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Sizing of Aortic Endografts in Chimney Technique
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:
- 1.
Identification and characterization of the aortic neck and the involved aortic branches
- 2.
Estimation of the length of the new sealing zone
- 3.
Caliber and length of the infrarenal neck, if present
- 4.
Total aortic length to the bifurcation
- 5.
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
- 7.
Descending thoracic aortic and iliac artery tortuosity centerline calculations (mandatory to assess length accurately in tortuous anatomy)
- 8.
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).
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