The Management of Thoracoabdominal Aortic Aneurysms by Branched Endograft Technology

This chapter specifically describes the management of aortic aneurysms that involve both the thoracic and abdominal segments of the aorta and its associated visceral branches. The reader should note that extensive aortic pathologies commonly require solutions incorporating combinations of open surgery and fenestrated and branched endovascular solutions. Since the publication of the first edition of this book, the total endovascular repair of thoracoabdominal aortic aneurysms (TAAAs) has reached a state of technical maturity. Nevertheless, although several single-center reports describe the excellent short-term efficacy of the technique and materials in suitable patients and anatomies, there is still little, if any, level 1 evidence to support the general use of the solutions that we describe here.

Although most patients who have infrarenal aortoiliac aneurysms can be relatively safely treated, the prognosis for patients who have large (>6 cm diameter) aortic aneurysms that involve the origins of the abdominal visceral vessels is less certain. Untreated, survival is only 17% at 5 years and the annual combined rupture/dissection/mortality rate is 14%.

Although the first successful open thoracoabdominal aortic aneurysm repair was reported more than 60 years ago, the risks of treatment remain considerable. In the preendovascular era, Crawford showed the relationship between the risks of operative mortality and of paraplegia and aneurysm extent, leading to the Crawford classification. This classification (now slightly modified ) still informs the technical approach to and assessment of the risks posed by TAAA repair. Even in the “least risk” subgroup (Crawford type IV TAAA), the physiologic demands of open surgical repair are likely to be beyond the reserve of many, and survivors face lengthy recuperations. Endovascular infrarenal abdominal aortic aneurysm (AAA) repair (EVAR) confers significantly lower 30-day mortality than anatomically equivalent open surgery. Similarly, endovascular repair of isolated thoracic aortic aneurysms (TEVAR) is associated with a lower risk of death than its conventional equivalent. Because the first generations of devices available for endovascular aneurysm repair were simple tubes or bifurcated grafts, initial attempts to extend the benefits of EVAR to patients with TAAA led to “hybrid” solutions. These involved laparotomy, extraanatomic bypass from iliac arteries to the visceral vessels to preserve blood flow to the vital organs, and then extended EVAR/TEVAR to exclude the aneurysmal aortic segment. Although the initial reports were greeted with enthusiasm, good results have not been universal and the approach does not exploit all of the potential (physiologic and recovery) advantages promised by total endovascular solutions. This deficit, together with rapid technologic advances, has driven the development of complex endovascular solutions for the challenging TAAA patient group, and the “hybrid” approach is now rarely used.

Total endovascular repair of a true aortic aneurysm using a branched device was first described in 2001.

Philosophy

The ultimate therapeutic goal in the endovascular treatment of TAAA is the same as that for all forms of EVAR: exclusion of the aneurysm wall from arterial blood pressure (to eliminate the risk of aortic rupture) while preserving distal perfusion. In the particular case of the endovascular management of TAAA, there is the additional specific requirement to preserve visceral perfusion and function. This is achieved by the provision of custom-made branches for extension into the visceral vessel ostia—branched endovascular aneurysm repair (BEVAR).

In common with all varieties of EVAR, the BEVAR technique relies on suitable proximal and distal aortic or iliac sealing zones. In common with most, it requires the intravascular assembly of several overlapping parts, each of which must seal with adjacent components. In common with the use of fenestrated devices used to proximalize the proximal aortic sealing zone in juxtarenal AAA (FEVAR), it requires cannulation of target visceral vessels and the placement of covered bridging stents that seal in the main device branch and each targeted visceral artery.

There is, however, an important philosophical and practical difference between FEVAR and BEVAR. In the case of FEVAR, much of the seal is provided by apposition of the main device against the wall of the (relatively normal) visceral-bearing aorta, with secondary sealing between the (balloon-expandable) extension stent and the device (by internal flaring) and then between the extension stent and the target vessel. In the case of BEVAR, there is no reliance on visceral vessel level apposition between the main device and the (aneurysmal) visceral vessel-bearing aortic wall. In this case, the branches are an integral part of the main device, with seal being required within the branch and between the extension stent and the target vessel. To emphasize the point, FEVAR is a technique used in the provision of a sealing zone for aneurysms where the main graft is in apposition to the aortic wall at visceral artery level; this is generally used in proximalization of the neck in AAA but equally may provide the distal seal zone in TAAA. In contrast, BEVAR is used specifically in the management of aneurysms where the visceral bearing segment is dilated and unsuitable for use as a sealing zone: TAAAs. These distinctions are not absolute and many aneurysms can be managed by either BEVAR or FEVAR, as evidenced by the approaches of either Chuter (BEVAR only) or Bicknell (FEVAR only). Furthermore, some aneurysm anatomies require combined FEVAR/BEVAR solutions. In the interests of clarity (and in keeping with the chapter title), this chapter concentrates on “pure” BEVAR solutions, but the principles are extendable to more complex problems.

Special Considerations

Although the physiologic demands that BEVAR imposes are less formidable than conventional open TAAA repair, these are not inconsiderable, particularly because the patient group is generally older and even less physiologically resilient than patients presenting with infrarenal AAAs.

BEVAR is a technique that requires careful device design, operative planning, and substantial skill from the operating, anesthetic, and perioperative teams. Deployment takes longer to complete than EVAR or TEVAR and imposes greater contrast and radiation burdens. It also requires additional arterial access (particularly subclavian/axillary/brachial) and may have greater potential for type III endoleaks because of the large number of overlapping components.

Success demands a team approach and high case volume to acquire and maintain all of the skills, categorized as patient and aneurysm selection/turn down, analysis of aneurysm anatomy and device design, surgical planning and execution, and close follow-up with timely reintervention where necessary.

Anatomic analysis and device design mandates high-quality arterial phase contrast-enhanced computed tomography imaging and access to software capable of 3D image manipulation. This permits vessel center line-based measurement of the true diameters of the aortic and target vessel landing zones and also of the relative true longitudinal and rotational distances between target vessels. In designing devices, the following absolute minimum sealing zones should be planned: 20 mm graft/artery (aorta, iliac, or visceral vessel), 40 mm (3 stent overlap) between central components, and full branch length between device branches and bridging stents.

Anatomic Considerations

Successful deployment and aneurysm exclusion require adequate proximal and distal aortic and target vessel sealing zones (length and straightness). For durable patency, the target vessels must be 5 mm in diameter or larger. The device design limitations concerning target vessel origin proximity encountered with FEVAR are less of a problem with BEVAR. Good torque, manipulation, and passage of relatively long covered stents demand techniques (and anatomy) that achieve reasonably straight routes from access vessels to target vessel ostia. Aortic tortuosity at the visceral bearing segment may occasionally preclude use of currently available devices.

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