It has been almost two decades since Juan Carlos Parodi deployed the first endovascular stent-graft in an abdominal aortic aneurysm, a procedure that ushered in a new era in the treatment of an often fatal vascular disease. Since then, significant evolution in devices and techniques has resulted in a dramatic expansion in the performance of endovascular abdominal aortic aneurysm repair (EVAR) and accumulating data about outcomes. In the US, according to Medicare Part B data sets the number of EVAR cases has shown 162% growth, from 11,028 cases in 2001 to 28,937 cases in 2006, while surgical abdominal aortic aneurysm (AAA) repairs decreased by 51% (from 31,965 to 15,665 cases) over the same period.1 Additionally, the steady increase in EVAR has been shown to be associated with a decline in the mean annual number of AAA ruptures and related mortality.2 However, although current technologies yield predictable and favourable outcomes in appropriately selected patients, it has been estimated that, using the current commercially available devices, 30–40% of patients with AAA are not candidates for endovascular repair.3 The success and failure of EVAR is heavily reliant on patient selection, pre-procedural planning, operator experience, the technique employed and the type and the generation of the endograft. In this article, the important factors that make AAA patients less ideal candidates for EVAR as well as the techniques to overcome these obstacles are briefly discussed.
Unfavourable Access
Despite the evolution of endograft designs and the introduction of more flexible and lower-profile systems, all of the devices developed to date for endograft repair of aortic aneurysms are deployed through relatively large (12–24 French [Fr]) sheaths, and access-related issues are a major reason for EVAR preclusion, failure or conversion to open repair among AAA patients. Since the combination of calcification, tortuosity and diminished calibre as well as endovascular sheath size contributes to the risk of iliac rupture during EVAR, these factors have to be meticulously evaluated prior to the procedure. Computed tomographic angiography (CTA) is generally considered the modality of choice, but for patients with renal insufficiency or contrast allergy, magnetic resonance angiography (MRA) can provide equally valuable information.4
Female gender and age have long been known to influence the quality of endovascular access, and a recent study demonstrated that ethnicity (Asian ancestry) may also play a role.4,5 The European Collaborators on Stent-Graft Techniques Abdominal Aortic Aneurysm Repair (Eurostar) registry reported access problems in 13% of patients selected for EVAR.4 Yano et al.6 reviewed their institution’s experience and demonstrated that 50 of 390 patients (12%) needed some type of adjunctive procedure to aid in endovascular access. Iliac artery angioplasty was the most common intervention in their series. Even with current imaging techniques and devices, many modern series still show a 10–15% incidence of difficult iliac artery anatomy that would necessitate a change or modification to the procedure or choice of access vessel.5 The increasing trend towards percutaneous suture-mediated endovascular access closure has also brought up additional factors such as arterial depth and obesity. However, the significance of these factors in comparison with the aforementioned issues remains to be verified.
Iliac artery tortuosity is one of the most frequently encountered challenges in EVAR access. Using super-stiff wires such as Amplatz and Lunderquist (COOK Inc., Indianapolis, IN) with or without a stiff Glidewire (‘buddy wire’ technique) is one of the techniques that could be used to straighten out a tortuous iliac artery access.5,7 Re-lining and dilation of narrowed iliac arteries with a covered stent graft (internal endoconduit) in order to accommodate large sheaths in small-calibre arteries has been successfully described as well.8 This technique could be accompanied by induction of controlled rupture of the narrowed segment.8 A potential drawback of this procedure is the fact that coverage of the hypogastric artery origin is often necessary.4 The importance of this issue will be discussed later in this article.
Use of alternative endovascular access (e.g. the carotid artery) and retroperitoneal exposure combined with the creation of an iliac or aortic external conduit or direct access to the iliac arteries are among other techniques used to overcome access problems.
Unfavourable Abdominal Aortic Aneurysm Anatomy
Unfavourable anatomy of the proximal aortic neck is the most common factor (seen in up to 40% of cases) precluding patients from an endovascular treatment option.9 Some of the most common predictors of endograft failure are angulated and short infrarenal necks, large-diameter necks and thrombus within the aneurysmal sac.
Unfavourable anatomical features are generally defined as neck diameter >28mm, angulation >60°, circumferential thrombus >50% and length <10mm. Endograft migration and type 1 endoleak – which are major problems after EVAR and significantly increase the incidence of aneurysm rupture and the need for conventional surgical repair – are highly associated with a short and angulated neck. Additionally, calcium or thrombus, or both, can compromise the fixation and sealing of the endograft at the implantation sites.10 Because the anatomical causes of stent migration cannot be changed, success in preventing such migration lies in modifying the deployment technique and the design of the stent-graft itself. Several techniques have been described to deal with short or hostile necks in order to decrease the risk of endoleak and distal migration. Modifications in technique should begin with how the delivery system is introduced. For instance, when the angulation is on one side of the coronal plane, introducing the delivery system from the side opposite the angle will facilitate passage at the top of the bend.10,11 Using a super-stiff guidewire may be necessary to provide enough stability during endograft deployment. On the other hand, attempting to reposition the endoprosthesis after deployment has been initiated increases the risk of migration and therefore is not advisable.10
In aneurysms with short or hostile necks, a Palmaz® XL stent (Cordis Corporation, a Johnson & Johnson company; Miami Lakes, FL) can be used in the infrarenal neck before or after deployment of the stent-graft.10 The ‘endowedge technique’ described by Minion and co-workers12 with the Excluder endograft (WL Gore & Associates, Inc., Flagstaff, AZ) also offers satisfactory juxtarenal sealing during endograft placement. This technique enables the scalloped proximal 4mm of the endoprosthesis to be wedged against the renal angioplasty balloons, which are placed via the brachial approach. The first two to three rings of the endograft are slowly deployed, then the device is advanced upwards against the inflated renal balloons for the completion of deployment.10 The ‘kilt technique’ has been described for aneurysms with a funnel-shaped or reversetapered proximal neck. In this technique, an aortic cuff is deployed in the distal infrarenal seal zone before the main body is deployed.12 It is important to emphasise that all of these techniques have to be carefully considered within the context of the operator’s experience and the availability of the alternative approaches.
