Parallel Grafts in Perspective: Definitions and a New Classification

The success of endovascular repair is predicated on the ability of the stent-graft device to exclude the target aortic segment. Achieving optimal landing at the proximal and distal necks and secure circumferential fixation are the most crucial aspects. In addition to aortic morphology and neck diameters, the presence of important branches within the target segment is a major potential impediment that was recognized as such at the inception of endograft therapy more than 20 years ago. The chimney graft techniques have evolved in the effort to preserve one or more such vessels and, at the same time, optimize or lengthen the landing zones. Revascularization and rescue of an unintentionally covered branch vessel is another important function for chimneys which, in fact, emerged initially in the setting of such occurrences.

Developments with branched and fenestrated endografts unfolded during the same timeframe. But they have so far failed to become ubiquitous because of persistent issues related to availability, procedural complexity, and cost. Chimneys, on the other hand, are deserving of the off-the-shelf label often used to describe them because of the wide availability of the necessary techniques and equipment.

Definitions

“Parallel graft” (PG) has emerged as an all-encompassing term that best captures the one feature all branch-preserving conduits share, and it can be applied to the various technical iterations so far described, such as chimneys, snorkels, periscopes, and sandwich grafts. In all cases the graft or stent is implanted within the target branch vessel and follows a path within the aorta that is parallel to the stent-graft device. It creates a flow channel that runs between the stent-graft and the aortic wall, thereby preserving or restoring branch-vessel blood flow. Conceptually, it could be said the chimney graft internalizes the branch vessel origin within the aortic lumen, lending support to the term “endobranching” that has also been used to describe the technique.

Historical Evolution

Greenberg was first to use a chimney stent to revascularize a renal artery in 2001 in the setting of an EVAR procedure performed for treatment of an abdominal aortic aneurysm featuring a difficult proximal neck.¹ Interestingly, the term “snorkel” was actually proposed before “chimney” by Robert Rutherford, then editor of the Journal of Vascular Surgery, during his prepublication review of the 2003 paper by Greenberg et al (Dr. R. K. Greenberg, personal communication, June 2010).²

Early in 2003, Criado placed a bare-metal stent in the left common carotid artery to reestablish antegrade flow after unintentional stent-graft coverage in the course of a TEVAR procedure for treatment of a distal aortic arch aneurysm.² Larzon performed an almost identical procedure in 2004 to lengthen the proximal fixation zone during TEVAR, although the procedure included intentional carotid artery coverage.³

In 2007, Criado reasoned that “longer chimneys” traversing a lengthier course inside the aorta would be feasible and likely successful.⁴ This suggestion set the stage for subsequent developments, including the Lachat-Mayer periscope technique⁵ first performed by Mayer and Hechelhammer in 2008 to revascularize the visceral arteries during stent-graft repair of a ruptured TAAA (Dr. M. Lachat, personal communication, November 2012), and Lobato’s sandwich graft procedures first done in 2008 for revascularization of the hypogastric artery⁶ and in 2009 for the visceral and renal arteries.⁷ In 2010, Kasirajan proposed a creative new approach (for treatment of TAAA) consisting of the use of two inverted abdominal bifurcated stent-grafts in the descending thoracic aorta with subsequent antegrade deployment of additional multiple long conduits extending from the iliac limbs into the visceral arteries.⁸ A technical modification of this idea by Galvagni et al in 2011 obviated the need to use two abdominal grafts.⁹

The term “chimney” has endured¹⁰ but it can no longer adequately describe the ever-growing constellation of related techniques and procedures.  Table 1 summarizes the historical evolution of all PGs to date.

Classifying Parallel Grafts

The parallel nature of PGs relates to the course and position of the conduit in relation to the intra-aortic stent-graft, but it is not a reflection of their mutual relationship when multiple. Given the current proliferation of technical modifications and new iterations emerging over the past few years, it is the authors’ view that proposal of a simple but clear classification should prove helpful for description, documentation, and reporting purposes. 

