An ideal access channel should have a lumen of at least 8 mm in diameter to allow larger devices to be introduced when needed. The smallest diameter is typically found in the proximal external iliac artery, an area that is also more prone to calcified plaque, which can restrict delivery of the device. This is a particular problem in women, whose iliofemoral arteries tend to be smaller.¹⁰ When the narrowest segment of the iliac artery falls below the expected minimum diameter required for the planned device insertion, access is generally shifted proximally to the common iliac artery by use of an iliac conduit. Rarely, the entire iliac system is hostile to delivery of the device on both sides, and consideration should then be given to access through the distal aorta if the risks associated with thoracotomy are too significant.

The Aortic Channel

The abdominal aorta rarely presents an anatomic hindrance to delivery of the device, but tortuosity of the thoracic aorta can be challenging in the presence of aneurysmal disease because it is frequently associated with elongation and exaggeration of the normal curvatures of the aorta in the chest. As illustrated in Figure 136-5, this is particularly relevant in the case of a large descending thoracic aorta when cephalocaudad elongation causes exaggeration of the posteroanterior course of the aorta as it enters the aortic hiatus. This tortuosity, when it is combined with increased angulation at the transition from the arch to the proximal descending aorta, may result in difficulty in tracking because of bending of the delivery sheath at flexion points as it negotiates severely angulated regions.

Adequate fixation of a stent-graft to both proximal and distal landing zones is essential for successful performance of TEVAR. To standardize comparisons, reporting standards for TEVAR were published by the Society for Vascular Surgery, and 11 landing zones have been defined for the management of thoracic aortic disease (Fig. 136-6).¹¹ This standardization aids in determining the risk and complexity of the procedure in relation to the site of fixation and length of aortic coverage. Several publications have shown that the frequency of neurologic events correlates with location of the proximal landing zone.^12,13 The original proposal had four attachment zones for the ascending aorta, arch, and proximal descending aorta. The updated and extended reporting system precisely defines the proximal and distal attachment zones, which in turn aids in the determination of device selection, operative risk, and complexity of the procedure. The proximal attachment zones are based on the location of the proximal edge of the TEVAR device in relation to branch vessels:

Atherosclerotic aneurysms typically are not limited to one focal area, and the degenerative process usually involves the apparently normal adjacent aorta. This is important in selection of landing zone length. Although 2 to 3 cm is considered adequate for various devices, a longer seal zone may be preferable, especially at angulated areas to decrease the occurrence of late endoleaks and to protect from late aneurysmal degeneration. This urge to cover more aorta, however, should be tempered by the increased risk for spinal cord ischemia with excessive coverage. If it is anatomically possible, the landing zone should ideally be parallel in configuration and distal to the left SCA (zones 3 and 4) and proximal to the celiac artery (Fig. 136-7). This results in the best outcomes. A sealing zone in a curved portion of the aorta, generally near the left SCA (zones 2 and 3), presents added complexity. The longitudinal stiffness of the devices often limits complete apposition to the inner curve. The minimum sealing length in those regions should apply to the shorter inner curve rather than to the longer outer curve. The proximity of the sealing zones to major branches should be carefully evaluated to avoid occlusion of a necessary artery. Alternatively, a bypass could be planned to maintain patency of the branch vessel or a chimney/snorkel technique used to preserve in-line branch vessel flow. The decision whether to revascularize the left SCA is discussed under Management of the Left Subclavian Artery.

Preoperative planning begins with imaging studies to delineate the morphologic features of the aneurysm, its relationship to adjacent arch and mesenteric vessels, and the quality of access vessels. The primary modality for such planning is computed tomographic angiography (CTA), which has mostly supplanted the use of catheter angiography in preoperative planning and is capable of providing all preoperative anatomic information. Although fine reconstructions (<2.5 mm) from older-generation scanners are acceptable, the 64-slice computed tomography (CT) scanner is the latest technology and provides high definition and very fast scan times. It also offers three-dimensional volume rendering as well as maximum intensity projections and sagittal and coronal reconstructions, which aid in the analysis of angles and relationships to adjacent vessels. Software such as M2S (M2S, West Lebanon, NH), Vitrea (Vital Images, Minnetonka, Minn), and Aquarius (TeraRecon, San Mateo, Calif) offers additional tools, such as reconstructions orthogonal to the line of flow and virtual grafts to aid in measurements and procedure planning. When patients are unable to undergo CTA because of life-threatening allergies to contrast material or renal insufficiency, gated magnetic resonance angiography (MRA) can be substituted for additional information and is usually added to a CT scan without contrast enhancement.

Accurate measurements of the diameter of the access vessels and the sealing zones are important to determine the feasibility of the procedure and appropriate graft sizes, which are typically oversized by 6% to 36%, depending on manufacturers’ recommendations. These recommendations should be followed closely because they form the range in which fatigue testing of the stent-grafts was performed in the preclinical assessment phase. Too small a stent-graft can result in an endoleak or migration, whereas excessive oversizing may cause infolding, poor sealing because of gutter formation and excessive radial force with accelerated degeneration of the neck, and possibly earlier material failure. Diameters are best measured on reconstructions orthogonal to the line of flow, especially in areas of tortuosity and angulation, to avoid erroneous measurements. If reconstructions are not available, the short axis is a reasonable estimate of the true diameter.¹⁴ Neck diameters between the proximal and distal sealing zones can be different, which may lead to inappropriate oversizing on the smaller end. Strategies for overcoming this problem include tapered grafts and shorter devices deploying a larger device inside the smaller one to prevent a type III endoleak.

