Management of Thoracoabdominal Aneurysms by Branched Endograft Technology

Mark R. Tyrrell, C. Jason Wilkins and Stéphan Haulon

Introduction

This chapter is concerned specifically with the management of aortic aneurysms that involve both the thoracic and abdominal segments of the aorta and its associated visceral branches. The aortic arch, aortic dissection, isolated thoracic, infrarenal, juxtarenal and suprarenal aneurysms are dealt with elsewhere in this book. The reader should note that extensive aortic pathologies commonly require solutions incorporating combinations of open surgery and fenestrated and branched endovascular solutions. It should be recognized that the total endovascular repair of thoracoabdominal aortic aneurysms (TAAAs) is currently a technique and technology in evolution. There is little if any level 1 evidence to support the general use of the solutions that we will discuss.

The majority of patients who have infrarenal aortoiliac aneurysms can now be relatively safely treated, but the prognosis for patients who have large (>6 cm diameter) aortic aneurysms that involve the origins of the abdominal visceral vessels remains grave. The untreated natural history is only 17% survival at 5 years,¹ and the annual combined rupture/dissection/mortality rate is 14%.²

Although the first successful open repair was reported 56 years ago,³ the risks of treatment remain considerable. In the pre-endovascular era, Crawford’s considerable experience showed that operative mortality risk and the risk of paraplegia relates to aneurysm extent, leading to the Crawford classification.⁴ This classification (now slightly modified)⁵ still forms the foundation underpinning the technical approach and risk assessment of TAAA repair. Even in the “best risk” subgroup (Crawford type IV TAAA), the physiologic demands of open surgical repair place considerable stresses on patients that are likely to be beyond the reserve of many, and the recovery time is long in survivors. Endovascular infrarenal aortic aneurysm (AAA) repair results in significantly lower 30-day mortality than anatomically equivalent open surgery.^6–8 Similarly, endovascular repair of isolated thoracic aortic aneurysms (TEVAR) is associated with a lower risk of death than its conventional equivalent.^9–11 Since the first generations of devices available for endovascular aneurysm repair (EVAR) were relatively simple tubes or bifurcated grafts, initial attempts to extend the benefits of EVAR to patients with TAAA led to “hybrid” solutions. These involve laparotomy, extra-anatomic bypass to the visceral vessels, and then extended EVAR to exclude the aneurysmal segment while preserving blood flow to the vital organs. Although initial reports were greeted with enthusiasm,^12–14 good results have not been universal,^15,16 and the approach does not exploit all of the potential advantages of a “pure” endovascular approach (e.g., lesser surgical insult, better physiologic control, rapid recovery). This unmet need, together with rapid technologic advances, has encouraged the development of more ambitious endovascular solutions to extend the principles and potential benefits of EVAR to the challenging TAAA patient group.

Total endovascular repair of a true aortic aneurysm using a branched device was first described in 2001.¹⁷

Philosophy

The ultimate therapeutic goal in the treatment of TAAA is the same as that in the endovascular treatment of any aortic aneurysm: exclusion of the aneurysm wall from arterial blood pressure, thereby eliminating the risk of aortic rupture and exsanguination, while preserving distal perfusion. In the special case of endovascular management of TAAA, there is the additional requirement to preserve organ blood flow and function. The latter 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 technique relies on adequate proximal and distal aortic or iliac sealing zones. In common with most, it requires 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 extension from the main device into each of these using covered bridging stents.

Again, there is a requirement for hemostatic seal between the extension stent and the main device and also between the extension stent and target vessel.

There is, however, an important philosophic 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 between the extension stent and the target vessel. In the case of BEVAR, there is no 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 (and their origins are therefore manufactured as hemostatic), with seal being required within the branch and into the target vessel.

To emphasize the point, FEVAR is a technique used in the proximalization of sealing zones for AAA with short aortic “necks”; BEVAR is used specifically in the management of aneurysms where the visceral bearing segment is dilated and unsuitable for use as a sealing zone—the thoracoabdominal aortic aneurysms (TAAAs). In many situations, instances could be applied as evidenced by the approaches of either Chuter (BEVAR only)¹⁸ or Bicknell (FEVAR only).¹⁹

In practice, some aneurysm anatomies require combined FEVAR/BEVAR solutions. In the interests of clarity (and in keeping with the chapter title), this chapter will concentrate on “pure” BEVAR solutions, but the principles are extendable to more complex problems.

Special Considerations

It should be borne in mind that BEVAR requires considerably more complex device design and planning than conventional EVAR. Deployment is more technically demanding and takes longer to complete (a greater contrast and radiation burden). It also requires different arterial access (particularly subclavian/axillary/brachial) and has more potential for type III endoleak (because of the large number of overlapping components).

In short, this is a technique that places unusual demands on both the operating/anesthetic team and the patient. It is hoped that these demands will prove less arduous than conventional open TAAA repair, but it remains the case that the risks are appreciable. This is all the more acutely felt because, in most authors’ experience, the patient group is even less physiologically resilient (and often older) than patients presenting with AAA in general, and the “turn-down rate” is consequently high.

It is self-evident that patient, aneurysm anatomy, and surgical team selection are paramount.

Device design requires high-quality, arterial phase contrast-enhanced computed tomography (CE-CT) imaging and access to software capable of three-dimensional (3D) image manipulation. This permits measurement of the true diameters of the aortic and target vessel landing zones and of the relative true longitudinal and rotational distances between target vessels based on calculated center lines. In designing devices, the following 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 branches and bridging stents.

Anatomic Considerations

Successful deployment and aneurysm exclusion require adequate proximal and distal aortic and target vessel sealing zones. Each of these has to be of sufficient length and straightness. For durable patency, the target vessels have to be of a minimum diameter (5 mm). Target vessel cannulation may be problematic, but the limitations concerning target vessel origin proximity are less of a problem than with FEVAR. Good torque and manipulation and passage of relatively long covered stents demand techniques (and anatomy) to achieve reasonably straight routes from access vessels to target vessel ostia. Aortic tortuosity at the visceral bearing segment may preclude use of currently available devices.

Technique

Anatomic considerations are most easily explained in the context of an understanding of the steps involved in the deployment of a BEVAR device (the use of mixed FEVAR/BEVAR devices requires modifications to the steps delineated below):

1. Before anesthesia, it is important to check the device against the printed plan and description.

2. The chosen access vessels are cannulated by surgical cutdown and percutaneous access as required. In practice, the most common routes are surgical exposure of one femoral artery (the side associated with the larger diameter and least tortuosity), percutaneous puncture of the contralateral femoral artery, and open exposure of the left distal subclavian, axillary, or brachial.

3. Full anticoagulation is established using sodium heparin. This is monitored, corrected, and maintained until arterial closure is complete (target activated clotting time 300 seconds).

4. If necessary, proximal (tube) extension devices are passed via the iliac system and abdominal aorta and are deployed to seal at the planned proximal landing zone.

5. The main body is prepared, oriented correctly (using the various check markers and other radiopaque markers integral to the device) (Fig. 49-1, A), and passed to the planned proximal sealing zone (aorta, proximal TEVAR extension device, or previously placed elephant trunk, according to the case), reoriented if necessary, deployed, and released from the delivery system, which is then withdrawn. We have found the use of a centimeter-marked pigtail catheter is useful in positioning the device accurately—the objective being to “land” with the branch ends 15 to 20 mm superior to (or inferior to in the case of upward-pointing branches) the target vessel.