The new classification recognizes 2 basic PG types depending on where the proximal (inflow-taking) end of the conduit resides, whether in the lumen of the native aorta or in a stent-graft device (Table 2):

1. Type I PG conduits go from the native aorta to a branch vessel, coursing along a short path (type Ia) or one that is several centimeters in length (type Ib). A left carotid artery or renal artery PG (Figures 1A, 1B) are good examples of the former, and a periscope graft is an example of the latter (Figure 2).
2. Type II PG, on the other hand, designates a PG conduit that travels (usually for several centimeters or more) from the lumen of an aortic stent-graft to the target branch vessel, including a segment of 5 cm or more where the conduit is sandwiched between two aortic endografts. The sandwich graft (and variations thereof) and the multiple thoraco-abdominal visceral/renal conduits can be cited as prototypical examples (Figure 3).

Current Evidence and Results

Two recently published review papers provide a useful summary of currently available evidence.^11,12 Overall, the authors noted the high technical success rate of the chimney technique and relatively few complications and low risk but also pointed to the many unknowns, including choice of an optimal stent device for a given case. Clearly we need more information on a larger pool of patients and with longer term follow-up.

Data on sandwich grafts and other long thoraco-abdominal PGs are also limited.^7,8 Recent publications offer a glimpse into the actual potential of these techniques, but it is only the beginning.

Future Perspective

Clinical use of PGs is clearly growing. But amid the heightened attention we must remember their currently imperfect nature and important limitations. Worthy of mention among these are the relatively small clinical dataset and insufficient information on long-term follow-up, the gutters that result from their interaction with the aortic stent-graft (Figure 4) and resultant potential for endoleak, and the absence of clear guidelines on best choices for device type in a given situation. Some, if not all, of these issues may be resolved or improved upon in the foreseeable future through better techniques and – perhaps – the development of better suited PG conduit technologies. This may become an imperative of sorts as long PGs and sandwich grafts become ever more popular in the hands of operators managing complex aortic pathologies (Figure 5).   

References

1. Greenberg RK, Clair D, Srivastava S et al. Should patients with challenging anatomy be offered endovascular aneurysm repair? J Vasc Surg. 2003;38(5):990-996.
2. Criado FJ. A percutaneous technique for preservation of arch branch patency during thoracic endovascular aortic repair (TEVAR): retrograde catheterization and stenting. J Endovasc Ther. 2007;14(1):54-58.
3. Larzon T, Gruber G, Friberg O, Geijer H, Norgren L. Experiences of intentional carotid stenting in endovascular repair of aortic arch aneurysms–two case reports. Eur J Vasc Endovasc Surgery. 2005;30(2):147-151.
4. Criado FJ. Chimney grafts and bare stents: aortic branch preservation revisited. J Endovasc Ther. 2007;14(6):823-824.
5. Lachat M, Frauenfelder T, Mayer D, et al. Complete endovascular renal and visceral artery revascularization and exclusion of a ruptured type IV thoracoabdominal aortic aneurysm. J Endovasc Ther. 2010;17(2):216-220.
6. Lobato AC. Sandwich technique for aortoiliac aneurysms extending to the internal iliac artery or isolated common/internal iliac artery aneurysms: a new endovascular approach to preserve pelvic circulation. J Endovasc Ther. 2011;18(1):106-111.
7. Lobato AC, Camacho-Lobato L. Endovascular treatment of complex aortic aneurysms using the sandwich technique. J Endovasc Ther. 2012;19(6):691-706.
8. Kasirajan K. Branched grafts for thoracoabdominal aneurysms: off-label use of FDA-approved devices. J Endovasc Ther. 2011;18(4):471-476.
9. Silveira PG, Galego GN, Bortoluzzi CT, Franklin RN. RE: Branched grafts for thoracoabdominal aneurysms: Off-label use of FDA-approved devices. J Endovasc Ther. 2012;19(1):130.
10. Malina M, Resch T, Sonesson B. EVAR and complex anatomy: An update on fenestrated and branched stent grafts. Scand J Surg. 2008;97(2):195-204.
11. Yang J, Xiong J, Liu X, Jia X, Zhu Y, Guo W. Endovascular chimney technique of aortic arch pathologies: a systematic review. Ann Vasc Surg. 2012;26(7):1014-1021.
12. Tolenaar JL, van Keulen JW, Trimarchi S, Muhs BE, Moll FL, van Herwaarden JA. The chimney graft, a systematic review. Ann Vasc Surg. 2012;26(7):1030-1038.