Volume 43, Issue 2, Supplement , Pages A48-A52, February 2006
Endovascular management of thoracic aortic aneurysms. Preoperative imaging and device sizing
Article Outline
- Introduction
- Preoperative imaging
- Device sizing
- Difficult scenarios
- Conclusion
- Supplementary data
- References
- Copyright
Introduction
Thoracic aortic pathology in a variety of forms continues to represent a significant health challenge. The incidence of thoracic aortic aneurysms (TAAs) is estimated to be as high as 10 cases per 100,000 population per year,1, 2 acute aortic dissection occurs in 10 to 20 individuals per million population,3 and traumatic aortic tears occur in up to 18% of motor vehicle accidents.4 Until recently, the only effective treatment method was surgical graft replacement, involving high morbidity and, at times, technically complex operations. Endovascular stent-graft repair recently emerged as viable alternative and has now gained acceptance as an innovative, safe treatment method associated with substantially less morbidity and reduced hospital stay.1
Unlike open TAA repair, where the surgeon can make decisions on graft size at the time of surgery, endoluminal repair requires meticulous preoperative imaging to precisely define the aneurysm morphology and choose the appropriate size graft. This quickly became evident in the early era of endovascular abdominal aortic aneurysm (AAA) repair, when failure to correctly measure the aneurysm led to endoleaks, graft thrombosis, graft misalignment, and failure to exclude the aneurysm.5 It is particularly true for aneurysms of the thoracic aorta, where high flow velocities and acute arch angles pose a formidable challenge in accurate graft placement and aneurysm exclusion. Evaluation must include measurements of various diameters and lengths of the aortic arch and proximal and distal thoracic aorta. In addition, iliac artery size, tortuosity, angulation, and calcification may impact delivery and need to be taken into account. This chapter summarizes the most important aspects and highlights common pitfalls in endograft sizing before endovascular repair of TAA.
Preoperative imaging
An ideal preoperative imaging modality to evaluate a patient for endovascular TAA repair would be accurate in depicting aneurysm morphology, minimize radiation and contrast exposure, and be noninvasive, inexpensive, and easily tolerated by the patient. Such a modality is not available. Techniques that are available and currently in use, sometimes in combination, include computed tomography (CT) scans, aortography, intravascular ultrasound scanning, and magnetic resonance angiography. The most commonly used diagnostic modalities are CT scan and marking aortography.6
CT angiography with intravenous administration of contrast and 3-mm-thick slices, widely used in the assessment of TAAs, may be the only study required before an open repair. Traditional axial CT images over a tortuous portion of the aorta will overestimate the true diameter of the vessel and should thus be interpreted cautiously when a thoracic endograft is being sized. Digital modification of CT-acquired images has been used with success for both AAAs and TAAs. Reconstruction of the image with curved linear reformats allows visualization of the vascular lumen in a plane perpendicular to the central arterial axis. To achieve this, central lumen lines are created by placing markers in the center of the vessel of interest (Fig 1). The reconstruction obtained by this method may provide more accurate information on aortic length than traditional arteriography with a marking catheter, because the position of the central lumen line can be adjusted to the anticipated position of the endograft. If the aorta is elliptical at the level of the measurement, then the mean diameter can be used to select the appropriate graft.7
Fig 1.
Three-dimensional reconstruction of computed tomography scans is invaluable in permitting accurate measurements of thoracic aorta aneurysms. Length measurements are optimized; however, lumen diameters are best measured directly from transverse slices.
In addition, three-dimensional (3D), interactive digital reconstruction is possible with proprietary software (Preview Software, Medical Media Systems, West Lebanon, NH). Encouraging results with this technology indicate that 3D reconstruction might be the only diagnostic modality necessary for preoperative planning in patients with AAAs, eliminating the need for preoperative angiography.8 Others take a more conservative approach, however, suggesting that for the straightforward aneurysm, 3D reconstruction does not really alter or improve decision-making compared with traditional CT angiography.6
Studies comparing 3D CT reconstruction with traditional CT angiography and other imaging modalities in terms of preoperative thoracic endograft planning are not currently available. The complex structure of the aortic arch and the tortuosity present in the distal descending aorta—the two common landing zones of thoracic endografts—lend themselves to a more sophisticated level of imaging, and most authors use 3D reconstruction routinely.1
Aortography provides important information on arterial tortuosity, length of the various segments, and the presence of concomitant occlusive disease. Distinct disadvantages include complications associated with the invasive nature of the procedure and inability to reliably measure diameter because of the presence of mural thrombus or detect calcification that can be a major cause of fixation and delivery problems. When performed as a separate procedure before endograft placement, arteriography also adds to the overall cost of the procedure. In our experience, sizing the endograft with CT angiography and performing the arteriogram with a marking catheter just before the deployment of the stent-graft is preferable, except for the most complicated aneurysms or those associated with chronic dissection.
