Endovascular Repair of the Aortic Arch and Thoracoabdominal Aorta

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