Sizing

Accurate measurements of the proximal and distal seal zones and access vessel diameter are of the utmost importance in planning TEVAR. Most endografts require oversizing by 10% to 20% of the seal zone diameter, depending on device parameters. Excessive oversizing can lead to folding, gutter formation, and accelerated aneurysm degeneration. Undersized grafts can lead to migration and endoleaks. Diameter measurements are best obtained in a plane orthogonal to the centerline of flow for accurate sizing of the endograft. Distal landing zone diameters are often smaller than the proximal seal zone, requiring the use of multiple grafts to correct the discrepancy. Use of tapered grafts can alleviate this problem if the length of the aneurysm can be properly bridged with one graft. Aortic coverage lengths can be difficult to approximate, however, and are usually underestimated by centerline measurements. The natural tortuosity of the aorta, coupled with the large cavity of an aneurysm and the tendency of the endograft to sit against the greater curvature, usually result in a longer trajectory for graft coverage. These factors should be taken into consideration during graft planning and deployment.

Access

The thoracic aortic diameter is typically larger than the abdominal aorta and requires larger devices and delivery systems. Many devices have a 24-French profile and require at least an 8-mm access channel for safe delivery and deployment. Particular attention should be given to the proximal external iliac artery; this area is prone to atherosclerotic narrowing and calcification, particularly in women. In such a setting, the common iliac artery can be accessed by a small retroperitoneal incision and the use of an iliac conduit. Conduits measuring 10 mm in diameter are preferred and can be used through the retroperitoneal incision or tunneled to the femoral region to avoid angulations in large individuals. Occasionally, this conduit can serve the function of a bypass at the end of the procedure in case of severe external iliac artery disease. On rare occasions, the infrarenal aorta can be accessed directly at the time of exploratory laparotomy. A pure endovascular approach consisting of aligning the iliac system with a graft limb, followed by aggressive balloon dilatation, has been described as a means to obtain satisfactory access. One disadvantage to this approach is the loss of patency of an internal iliac artery.

Landing Zones

The tortuosity and high shear force in the thoracic aorta mandate a longer seal zone than infrarenal aneurysm repair. It is recommended that at least 2 cm of seal zone be used at either end of the graft to prevent migration. These seal zones should be longer in areas of severe angulation, such as the apex of the arch, and may also have to be adjusted upward for thrombus or calcification in the wall. It is also important to recognize that the longer the aortic coverage, the greater the number of excluded intercostal arteries, which affects paraplegia rates.

The proximal seal locations have been classified into five zones (Figure 35-1). Each zone is divided by a tangential line along the distal side of each great vessel; zone 0 involves the origin of the innominate artery, zone 1 the origin of the left common carotid artery (LCCA), zone 2 the origin of the left subclavian artery (LSA), zone 3 the proximal descending thoracic aorta down to the T4 vertebral body, and zone 4 the remainder of descending thoracic aorta. The location of the proximal landing zone depends on the aneurysm morphology; however; the ideal location in terms of anatomic accommodation of the graft is in zones 3 and 4. An important consideration for the proximal seal zone is the angle of the aortic arch. Grafts that are placed within the aortic arch (zones 2 and 3), especially one with an acute angle, may fail to properly appose to the inner curvature of the arch (Figure 35-2). This configuration has occasionally resulted in untoward effects such as migration and collapse, mostly in nonaneurysmal applications. Recent endograft modifications such as the Zenith Pro-Form (Cook, Bloomington, Ind.) and the C-TAG (W.L. Gore, Flagstaff, Ariz.) have been designed to specifically address this problem.

FIGURE 35-1 Anatomic description of the proximal landing zones.

FIGURE 35-2 Angiogram showing “bird beaking” of the proximal endograft.

Management of the Left Subclavian Artery

In approximately 20% of cases, adequate coverage of the TAA can be achieved only by extending the graft into zone 2, effectively excluding the LSA from circulation⁹ (Figure 35-3). The management strategy of the LSA has evolved since the inception of TEVAR. Prophylactic revascularization of the LSA, either through a carotid subclavian bypass or a transposition, was used routinely in the first study of TEVAR. Intentional coverage without revascularization was later tolerated as a relatively safe practice especially in emergency settings and in the presence of good collateral circulation. Absolute contraindications include an occluded or atretic right vertebral artery or a left internal mammary bypass to the coronary circulation. A review of 22 patients who underwent intentional coverage of the LSA found that almost 70% of the patients remained asymptomatic, with the remaining 30% suffering only mild left arm claudication symptoms.

FIGURE 35-3 Predeployment angiogram (A), and postdeployment (B). Note the complete coverage of the left subclavian artery.

The practice of LSA coverage has come under severe scrutiny lately with EUROSTAR data linking it to an increased risk of spinal cord ischemia. Several reports of brain stem strokes and other complications have also surfaced. Peterson and colleagues¹¹ observed a 63% (5/8) stroke and upper extremity ischemic symptom rate in patients who had LSA coverage without revascularization. A review of a random segment of the literature disclosed a 23% complication rate with LSA coverage compared with a 3% rate when flow to the LSA was maintained.^11,14 The current Society for Vascular Surgery (SVS) guidelines recommend routine preoperative revascularization in elective TEVAR and expectant management in acute settings excluding absolute contraindications.¹⁵

In order to safely deploy a stent-graft at the level of the left subclavian artery, it is often helpful to gain access to the aortic arch through the left brachial. This allows visualization of the junction between the subclavian and the aorta easier and also allows for salvage stenting in the event of inadvertent coverage.

