In the last decade the endovascular treatment of complex aortic disease was revolutionized with the introduction of the chimney technique, an alternative approach to create an adequate proximal/distal landing zone in the event of an insufficient aneurysm neck. The main issue that determines a successful outcome with the chimney technique is the degree of main endograft oversizing in relation to the parallel grafts. A correct oversizing is crucial to achieve an appropriate balance of the opposing radial forces among the chimney, endograft, and aortic wall. An appropriate amount of oversizing allows the endograft to conform optimally around the stent and avoids both type I endoleaks through large gutters and compression of the parallel graft(s).
Type I endoleaks caused by gutters were often described as the “Achilles’ heel” of the chimney procedure. Persistent type I endoleaks requiring reintervention range between 1.6% (PROTAGORAS) and 2.9% (PERICLES). The main reason for this relatively high rate of gutter leaks is a lack of consensus regarding the optimal configuration, combination of devices, and operative technique. It is evident, however, that appropriate planning and selection of devices are crucial to minimize the risk for gutter leaks.
Preoperative Imaging
Thoracoabdominal computed tomography angiography (CTA) is essential for preoperative planning. CTA scans should have a thickness of 1 mm to achieve a very high imaging resolution. The imaging should be analyzed using dedicated vascular software for three-dimensional reformatting and centerline reconstructions for exact measurement of vessel length and diameter.
The configuration of the vessels should be carefully studied by multiplanar reconstruction (MPR), particularly at the level of the juxtarenal aorta, to assess the renal and visceral vessels. Also, imaging should include adequate views of the access vessels, such as the iliac and femoral arteries, because these are essential to plan the chimney procedure. Similarly, imaging must include an evaluation of the arch vessels. The subclavian artery is used as the main vessel for the upper extremity access, as well as to rule out high-grade carotid stenoses, which can be a risk for perioperative neurologic complications.
The presence of thrombus in the descending aorta can also be a source of cerebral or visceral embolization. In this context, additional administration of heparin and monitoring of the activated clotting time are strongly recommended.
New Sealing Length
The total neck length in the chimney technique is defined as the sum of a seal neck and new neck. The new neck is defined as the segment between the proximal fabric edge of the aortic endograft and the orifice of the lowest target vessel in which the chimney graft will be deployed. The seal neck reflects the native portion of the infrarenal aortic neck, always shorter than 10 mm and often completely absent. The goal is to have a total neck length of at least 20 mm. This length was required in the PERICLES Registry and the PROTAGORAS Study. Each of the following parameters should be evaluated preoperatively:
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Identification and characterization of the aortic neck and the involved aortic branches
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Estimation of the length of the new sealing zone
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Caliber and length of the infrarenal neck, if present
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Total aortic length to the bifurcation
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Anatomic characteristics of the target vessels:
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Angulation
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Curvature
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Origin along the aortic circumference (clock face)
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Diameter of the branch vessel
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Length, with consideration of relevant branches that perfuse a significant segment of the kidney or the bowel, and whether these could be overstented after deployment of the chimney graft in the main branch
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- 6.
Presence of accessory renal arteries and estimation of the renal mass supplied by the vessel for preoperative decision making, regarding whether overstenting or maintaining perfusion is preferable
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Descending thoracic aortic and iliac artery tortuosity centerline calculations (mandatory to assess length accurately in tortuous anatomy)
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Caliber and presence of calcification of the common femoral arteries and the external iliac arteries
Planning The Procedure
Aortic Neck Sizing
Current devices and technical skills have increased the applicability of standard endovascular aneurysm repair (EVAR) to a broad spectrum of patients with aortic aneurysms. Regarding the infrarenal aortic neck, current instructions for use (IFU) include evaluation of not only standard parameters, such as the length and diameter, but also the shape (conical, bubble, or straight) and the angulation (both suprarenal and infrarenal).
Most latest-generation endograft IFUs specify a required neck length of 15 to 20 mm, but lengths of 10 mm may be acceptable when the neck is not severely angulated (<60 degrees). The use of chimney technique to treat patients with an inadequate neck length is based on the creation of a new aortic neck of a similar required length, which is at least a 15-mm sealing zone for an abdominal device.
Aortic Neck Diameter
When choosing the size of the aortic endograft, one must assess the diameter of the pararenal aorta at the intended segment for the deployment of the endoprosthesis. In vitro analysis showed that oversizing of 20% to 30% minimizes the risk for persistent gutter-related endoleaks. This means that the maximum neck diameter in single chimneys should be no more than 30 mm. In this case, an aortic device of 36 mm is at least 30% oversizing. It is important to measure the neck diameter in three parts—proximally, medially, and distally—and use adventitia-to-adventitia (outer wall–to–outer wall) measurements, with exception of the Gore endografts (recommend “inner-to-inner” measurements).
