Historical Background
The first multibranched stent grafts, like the first bifurcated stent grafts, were of unibody design, whereby the entire stent graft was inserted whole and deployed using a system of catheters. Downstream access to the branches made the arch version slightly simpler than the thoracoabdominal version, but both showed a high degree of irreducible complexity that has limited application to a small group of highly skilled users in Japan.
Modular multibranched stent grafts, combining a primary stent graft with one or more covered stents, vary mainly in the type of intercomponent connection. The first modular systems for thoracoabdominal aortic aneurysm (TAAA) and aortic arch aneurysm (ArAA) repair used longitudinally oriented cuffs, like the attachments sites on a typical bifurcated modular stent graft. The first modular system for ArAA repair was simply an upside-down bifurcated stent graft, with one long leg extending back along the line of insertion into the innominate artery and the other opening into the aorta as an attachment site for a descending thoracic aortic extension.
The first hybrid operations for TAAA and ArAA were reported more than a decade ago, and these techniques have probably been used more widely than other methods of endovascular repair. The snorkel technique was first described as a way to treat juxtarenal aneurysm. The expanded use of this technique for ArAA and TAAA is a relatively recent phenomenon.
Indications
The most common indication for endovascular repair is the presence of a large asymptomatic aortic aneurysm. Although data on the natural history of TAAA and ArAA are not as robust as those for abdominal aortic aneurysm, it is clear that the risk of rupture depends mainly on aneurysm size; in addition, female sex, a positive family history, symptoms related to the aneurysm, and the presence of chronic obstructive pulmonary disease have been demonstrated to increase rupture risk in natural history studies. At the University of California at San Francisco (UCSF) procedures of this type are generally performed under research protocols, which require a minimum aneurysm diameter of 60 mm. Factors that increase the diameter threshold include serious comorbid conditions that raise the risks of surgery or shorten life expectancy, thereby reducing the benefit of freedom from rupture. Factors that reduce the diameter threshold include female gender, pseudoaneurysm, saccular aneurysm, and symptoms or signs of imminent rupture.
The long-term effects of aortic dissection include false lumen dilatation and aneurysm formation, yet dissection remains an uncommon indication for endovascular repair of the aortic arch and thoracoabdominal aorta for the following reasons: the true lumen is often narrow, and origins of the aortic branches may be separated by the septum between the true and the false lumens, especially within the dissected thoracoabdominal aorta. These factors do not usually impede hybrid repairs, which use bypass grafts to circumvent complex luminal anatomy. Often patients with aneurysms of chronic dissection etiology are younger, afflicted with syndromic conditions, or both such that open repair remains the mainstay of treatment.
Patient Selection
Patient selection depends on an assessment of the relative risks of observation, open surgical repair, and various endovascular alternatives. The lack of good long-term data on outcome relegates endovascular repair to a subsidiary role in the management of ArAA and TAAA. Endovascular repair offers a last resort for patients whose large aneurysms preclude observation and whose poor physical condition precludes open surgery. Most candidates for endovascular repair of ArAA or TAAA have already undergone a full assessment of their fitness for operation, including tests of cardiac, pulmonary, and renal function. These patients usually present with well-documented indications for treatment in the form of aneurysm diameter measurements. The feasibility of endovascular repair depends on general anatomic factors, such as the state of the implantation sites, the proximity of aortic branches, the diameter of the iliac arteries, and the presence of mural thrombus. Other site- and device-specific anatomic factors relate to the potential pitfalls of a particular technique. For example, transcarotid insertion of an ascending aortic bifurcated stent graft requires a large right carotid artery, and the multibranched repair of a TAAA requires a luminal diameter of at least 20 mm at the level of the visceral arteries.
Preoperative Preparation
- •
Cardiopulmonary function. Cardiac risk stratification for TEVAR procedures in general is neither evidence-nor consensus guidelines-based; applying the same paradigm used in open repair appears illogical. Recent placement of a coronary stent, especially a drug-eluting stent, requires antiplatelet therapy such as clopidogrel to be maintained throughout the perioperative period. The risk of intraoperative hemorrhage is lower for a completely endovascular technique, but a hybrid repair may be contraindicated.