Management of the Celiac Axis

The ideal distal landing zone places at least 2 cm of graft proximal to the celiac axis. When the aneurysm is more extensive, coverage of the celiac artery is a possibility, allowing for a longer seal zone. Concerns for bowel ischemia and hepatic failure should prompt careful assessment of the collateral network, especially through the gastroduodenal and pancreaticoduodenal arcades. A recent review reported selective coverage of the celiac axis in 31 patients during TEVAR.¹⁶ Of these patients, two (6%) patients had bowel ischemia, and five (16%) had an endoleak that required reintervention. The findings illustrate the fact that celiac coverage is feasible in many cases, but potentially morbid in the absence of adequate collaterals, and should be undertaken with caution.

Gore TAG

The TAG graft (W.L. Gore, Flagstaff, Ariz.) was the first FDA-approved device for the treatment of DTAs (Figure 35-4). As such, it is the most common device implanted for both on- and off-label use in the United States. The graft is an expanded polytetrafluoroethylene (ePTFE) tubular graft reinforced with a layer of fluorinated ethylene propylene (FEP) material and external Nitinol self-expanding stents. The marketed TAG endograft has a lower porosity to help avoid the sac enlargement associated with the earlier version of the device.^17,18 The FEP wrap was also added as a replacement of the longitudinal deployment wire tested with the original device. The stent wire-forms are secured to the graft with a bonding tape made of ePTFE and FEP. Effective proximal and distal seals are achieved by flared edges of the graft as well as an additional ePTFE cuff. The graft has an inlaid gold band that marks the proximal and distal edges, although the flared ends protrude 7 to 9 mm beyond these edges.

FIGURE 35-4 Gore TAG device.

(Courtesy W.L. Gore & Associates.)

The TAG device is constrained in a sleeve of ePTFE/FEP film that is bound by a longitudinal seam running the entire length of the device. This seam is held together by an ePTFE line connected to a deployment knob located at the control end of the delivery catheter. The device is deployed by pulling on the string, which unzips the seam, thereby deploying the stent-graft. The device is available in diameters of 26 to 45 mm and lengths of 10, 15, or 20 cm. It is delivered through a 20- to 24-French sheath. The Gore TAG device is designed to be oversized by 7% to 22%, which is incorporated into the instructions for use (IFU) device sizing guide.

Advantages of the TAG device include a simple deployment mechanism and flexibility that allows easy accommodation of tortuous thoracic aortas. The endograft deploys from the middle of the graft out toward both ends. This design theoretically avoids the wind sock effect that can occur if the proximal end is released first. Once the graft is deployed, the seal zones are ballooned with a tri-lobed balloon that allows for continuous antegrade blood flow.

The new conformable TAG (C-TAG) device has been modified to increase conformability, especially to the lesser curvature of the arch by changing the wire form strength and pattern (Figure 35-5). The proximal covered scallops have also been replaced by short bare metal stents ranging in length from 3 to 6.5 mm. A 21-mm device has been added to the size range so that patients with smaller aortic diameters can be treated. Two tapered devices (26 to 21 mm and 31 to 26 mm) were also added, providing added flexibility in sizing. Other changes include the addition of a 46-mm device in 2010 and the introduction of a new pressurized hemostatic valve on its sheath to minimize blood loss.

FIGURE 35-5 The conformable Gore TAG thoracic endoprosthesis.

(Courtesy W.L. Gore & Associates.)

Medtronic Talent

The Talent device (Medtronic, Santa Rosa, Calif.) was approved for commercial use in the United States in June 2008. The device has been available worldwide since 1997 and was widely used outside the United States.

The graft is composed of a nitinol skeleton attached to a woven polyester graft. There is a bare metal component at the proximal end to allow for extended fixation while maintaining flow to the branch vessels. There are a number of tapered and nontapered configurations available as well as different configurations for the last stent (Figure 35-6). The support bar of the device is intended to be aligned along the greater curvature of the aorta, and can be identified by the radiopaque “8” that marks the bar. The Talent is available in a wide range of diameters, from 22 to 46 mm, but was initially limited to 112- to 116-mm lengths, which required many devices to be used to cover a long aortic segment, an average of 2.7 devices in the VALOR trial.^18,19 The device is usually oversized by 10% to 20% over the aortic landing zones and uses a 22- to 25-French delivery catheter.

FIGURE 35-6 Medtronic Talent device.

(From Moore WS: Endovascular surgery, ed 4, Philadelphia, 2011, Saunders.)

The talent device has undergone a series of modifications and improvements, including a much improved delivery catheter and a controlled deployment mechanism. In addition, graft lengths greater than 20 cm are available to overcome one major disadvantage of the earlier system.

The next generation of the device, the Valiant (Medtronic, Fridley, Minn), has finished all regulatory trials in the United States and is awaiting final approval. It uses the Xcelerant (Medtronic, Fridley, Minn) delivery system, which touts greater trackablity for the tortuous aorta because of a hydrophilic outer coating and shorter tip on a braided sheath. Changes to the implant were also introduced to improve flexibility and radial strength of the graft while reducing pressure points in the landing zones by increasing the number of shorter peaks on its uncovered stents from five to eight. The longitudinal support bar was removed, and longer devices were made (up to 227 mm).