Intravascular ultrasound (IVUS) has become an invaluable intraoperative imaging tool, particularly in difficult cases and those associated with aortic dissection. IVUS gives information on aortic diameter without the confounding magnification effect of arteriography, measures the length of proximal and distal landing zones, confirms aortic branch anatomy, verifies the optimal stent-graft placement after deployment, confirms wire passage in the true lumen and adequate coverage of the entry site in cases of dissection, and offers information on adequate graft apposition and relation to adjacent branches.9, 10
Device sizing
The selection of appropriate length and diameter of the endoprosthesis used in endovascular TAA repair is closely linked to the sophistication and accuracy of the preoperative imaging modalities. The importance of accurate sizing cannot be overemphasized. Some of the major postoperative complications, including type I endoleak, kinking and collapse, and obstruction at branch orifice points, are directly related to poor sizing.
Diameter considerations
CT scan imaging with or without 3D reconstruction indicates the aneurysmal portion of the aorta and the adjacent segments of normal caliber that can serve as landing zones (Fig 2). Diameter measurements of the true lumen from inner wall to inner wall at 1 and 2 cm from the proximal and distal implantation sites are recommended to assess for a conically shaped neck, which may increase the risk of graft migration. A graft diameter 6% to 19% larger than the aortic diameter is desirable to allow for good wall apposition. For the Gore Thoracic Aortic Graft (TAG) device (W. L. Gore & Assoc, Flagstaff, Ariz), this means that aortic diameters from 23 to 37 mm can be treated with six different prosthesis diameters from 26 to 40 mm (Fig 3).
Fig 2.
A typical sizing diagram for sizing of the descending thoracic aorta. A minimum of 6 diameters are used to characterize the proximal and distal landing zones. For the Gore Tag device, these are lumen-to-lumen measurements, not adventitial-to-adventitial measurements. Consequently appropriate sizing requires a contrast-enhanced computed tomography scan, angiogram, or intravascular ultrasound. A, Proximal implantation site; B, 1 cm from proximal implantation site; C, 2 cm from proximal implantation site; D, aneurysm; E, 2 cm from distal implantation site; F, 1 cm from proximal implantation site; G, distal implantation site; H, right common iliac artery; I, left common iliac artery; J, right external iliac/femoral; K, left external iliac/femoral; L, proximal neck, distance from aneurysm to left subclavian or carotid arteries; M, aneurysm length; N, distal neck, distance from aneurysm to celiac axis; O, total treatment length; P, proximal angle; Q, distal angle.
Fig 3.
Sizing chart for the W. L. Gore Thoracic Aneurysm Graft (TAG).
Further oversizing the graft does not add anything to the procedure and can be dangerous if the end point of the graft protrudes into the aortic lumen, especially if the proximal landing zone is just distal to an acutely angled aortic arch. This has the potential for graft collapse, which can lead to continuous pressurization of the aneurysmal sac. In an even worst-case scenario, high blood flows within the aortic arch can make the graft fold into itself, causing obstruction of the aorta and death. This is of particular concern in young patients with relatively small aortas who present with aortic trauma and for whom appropriately small endoprostheses are not available.
The size of the introducer sheath used to deliver the endoprosthesis is closely linked to the diameter of the endograft and may vary from 20F (7.6 mm) to 24F (9.2 mm). This is an important consideration when the operation is planned and mandates careful evaluation of the diameter of the access vessels. Small, calcified, and tortuous iliac and femoral vessels will not accommodate the large delivery devices. The placement of a temporary prosthetic conduit in the common iliac artery is recommended in these situations to avoid vascular injuries in the iliac and femoral territory. If a conduit is used, the graft selected should have a large enough inner for the sheath, such as a 10-mm Dacron graft.
Length considerations
At least a 2-cm neck length proximally and distally is recommended by most authors to allow adequate stent-graft seal and minimize the risk of type I endoleaks. In cases where this was not possible, continuous sac pressurization with deleterious consequences has been described.11 When the aneurysm starts at least 2 cm distal to the left subclavian artery, the endograft deployment is rather straightforward. Aneurysms with more proximal extent may necessitate complex debranching procedures that will allow coverage of one or more of the arch branches to achieve the desirable 2-cm sealing zone. Distal neck length, measured as the distance of the aneurysm from the celiac axis, also needs to be adequate, although debranching procedures have been described12 and can optimize the neck to the appropriate length (Fig 4, Fig 5, Fig 6).
