Surgery of Descending Thoracic Aorta (Endovascular)
T. Brett Reece
Ramesh Singh
Indications/ContraindicationsIndications for thoracic endovascular aortic repair (TEVAR) are identical to open repair of the same pathology. In the most recent guidelines, asymptomatic aneurysms >5.5 cm are indicated for repair. Indicated pathologies for TEVAR are as follows:
Degenerative aneurysms
Chronic aortic dissections leading to aneurysms or true lumen compression or “pseudo-coarctation”
Some arguments can be made that aneurysms <5.5 cm could be repaired in special situations including:
Growth would compromise adequate landing zones, thus ruling out endografting
Aneurysms close to the proximal or distal extent of the aneurysm where longitudinal growth of the aneurysm may reduce landing zone beyond the ability of the stent to seal
Smaller aneurysms growing at rates of more than 0.5 cm/year
Symptomatic pathologies independent of size
Traumatic aortic tears ranging from pseudoaneurysm to transection
Penetrating aortic ulcers >2 cm in width or depth
Acute aortic dissections
Acute complicated dissections are now initially approached with TEVAR
Rupture requires coverage from the subclavian to the celiac
Malperfusion requires re-establishing optimal blood flow to the true lumen
Acute uncomplicated dissections can be stented, but the risks and benefits must be weighed. The investigation of stent grafts in aortic dissection (INSTEAD) trial demonstrated that there is significant risk upfront with TEVAR, but that after 2 years, a survival benefit was present for patients undergoing TEVAR in the follow-up study.
Indications for TEVAR have expanded as quickly as the technology has been developed and introduced. Essentially, almost any pathology that can be excluded with adequate landing zones for seal can undergo successful TEVAR.
Contraindications to TEVAR are derived from the inability to gain a seal zone as well as adequate aortic tissue properties to accommodate the radial stress required to allow for graft to aorta opposition. Essentially, contraindications constitute modes of failure either acutely from the tissue graft interface or chronically from failure of that interface. The anatomic restrictions can arise from limitations of the current technologies.
Anatomic restrictions to TEVAR include:
Seal zone compromise
Length
Currently available devices require 1.5 to 2 cm of “normal” aorta for both the proximal and distal landing zones. Landing zones can be extended with safe coverage of major branches such as the left subclavian artery or the celiac, but this must be done thoughtfully. These branches can be covered safely if the collaterals can support adequate flow. For instance, the left subclavian artery (LSCA) can be covered in most patients, but this vessel cannot be covered without revascularization in patients with a patent left internal mammary artery (LIMA) to left anterior descending artery (LAD) bypass or patients with a dominant vertebral artery due to the risk of posterior circulation stroke. Fortunately, the revascularization of the subclavian is relatively easy with a carotid subclavian bypass or transposition. Occlusion of the base of the subclavian, below the vertebral and the internal mammary, can reduce the likelihood of a type 2 endoleak, but is not mandatory. The takeoff of the subclavian can be dealt with vascular plugs prior to stent deployment or even after the procedure through the arm for later issues. The celiac artery may be covered in patients with a patent gastroduodenal artery.
Landing zone quality
An intramural hematoma (IMH) may be the most unstable vascular wall that can be interfaced.
Atheroma can have two issues. The first is distal embolization with wire movement. This embolization may be amplified when the stiff delivery systems traverse the atheroma. The second is that atheroma do not have the rigidity to create a stable landing zone.
A dissected aorta does not constitute a great landing zone for a stent. The radial force can lead to another tear through the intima. Depending on the area of new tear, the new pathology can lead to retrograde dissection proximally or preserve the false lumen flow by creating another dominant tear distally. For these reasons, at least the proximal landing zone should be deployed in nondissected aorta.
Access for delivery
Current devices range from 18 to 24 Fr in external diameter. However, the terminology for size needs some discussion. First, the catheters are reported in French sizes. This can be converted to metric size, or vessel measurement, by dividing the French size by 3 to get the vascular diameter in centimeters.
Delivery catheters are reported in true outer diameter (OD). This would be the size of the catheter if no sheath is required, or desired, for delivery.
Sheaths, however, are reported as inner diameter in order to describe the catheter size that they will take. The OD, which dictates the size of vessel that will accommodate the sheath, is stated on the package. The sheath OD is always a few millimeters larger than the descriptor.
Specific devices are delivered with varied techniques. Three of the four currently available devices do not require a sheath, so their diameter should be true to the vessel size required. It must be understood that sheathless delivery is optimal for single stents, but becomes less desirable when multiple devices are necessary. While delivery through a sheath may cause less repetitive damage to the access artery, size restraints may require reaccessing the sites.