Aortic Neck Angulation and Thrombus
Presence of severe (>60 degrees) infrarenal neck angulation and a conical shape are predictors for early type Ia endoleak. These endoleaks can be reduced if the aortic device can be deployed in a straighter aortic segment, avoiding placement in highly angulated anatomy. The surgeon should adhere to this principle, even if doing so would require additional chimney graft placement into the upper aortic branches.
Calcification and mural thrombus are also factors that predispose to suboptimal deployment of the aortic device, poor wall apposition, and a higher risk of type Ia endoleaks, as in infrarenally deployed endografts.
Sizing of Chimney Grafts
A sealing zone of at least 10-mm length into the aortic side branches is recommended. Usually the diameter of the chimney graft is chosen based on the diameter of the target vessel. For example, a 6-mm chimney graft would be chosen for visceral vessels with a diameter between 5.1–6mm. Placement of a nitinol bare-metal stent in an already-deployed chimney graft lengthens the distal sealing zone while maintaining perfusion of a relevant side branch. The diameter of the additionally deployed nitinol stent is the same as the diameter of the previously deployed chimney graft.
Aortic Branch Stenosis or Angulation
Severely calcified stenoses of the visceral vessels are associated with a technically demanding procedure. Predilation with a 4-mm balloon catheter can be a good option. Alternatively, the use of the “lift technique” allows the transfemoral placement of self-expanding chimney grafts. This technique permits the creation of a standard chimney configuration from a femoral approach, moving the proximal aspect of the chimney graft cranially after deployment. Fig. 7.1 shows the already-deployed Viabahn self-expanding covered stent in the left renal artery. A short balloon is positioned at the distal aspect of the Viabahn chimney graft to fix its position. The sheath is then advanced cranially to reposition the chimney graft so that its proximal aspect is in a normal chimney configuration, proximal to the branch vessel for antegrade perfusion into the branch.
Oversizing of Abdominal Endograft
Four options can be considered to determine the proper amount of oversizing of aortic endografts for the chimney technique: standard oversizing, use of a mathematical formula, in vitro testing, and custom-made oversizing.
Standard Oversizing
The PERICLES Registry and PROTAGORAS Study recommend oversizing between 20% and 30%. This is based on the clinical experience with standard EVAR, with recommendations for 10% oversizing without a chimney graft. In order to have enough fabric material to wrap around the chimney grafts, additional oversizing is required.
Mathematical Formula
Lachat suggested during a LINC meeting an elliptic model for the estimation of the appropriate aortic stent-graft diameter. The formula is based on an ellipse configuration with major diameter (A) equal to the sum of the mean aortic diameter plus the chimney stent diameters, and minor diameter (B) equal to the mean aortic diameter. Mathematically, this is equal to (A + B)/2, if B is the sum of the chimney graft diameters.
Kölbel et al. proposed a mathematical formula using circular segments of the aorta and a single chimney graft of 6–8 mm, concluding that 30% is the best oversizing for a single visceral chimney graft.
In Vitro Testing
According to Mestres et al., 30% is the recommended oversizing when placement of a single chimney graft is planned. The gutters must have the smallest possible area between the chimney and abdominal endografts to minimize the risk for persistent type Ia endoleak. The choice of abdominal endograft also plays an important role.
Two abdominal endografts, the Endurant (Medtronic, Santa Rosa, California, USA) and Excluder (Gore, Tempe, Arizona, USA) were tested with placement of a single chimney graft, either Advanta V12 (Atrium Medical/Maquet, Hudson, New Hampshire, USA) or Viabahn (Gore). The configurations were tested with three different degrees of oversizing: normal (15% oversizing), excessive (30%), and overexcessive (40%). The best results with freedom from chimney graft compression and smallest gutter areas were observed in the combinations of Advanta V12 with a 30% oversized Endurant abdominal device and Viabahn with a 30% oversized Excluder. The increased radial force in the Endurant performs better with a balloon-expandable covered stent, which has also higher radial force compared with self-expanding stents.
Custom-Made Oversizing Technique
“Over-SIRIX” is a new patient-tailored method to select the appropriate abdominal endograft in order to minimize gutter formation and reduce the incidence of type Ia endoleak with the chimney technique ( Fig. 7.2 ). The ideal perimeter is drawn around the chimney graft directly on the CTA (based on Osirix imaging software), where the chimney graft is planned within the axial plane on MPR, at the level of the proximal aspect of the new neck.
