evaluation of the right vertebral artery and the adequacy of the circle of Willis is crucial in planning to cover the left subclavian artery. In those patients with inadequate collaterals through the circle of Willis, stenotic right vertebral artery, or dominant left vertebral artery, strong consideration should be given to bypass of the left subclavian artery before covering. One option is to transpose the left subclavian artery to the left common carotid artery with oversewing of the proximal left subclavian artery. A second option is to bypass from the left common carotid artery to the left subclavian artery. Either surgical ligation of the proximal left subclavian at the time of bypass or staged coil embolization of the proximal left subclavian artery at the time of graft delivery may be used. Bypass of the left subclavian may be preferable because it avoids any mediastinal dissection, and there is no interruption of antegrade flow to the vertebral artery or internal mammary artery branch vessels.¹⁸

Evaluation of the distal landing zone also requires careful preoperative workup. The distal landing zone should again have 15 to 20 mm of normal aorta with minimal calcification, angulation, and tapering. The celiac artery is the first distal branch vessel that must be avoided. Therefore, the length of the aortic graft must be correctly planned. Enough graft length should cover the aortic pathologic process while avoiding excessive coverage of the descending thoracic aorta. The goal is to preserve the distal vertebral artery branches and perfusion to the spinal cord.

Results

Multicenter Clinical Trials

The Gore TAG endoprosthesis was approved as a result of the phase II U.S. multicenter trial comparing the Gore TAG endoprosthesis with an open surgical control group in the treatment of descending thoracic aortic aneurysms.¹³ Between September 1999 and May 2001, 140 patients with descending thoracic aortic aneurysms were enrolled from 17 sites in the United States. All patients were required to have adequate landing zones of at least 2 cm in length of nonaneurysmal aorta distal to the left carotid and proximal to the celiac axis. At the same centers, the open surgical control cohort enrolled 94 patients; 44 patients were concurrent controls and 50 were historically and retrospectively acquired by selection of the most recent surgical patients in reverse chronologic order.

In the endograft group, statistically significant improvement was seen in the 30-day mortality, incidence of postoperative respiratory failure, renal failure, spinal cord ischemia, mean intensive care unit stay, and hospital stay. Thirty-day mortality was 2.1% and 11.7% in the Gore TAG group and the open surgical control group, respectively. Spinal cord ischemia was 2.9% in the Gore TAG group versus 13.8% in the control group. However, peripheral vascular complications were significantly higher in the endograft group (14% versus 4% in the control group).¹³

The mean duration of follow-up was 25.8 months in the endograft group and 24.9 months in the open control group. Kaplan-Meier 2-year survival was similar between the two groups (78% in the endograft group versus 76% in the control group). The incidence of endoleak at 1-year and 2-year follow-up was 6% (6 of 103) and 9% (7 of 80), respectively. There were no cases of aneurysm rupture in either group. At 2-year follow-up, 45% of the endograft group had aneurysmal regression of 5 mm or more, 42% had no change, and 13% had aneurysmal progression of 5 mm or more.¹³ At 5-year follow-up, all-cause mortality remained similar between the two groups (68%, endograft group; 67%, control). Aneurysm-related mortality was lower in the endograft group (2.8%) compared with the control group (11.7%).¹⁹

The Vascular Talent Thoracic Stent Graft System for the Treatment of Thoracic Aortic Aneurysms (VALOR) trial led to the approval of the Talent endoprosthesis for the treatment of thoracic aortic aneurysms. The Medtronic VALOR trial was a prospective, multicenter, nonrandomized, observational trial evaluating the use of the Medtronic Talent thoracic stent graft system in the treatment of thoracic aortic disease. The study was conducted from 2003 to 2005 and involved 38 sites. A total of 195 patients were enrolled, with 189 patients identified as retrospective open surgical controls. In the Talent group, the 30-day mortality was 2.1%, with an incidence of 1.5% of paraplegia and 3.6% of stroke. One-year mortality was 16.1%, with aneurysm-related mortality of 3.1%. Compared with the surgical control group, the Talent arm demonstrated statistically superior outcome in perioperative mortality (2% versus 8%; P < .01), 30-day major adverse events (41% versus 84.4%; P < .001), and 12-month aneurysm-related mortality (3.1% versus 11.6%; P < .002).

Published in 2008, the international controlled clinical trial of the TX2 device is a nonrandomized, controlled, multicenter, international trial comparing the treatment of thoracic aortic aneurysm with the TX2 endoprosthesis versus open repair. Enrollment began in March 2004 and was completed by July 2006. Involving 42 institutions, a total of 160 patients were enrolled in the endovascular group and 70 patients in the open group. The 30-day survival rate was noninferior for the TX2 group compared with the control group (98.1% versus 94.3%). The 30-day cumulative major morbidity scores were significantly lower in the TX2 group than in the control group (1.3 versus 2.9). Although not statistically significant, the incidence of neurologic complications trended in favor of endovascular repair. The incidence of stroke in the TX2 group was 2.5% at 30 days compared with 8.6% in the control group. The incidence of paraplegia in the TX2 group was 1.3% versus 5.7% in the control group. At 12 months of follow-up, aneurysmal growth was seen in 7.1%, endoleak in 3.9%, and device migration in 2.8%. One-year survival estimated from all-cause mortality was similar between the TX2 group (91.6%) and the control group (85.5%). The 1-year survival estimate from aneurysm-related mortality was also similar between the TX2 group (94.2%) and the open surgical control group (88.2%).¹⁶

< div class='tao-gold-member'>

Only gold members can continue reading. Log In or Register to continue