Nonanatomic contraindications
Aortopathy: In many people’s minds, IMH at the landing zone is unstable and should not be considered a viable purchase for the stents. The aortic wall is not structurally normal, so the radial force required for the stent to remain static can actually tear through the aorta leading to pseudoaneurysm or even rupture. A similar potential issue must be acknowledged with patients with connective tissue disorders. The abnormal aorta of Marfan, Ehlers–Danlos, etc., can break down from the radial force of the stent on the aorta.
Infection/aerodigestive fistulas can be stented, but by definition are infected. TEVAR in this setting should be limited to the acute setting with extreme prejudice for a staged open procedure with removal of the device and all infected tissues. Prolonged treatment with antibiotics should be closely followed with imaging as rupture or refistulization may be the first indication of progressing pathology.
Preoperative PlanningThe initial planning for TEVAR begins with sizing the aneurysm for the endovascular appliance. The proximal and distal landing zones should be measured. Landing zones are defined by the graft indications for use, but generally they require 2 cm as a minimum length of normal aorta. The landing zones are defined by the portion of covered graft that interfaces with normal aorta. Coverage of the subclavian or celiac may be suitable depending on the rest of the circumference of the aorta. The diameter of landing zone that can be treated is also defined by the device indications for use. The current devices can treat a wide range of diameters, essentially from 16 mm to 42 mm. The characteristics of the currently available grafts are summarized in Table 14.1. The diameter of the graft is generally 10% to 20% larger than the landing zone diameters for aneurysms. However, some pathologies are sized more than the size of the landing zone because oversizing can lead to more problems, like intimal tears in oversized dissections. Partial circumference pathologies (PAU) may require less landing zones for exclusion of flow. The length of coverage depends on the length of the pathology. The length of coverage can vary widely in tortuous aortas. Multiple grafts with overlap of 2 to 3 cm can help make up for the distance taken up by the tortuosity.
Imaging for the access vessels should be included on the preoperative imaging. Access vessels have to accommodate not only the delivery device but also serial delivery of devices. It is important to understand the terminology utilized for these devices. While this was mentioned previously, the differences in description of catheters and sheaths can widely change the access needs. For instance, catheters are reported by external diameter. Sheaths are reported by internal diameter. The sheaths will tend to be the largest structures put into the access vessel, but they are described by sizes that
are not the true diameter placed within the vessel. This requires that the imaging includes cuts through the common femoral arteries. While the common femoral arteries can be too small to accommodate the 24-Fr sheaths required for the bigger devices, the external iliac is commonly the vessel too small for the sheaths rather than the femoral. Alternative accesses to the femoral arteries in TEVAR have traditionally been retroperitoneal exposures of the common iliacs or aorta. The utilization of 10-mm conduits to these access points provides a more stable exchange platform for the large sheaths, but direct puncture of the vessels has been described. The 10-mm conduit is large enough to deliver any available stent as well as put up another catheter for imaging, usually a pigtail.
are not the true diameter placed within the vessel. This requires that the imaging includes cuts through the common femoral arteries. While the common femoral arteries can be too small to accommodate the 24-Fr sheaths required for the bigger devices, the external iliac is commonly the vessel too small for the sheaths rather than the femoral. Alternative accesses to the femoral arteries in TEVAR have traditionally been retroperitoneal exposures of the common iliacs or aorta. The utilization of 10-mm conduits to these access points provides a more stable exchange platform for the large sheaths, but direct puncture of the vessels has been described. The 10-mm conduit is large enough to deliver any available stent as well as put up another catheter for imaging, usually a pigtail.
TABLE 14.1 Thoracic Stent Grafts Currently Available in the United States. The Chart Depicts Characteristics of the Devices as Well as Indications Derived From Each Product Instruction for Use Catalogue | |||||||||||||||||||||||||
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TABLE 14.2 Access Possibilities for TEVAR Delivery | ||||||||||||||||||||||||||||||
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If for any reason, the preoperative imaging is not clear, intravascular ultrasound or intraoperative angiography can be utilized to measure everything from the adequacy of the landing zone diameter and length, the length of coverage, and identification of branch vessels.
No discussion of thoracic aortic surgery is complete without a discussion of postoperative paraplegia. The risk of paraplegia must be assessed preoperatively. While this does not happen commonly, the implications of paraplegia on the patient are extraordinary. The patient must understand the risk as well as the signs of injury as they will most likely be the first to recognize the complication. Table 14.2
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