Appropriate length measurements are also necessary to estimate the required device lengths. This is difficult in the thoracic aorta because the aorta moves posterolaterally in its proximal portion and anteromedially near the diaphragm. The presence of a DTAA exacerbates this native tortuosity and adds complexity in that the aorta and aneurysm have both inside and outside measurements that can be different from each other. The best estimate of length is a centerline flow measurement from a three-dimensional reconstruction, although this will commonly underestimate the length of endografts needed because the graft will tend to gravitate toward the greater curvature, especially when a large cavity exists in the aneurysm sac.

Selection of Access

Planning for access clearly depends on device size and access arteries. With TEVAR requiring larger device sizes and with DTAA being more prevalent than infrarenal AAA in women,¹⁵ an iliac conduit is often needed, in up to 15% of patients, and is better done prophylactically than after an iliac artery injury.¹⁶ Creation of an internal endoconduit has been described to address unfavorable iliac artery anatomy during TEVAR.¹⁷ This technique uses the controlled deployment of a stent-graft across a stenotic iliac artery, followed by balloon angioplasty and controlled rupture of the diseased segment. In turn, this facilitates safe passage of the TEVAR device. Technical advancements of introducer sheaths have also aided in the management of challenging iliac access. The Terumo SoloPath Balloon Expandable Transfemoral Access Sheath (Terumo Medical Corporation, Somerset, NJ) is a larger size hydrophilic introducer sheath (14F-21F) with an external noncompliant radially expanding balloon.¹⁸ This device eliminates the need for tapered dilators and facilitates delivery though difficult anatomy, and the balloon passively collapses before the removal of the introducer sheath, which may reduce the risk of iliac artery avulsion.

Identification of Landing Zones

The lack of adequate landing zones both proximally and distally is encountered frequently. Extension into zone 2 to achieve adequate sealing proximally was required at least 20% of the time in some regulatory trials.¹⁶ The decision to cover the left SCA or to bypass it has undergone a substantial paradigm shift over the years. Although all early cases underwent a left carotid–subclavian bypass or transposition, later reports of safe coverage resulted in indiscriminate coverage of the left SCA.^17–21 Recently, however, renewed interest in preserving flow in the left SCA developed after reports of brainstem stroke, increased risk for spinal cord ischemia, and severe arm ischemia.^22,23 It has been shown that a dominant left vertebral artery is found in more than 60% of patients, which coincides with an absent or atretic right vertebral artery. Intuitively, this finding places the patient at a higher risk for a posterior circulation infarct. Intentional covering of the left SCA without revascularization has been shown in two studies to significantly increase the overall risk of stroke and specifically the risk of posterior circulation stroke (5.5% vs. 1.2%).^24,25

Management of the Left Subclavian Artery

The left SCA is an important vessel for perfusion of both the spinal cord and the brain through the left vertebral artery.²⁶ It also provides blood flow to the spinal cord through the internal mammary and its anterior intercostal branches.²⁷ When it is feasible, it should be preserved during TEVAR when a stent-graft is deployed at or near the left SCA. Placement of a transbrachial left SCA access wire in these situations can be helpful. It can serve as a guide for safe and precise deployment of the device at the level of the left SCA and as a rescue procedure to restore flow should inadvertent coverage of the vessel occur.²⁸ It can also be used for proximal SCA embolization at completion of the procedure to prevent a type II endoleak after intentional coverage of the vessel.

Thoracic aneurysms encroaching on the left CCA are not uncommon and can be technically challenging to manage. If coverage of the left carotid artery is needed, several options are available to maintain carotid perfusion. The most commonly performed procedure is an extra-anatomic carotid-carotid bypass. Right common carotid exposure is obtained through an oblique right-sided neck incision. A similar exposure is obtained on the contralateral side. A ringed 6-mm PTFE graft is tunneled retroesophageally to connect the two carotid arteries. If it is too large, the retroesophageal graft may compress the esophagus and cause dysphagia. These bypasses have been found to be a safe and durable (3-year primary patency rate of 88%) procedure.³⁷ In select situations when inflow from the right side (CCA or SCA) is not adequate and the patient is found to be at too high a risk for open arch reconstruction, sequential extra-anatomic bypasses are an option. This would involve performing a series of bypasses from the left common femoral to the left axillary artery and from the left SCA to the left CCA with prosthetic conduits. The length of the multiple bypasses does create concern in trying to define patency rates.³⁸ Endovascular debranching techniques for the left CCA have been described.³⁹ Exposure of the left CCA allows retrograde access into the aortic arch and the creation of a left carotid–SCA bypass if needed. TEVAR deployment from the femoral access and retrograde covered stent placement into the arch are then performed; this has been described as the chimney technique.⁴⁰ The covered stent is then deployed at the same level as the TEVAR stent graft. Early results have shown this technique to be an acceptable option for high-risk patients.