- •
Preoperative imaging. Preoperative imaging provides the anatomic data needed for patient selection, operative planning, and stent-graft sizing. Moreover, preoperative imaging is necessary to identify areas of stenosis or branching that might limit access to the aorta and its branches. In general, the more complicated the stent graft, the greater the need for precise anatomic data. This is particularly true of fenestrated stent grafts, because the distribution of fenestrations has to match the distribution of aortic branches. Modular branched stent grafts are more forgiving. Imaging techniques include computed tomography (CT), magnetic resonance imaging or magnetic resonance angiography, catheter angiography, and intravascular ultrasound (IVUS).
- •
Contrast-enhanced spiral CT imaging. High-resolution three-dimensional (3D) data sets yield orthogonal reconstructions for diameter measurements, multiplanar reconstructions for length measurements, and 3D representations such as shaded surface displays for assessments of angulation, profile, and relative position. Generic 3D reconstructions seldom provide the necessary level of anatomic detail. Image processing software such as that provided by TeraRecon (iNtuition) and OsiriX (OsiriX MD) allow the operator to make measurements and identify potential pitfalls by processing raw digital imaging and communications in medical files. Alternatively, services such as M2S can provide preprocessed data in an accessible form, together with the software for analysis and display.
- •
Magnetic resonance imaging. Magnetic resonance imaging yields a volumetric data set suitable for 3D analysis but lacks spatial resolution. The quality of magnetic resonance angiography is enhanced by the intravenous administration of gadolinium. However, rare, but serious side effects of gadolinium have almost eliminated its role in patients with poor renal function.
- •
Angiography. Catheter angiography is reserved for the evaluation and possible preoperative treatment of specific CT findings, such as renal artery or celiac stenosis.
- •
IVUS. Although IVUS is a potentially useful intraoperative adjunct in the presence of aortic dissection, it has no preoperative role. IVUS can provide accurate measurements of implantation site diameter, but so can CT angiography. IVUS-derived length measurements are unreliable.
- •
- •
Hybrid surgical and endovascular repairs. The physiologic stress of a hybrid repair may be reduced by staging the open surgical and endovascular procedures or by performing interventions to avoid the need for surgical intervention.
- •
Preoperative intervention of a branch artery stenosis. The goal of preoperative intervention is to create a wide, metal lined, radiopaque arterial orifice that lies flush to the surface of the artery.
Endovascular Strategy
Anatomic Considerations
ArAAs and TAAAs are more difficult to treat than aneurysms of the descending thoracic and infrarenal abdominal aorta. The surgeon cannot simply exclude an aneurysm from the circulation when its branches supply organs such as the brain or the abdominal viscera, which cannot tolerate ischemia.
Although four basic methods of branch preservation are used in all cases involving endovascular repair of ArAA, TAAA, and common iliac aneurysm, each branched segment has specific anatomic features, which affect the choice of one endovascular technique over another. The aortic arch is wide, curved, close to the aortic valve, and far from the femoral arteries. Its branches are accessible in the root of the neck. The thoracoabdominal aorta is narrower, straighter, farther from the aortic valve, and closer to the femoral arteries. Its branches stay within the abdomen. The common iliac artery has only two branches: one remains within the pelvis, whereas the other passes over the rim of the pelvis into the groin.
Basic Techniques for Branch Artery Preservation
Hybrid repair involves surgical bypass from a remote artery to each branch of the aneurysmal segment. The “debranched” aneurysm can then be treated using standard endovascular techniques. Alternatively, if the aneurysm involves only part of the branched aortic segment, the bypass may originate from one of the branches and terminate on another. The most common example of this approach is a left carotid-subclavian bypass before TEVAR repair (see Chapter 20 ), which is a modest “surgical component” to the hybrid procedure. At the other end of the spectrum would be a four-vessel renal or visceral debranching procedure to permit endovascular graft repair of a thoracoabdominal aorta. Debranching is a major surgical procedure.