Fig 4.
A, Measure the length of the aneurysm using angiography with marker catheter present. B, Measure the maximum lumen-to-lumen diameter using computed tomography.
Fig 5.
Diameter measured in angled necks as shown by aortogram and computed tomography (CT) scan: measure the smaller width of the ellipse.
Fig 6.
Compensating for tortuosity at landing zones: Use more than 2 cm of neck if available and appropriate. More than 2 cm of neck is recommended when a severe angle exists (<60° angle).
Lengths of thoracic stent-grafts are limited; the TAG endoprosthesis, for instance, is available in lengths of 10, 15, and 20 cm. Therefore, more than one piece of stent-graft is often required to completely exclude the aneurysm. Several factors should be considered in the decision-making process:
Difficult scenarios
Small iliac arteries
Vascular trauma or thrombosis occurred in 14% of patients in the Gore TAG pivotal trial, most of them access-related iliac artery injuries. Iliac artery rupture is the most commonly reported complication since US Food and Drug Administration (FDA) approval of the device. Judging the ability of the iliac artery to accommodate a sheath is very difficult. Size requirements vary depending on the diameter of the graft to be used, which in turn determines diameter of the sheath to be inserted:
Typically, a rupture will not be identified until the sheath is being removed. If sheath insertion has been difficult, have an occlusion balloon prepared and on the table or insert one into the distal aorta while slowly withdrawing the sheath and injecting dye. Some ruptures can be salvaged by insertion of a stent-graft. However, usually the external iliac is avulsed and back bleeding can continue from the internal iliac artery. In our experience, open surgical repair is usually required. As a result, we have adopted a very liberal policy for insertion of a retroperitoneal conduit: if in doubt, place a conduit.
Significant aortic tortuosity
Numerous curves can develop within the aorta. The double curves that occur when angulation and tortuosity at the arch is coupled with tortuosity immediately before the diaphragmatic hiatus is penetrated can lead to difficulty in advancing the device over the arch. The device tends to buckle into the curve above the diaphragm and fails to advance in the arch. Ensuring that the wire is advanced as much as possible, advancing the wire and catheter together, or placing a second wire to straighten the aorta as much as possible are adjunctive techniques for this problem.
Debranching techniques to extend the application of thoracic endografts
Selective carotid subclavian bypass was the first technique used to extend the proximal landing zone to permit endograft deployment up to the origin of the left common carotid artery. Since device approval, clinicians have developed techniques to further debranch the aorta. Carotid-to-carotid bypass with selective carotid-subclavian bypass permits device deployment up to the innominate. In patients with a normal ascending aorta or a prior ascending graft, an aortoinnominate bypass with carotid-to-carotid and carotid-subclavian bypass permits total debranching of the aortic arch.
Debranching of the abdominal aorta can also be performed from an infrarenal graft, infrarenal aorta, or iliac arteries. Retrograde bypass to the celiac, superior mesenteric artery, and renal arteries allows a sequential increase in the length of the distal landing zone.
Conclusion
Careful preoperative planning is essential for the success of thoracic endovascular aortic repair (TEVAR). CT scans with or without 3D reconstruction in conjunction with arteriography provides the information needed for graft selection. Careful diameter oversizing and adequate coverage of the proximal and distal landing zones will allow for aneurysm exclusion. Debranching procedures enable treatment of aneurysms adjacent to or involving the aortic arch branches and provide flexibility on landing zone selection. Characteristics of the iliofemoral vessels need to be taken into account, in conjunction with the delivery sheath size, to avoid serious injury to the access vessels.
Supplementary data
References
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- Endovascular stent-graft placement for the treatment of acute aortic dissection . N Engl J Med . 1999;340:1546–1552
- . Stent graft repair in the aortic arch and descending thoracic aorta (a 4-year experience) . J Vasc Surg . 2002;36:1121–1128
- . Endovascular stent-graft repair of pararenal and type IV thoracoabdominal aortic aneurysms with adjunctive visceral reconstruction . J Vasc Surg . 2005;41:191–198
Competition of interest: none.
PII: S0741-5214(05)02056-2
doi:10.1016/j.jvs.2005.11.032
© 2006 The Society for Vascular Surgery. Published by Elsevier Inc. All rights reserved.
Volume 43, Issue 2, Supplement , Pages A48-A52, February 2006