Thoracic aneurysms encroaching on the innominate artery are the most challenging to manage. Hybrid endovascular repair options are well described. Involvement of zones 0 and 1 are often best managed with an open arch debranching originating from the ascending aorta (Fig. 136-9A). Through a median sternotomy, a bifurcated prosthetic graft is sewn to the ascending aorta, the innominate artery, and the left CCA. After the debranching, a TEVAR stent graft can be advanced to the origin of the conduit and deployed.⁴¹ Hybrid options for Zone 0 and 1 aortic arch debranching are shown in Fig. 136-9 B-D. Andersen proposed an algorithmic approach to the management of aortic arch disease. This technique was found to be successful but did carry a higher than expected rate of type A dissection and 30-day mortality (14.9%).⁴²

If the distal landing zone is inadequate, the operator faces options similar to those with the left SCA. When adequate collateral circulation between the celiac axis and SMA, especially through the gastroduodenal artery, is ascertained by preoperative angiography, the celiac axis may be covered safely to extend the distal landing zone during TEVAR.⁴³ Criado et al¹⁰ observed no mesenteric ischemic complications with three cases of celiac axis coverage. Vaddineni et al⁴⁴ reported seven such cases without any mesenteric or spinal ischemic complications. If the collateral network is not adequate, a bypass to the celiac circulation must be provided before occlusion. At the 2008 annual meeting of the Society for Vascular Surgery, Darling et al⁴⁵ presented their series of 21 patients with celiac axis coverage during TEVAR. They observed one fatal hepatic ischemic complication in a patient who had no demonstrable SMA-celiac collaterals, but no late complications were noted. They concluded that celiac coverage can be performed safely in the presence of SMA-celiac collaterals. The periscope graft is an acceptable option in situations in which celiac coverage is necessary and open revascularization is not (Fig. 136-10). This technique has proved useful as a means of preserving or rescuing aortic branch vessels in stent-graft seal zones. A self-expanding covered stent is inserted into the celiac artery from a femoral access. The TEVAR stent graft is deployed in the standard manner. Reinforcement of the self-expanding stent may be necessary because of the steep downward angle created by the covered stent and TEVAR.⁴⁰

As reviewed later, TEVAR is not without risk for spinal ischemic complications, and there is consensus that patients with long-segment descending thoracic aortic coverage and antecedent or concomitant abdominal aortic grafting are at particular risk. In such circumstances, adjuncts are often used, albeit without a firm evidence base. Such techniques have been evaluated mostly during open repair of TAAA. For a more detailed discussion, the reader is referred to Chapter 135.

One of the most commonly used adjuncts with TEVAR is drainage of cerebrospinal fluid (CSF). Although some use it routinely, most centers use it selectively in patients with an elevated risk for paraplegia, such as extensive coverage of the thoracic aorta in association with prior aortoiliac intervention. The spinal drain is typically inserted preoperatively and maintained for 48 to 72 hours while CSF pressure is kept below 10 cm H₂O. The advantage of selective drain use with TEVAR relates to an obligatory intensive care unit admission, but it is also desirable for arterial line monitoring and maintenance of relative hypertension. The incidence of hemorrhagic complications from catheter placement is also not negligible, and such complications are reported in 0% to 3% of patients.⁴⁶ Regardless of whether it is used preoperatively, CSF drainage has been shown to be effective in reversal of delayed-onset paraplegia after TEVAR and should be used whenever spinal cord ischemia is suspected.⁴⁷

Besides the anatomic features noted earlier, hemodynamic stability seems to play a critical role in spinal cord protection because the underlying pathologic process appears to be poor collateral flow and decreased perfusion of the spinal cord. A low intraoperative mean arterial blood pressure (≤70 mm Hg) has been linked to spinal cord ischemia in one review.⁴⁷ Except for the short time required for endograft deployment, when relatively low blood pressure may be beneficial, an elevated pressure is preferred intraoperatively. Similar to the open DTAA repair paradigm, postoperative hypotension has been noted to precipitate spinal cord ischemia events in patients after TEVAR.

Of pharmacologic adjuncts, naloxone has been studied the most. Intravenous naloxone has been shown to be effective in decreasing circulating levels of excitotoxic amino acids and antagonizing opiate-mediated spinal cord vasoconstriction after open TAAA repair. Its beneficial effects have been observed in open procedures,^48,49 and it may be used in selected cases of TEVAR.

The importance of the artery of Adamkiewicz, which is usually located between T8 and L2, has been debated extensively but has not clearly been linked to spinal cord ischemia. The EUROSTAR review found an association between T10 coverage and spinal cord ischemia; 40% of patients with spinal cord ischemia exhibited T10 occlusion, and only 18% of this 40% did not suffer periprocedural paraplegia or paraparesis.²³ The association, however, was no longer present on multivariate regression analysis. Accordingly, no recommendation can yet be made to avoid covering the T10 region with a stent-graft; the variable origin of this vessel along the length of the thoracic aorta and its multiple contributing collateral vessels form the essence of this position.