The following types of stent grafts are used:
- •
A fenestrated stent graft has strategically located holes in its wall (see Chapter 27 ). The goal of using a fenestrated stent graft is to perfuse vital arterial branches without perfusing the aneurysm (type III endoleak).
- •
A branched stent graft has small side branches, each of which conveys blood from the lumen of the stent graft to the lumen of the corresponding arterial branch. A fenestrated stent graft can be converted into a branched stent graft by substituting a covered stent for the usual uncovered bridging stent.
- •
A snorkel, or chimney, stent runs alongside the stent graft from the nondilated aorta into a branch artery. The aortic stent graft seldom conforms perfectly to the outer surface of the stent, leaving channels through which blood can flow past the target artery into the aneurysm (type I endoleak). Multiple snorkels create multiple channels and multiple opportunities for endoleak.
Choice of Technique
There are too few published data regarding the results of endovascular ArAA and TAAA repair to support definite statements about the relative merits of hybrid repair, fenestrated stent grafts, branched stent grafts, and snorkels. Nevertheless, the following generalizations are probably valid: The surgical portion of a hybrid repair is familiar and well tested, and the endovascular portion is relatively simple and predictable. Snorkels and fenestrations both rely upon the creation of a hemostatic seal between the wall of the stent graft and the wall of the aorta. They tend to leak when the branches originate from a dilated segment of the aorta. In the creation of a branch, the intercomponent connection between the margin of a fenestration and a balloon-expanded covered stent is less secure, less hemostatic, and less forgiving than the connection between the lumen of a cuff and a self-expanding covered stent. The axially oriented branches of a cuffed stent graft are able to vary in length and orientation to accommodate variations in branch distribution. As a result, off-the-shelf cuffed stent grafts can be combined with other stent grafts and covered stents for multibranched endovascular repair of a symptomatic TAAA.
Device characteristics are often less important in the choice of technique than the availability of high-level endovascular skills and complex endovascular technology. The stent grafts used for hybrid and snorkel techniques are simple and widely available. Thousands of fenestrated stent grafts have been implanted by hundreds of surgeons. The technique may be complicated, but it is at least familiar. Simple fenestrations can be converted to branches with nothing more than the substitution of a covered stent for an uncovered stent. Few centers worldwide have used more than a dozen cuffed branched stent grafts. Unfamiliarity with the necessary techniques is one barrier to more widespread use; high cost is another. The multibranched stent graft is assembled from many components, each of which is sold separately. Fenestrated and cuffed stent grafts are now commercially available throughout the world with several notable exceptions, including the United States and Japan.
Modular Branched Repair of Aneurysms of the Aortic Arch
The following description focuses on modular multibranched techniques of ArAA repair because both snorkel technique and hybrid repair use standard endovascular devices and standard techniques, all of which are adequately described in other chapters. Only modular multibranched stent grafts have the potential to replace open surgery as the mainstay of treatment. Hybrid repair involves too much surgery, and snorkels and fenestrations involve too great a risk of endoleak. Small fenestrations require precise alignment, which is difficult to achieve in the curves of the aortic arch. Stent-graft misalignment can occlude cerebral flow, causing a stroke. Large fenestrations and scallops require less precise placement with a lower risk of stroke, but are more likely to be associated with an increased risk of endoleak. Unibody multibranched stent grafts are too complicated for widespread use.
Modular, multibranched stent grafts fall into two groups, transcervical and transfemoral, based on the route of insertion. Both require the tip of the delivery system to cross the aortic valve. The transcervical route is shorter and straighter. The delivery system does not have to navigate the aortic arch, and stent-graft orientation is easier to control. The basic form of transcervical stent graft, with only one supraaortic branch, requires little aortic instrumentation once the stent graft is in place, because blood flows to the innominate artery through a limb of the primary (bifurcated) stent graft. However, insertion of a large-caliber delivery system often requires access to the innominate artery. The necessary exposure negates some advantages of endovascular repair.
The transfemoral route is long and curved, and the delivery system tends to enter the aortic arch in one orientation and exit in another. In addition, the large-caliber delivery system with a short soft tip tracks poorly around the tight bends of the aneurysmal aortic arch. These barriers to transfemoral delivery require low-profile stents and fabrics, kink-resistant sheaths, precurved sheaths, and twist-prevention mechanisms, all of which have taken years to develop. Extensive in vitro testing and short-term results of early clinical experience suggest that transfemoral grafts have great promise.
Modular Transcervical Bifurcated Stent Graft
Device Design
The bifurcated stent graft ( Fig. 19-1 ) is an upside-down version of the type of long- and short-leg devices used to treat abdominal aortic aneurysm. The long, thin leg trails back along the route of insertion into one of the supraaortic trunks, usually the innominate artery ( Fig. 19-2 , A ). Carotid-carotid and carotid-subclavian bypass grafts distribute blood to the left carotid and left subclavian arteries ( Fig. 19-2 , B ). The short, wide leg opens into the aorta to form an attachment site for an extension to the descending thoracic aorta.
The delivery system is a short (30-40 cm), wide (22-24 Fr) version of the TX2 delivery system (Cook Medical, Bloomington, Ind.). Trigger wires attach the trunk of the stent graft and the long limb to the central portion of the delivery system.
Implantation Procedure
A bifurcated stent graft is inserted into the aortic arch as follows:
- 1.
A pacing catheter is inserted into the right ventricle through a femoral or subclavian vein. Ideally, capture rates in excess of 200 beats per minute allow the heart to be paced to standstill. Alternatively, the pacing catheter provides a means of restoring cardiac rhythm after adenosine-induced arrest. Either way, the catheter’s performance should be tested at the time of insertion.
- 2.
The patient is placed in the supine position on a radiolucent operating table. A left brachial access site is included in the surgical field and covered before tucking the arms at the patient’s sides to improve supraclavicular exposure of the supraaortic trunks. A draped table to the right of the patient’s neck serves as an angiographic runway.
- 3.
Left carotid-subclavian bypass and carotid-carotid bypass are performed in the standard fashion. The wounds can be left open or closed loosely with a few staples. Definitive closure is performed at the end of the operation.
- 4.
If the right carotid artery is too small to admit the primary delivery system, a conduit to the distal innominate artery is needed. Transcervical exposure of the innominate artery is sometimes difficult. A little traction on the carotid and subclavian arteries helps. Access is not through the end of the conduit but through a puncture site on the side of the conduit.
- 5.
Heparin (100 units per kilogram of body weight) is given intravenously and supplemented throughout the rest of the operation to keep the activated clotting time (ACT) above 300 seconds.
- 6.
One pigtail catheter is inserted through a sheath in the surgically exposed common femoral artery, advanced up to the proximal ascending aorta, connected to the contrast injector, and flushed carefully. Another pigtail catheter is advanced through the right carotid (or the innominate conduit) and spun through the aortic valve into the left ventricle, where it is exchanged for a double-curved Lunderquist (Cook Medical) wire. The intersection of the pigtail catheter and the Lunderquist wire marks the proximal margin of the innominate artery.
- 7.
The primary delivery system is inspected under fluoroscopy before insertion to check the orientation of the stent graft and identify all markers. The delivery system is inserted over the Lunderquist wire into the left ventricle. Only one radiopaque marker matters, the one at the distal end of the short leg of the bifurcated stent graft. This should be proximal to the innominate orifice and oriented toward the lesser curvature of the aortic arch. At this point in the procedure blood flows through the carotid-carotid bypass from left to right.
- 8.
Angiograms are performed through the femoral pigtail to confirm the position of the innominate artery.
- 9.
During a brief period of adenosine-induced asystole (or pacer-induced ventricular standstill), the stent graft is deployed by withdrawing the sheath to the innominate bifurcation ( Fig. 19-3 , A ), the proximal attachment is released by removing the trigger wire, and the tip of the delivery system is removed from the ventricle by loosening the pin vise and withdrawing the central cannula.