There have been some concerns about the short- and long-term risks of renal and mesenteric artery embolisation and occlusion after EVAR with suprarenal endograft fixation. However, suprarenal fixation was used in 50 and 87% of endografts in the Dutch Randomized Endovascular Aneurysm (DREAM) and Endovascular Aneurysm Repair (EVAR) trials, respectively, with no difference in the incidence of renal dysfunction compared with infrarenal fixation.13
Although the newer generations of endografts, such as the Endologix PowerLink® System (Endologix, Irvine, CA), the Aorfix™ Endovascular AAA Repair System (Lambard Medical Tech, Tempe, AZ), the Aptus stent-graft (Aptus Endosystems, Inc., Sunnyvale, CA) and Anaconda™ (Vascutek, part of TERUMO Cardio-Vascular Systems Corp., Ann Arbor, MI) as well as fenestrated endografts, along with innovative techniques such as parallel (or ‘snorkel’) grafts for branch vessel preservation during EVAR, have yet to be fully validated, all have been introduced to address the technical obstacles of EVAR in aneurysms with unfavourable anatomies. Single-centre reports of fenestrated devices have demonstrated promising intermediate-term outcomes comparable to infra-renal fixation devices.14 A recent study from Sweden reviewed 54 patients treated with a fenestrated device with a median follow-up of 24 months. Three type I endoleaks during the procedure and three type II endoleaks at one-year follow-up were detected in their patient population. Additionally, 96% of the 134 targeted visceral vessels remained patent during their study period.15 A multicentre clinical trial in the US using the Zenith fenestrated endograft (COOK, Bloomington, IN) in 30 patients reported 100% technical success with no aneurysm-related mortality or conversion throughout 24 months of follow-up. No incidences of type I or type III endoleaks were detected in this study. Type II endoleak was found in six (26.1%) and four (20%) patients at 12 and 24 months, respectively. Eight patients in this trial experienced renal events, five of whom required secondary interventions. However, no renal failure developed requiring dialysis.14
Several studies have shown minimal adverse events associated with unilateral coverage or embolisation of the hypogastric artery to achieve a sufficient distal landing zone.13 Bilateral hypogastric artery occlusion with endograft extension into both external iliac arteries is also acceptable under special circumstances such as existence of concomitant extensive iliac artery aneurysms. However, patients should be informed that in at least one large series, buttock claudication persisted in 12% of unilateral and 11% of bilateral hypogastric artery interruptions, whereas impotence occurred in 9 and 13%, respectively.13,16
Ruptured Aneurysm
Since the first attempts in the mid-1990s, successful treatment of ruptured AAA (RAAA) with EVAR has been demonstrated in several studies, yielding survival results comparable to open repair for rupture.17–19 A recent study on National Inpatient Samples (NIS) in the US showed that from an estimated 27,750 hospital discharges for RAAA between 2001 and 2006, 11.5% were treated with EVAR. Additionally, despite the relatively insignificant change in incidence of aneurysmal rupture, the percentage of patients undergoing EVAR for RAAA increased from 5.9% in 2001 to 18.9% in 2006. Furthermore, the overall crude mortality rate following EVAR and open repair for ruptured AAA was 31.7 and 40.7%, respectively. These mortality rates were significantly lower at centres with a high annual volume of AAA repairs. This study suggests that the experience of the centre is a strong predictor of success in emergency EVAR.20 However, there are several obstacles to the use of EVAR for many RAAA patients, and the selection of patients for emergency EVAR is still heavily debated. Factors such as haemodynamic instability, fear of treatment delay for patient transfer or imaging procedures and logistical issues often lead to the exclusion of anatomically suitable patients from emergency EVAR.21
Successful treatment of RAAA patients first and foremost requires a multidisciplinary approach with clear protocols that start in the emergency room. The layout of the treatment suite has to be customised in order to accommodate endovascular as well as open surgical approaches. The importance of early diagnosis of AAA rupture and identification of EVAR candidate can not be over-emphasised. A pre-procedural CT scan is vital for evaluating the feasibility of EVAR as well as for endograft sizing.18 Although the haemodynamic status of the patient generally dictates the need for a pre-operative CT scan, some data show that the majority of patients with RAAA have enough time to undergo a CT scan prior to the treatment.22 Additionally, it is imperative to have all the necessary devices in stock in order to effectively deal with different access and aneurysm anatomies. Since detailed discussion of all of the techniques relating to emergency EVAR is beyond the scope of this article, only some of the major issues are highlighted here.
Emergency EVAR has been described both under local anaesthesia and a percutaneous approach and under general anaesthesia with a femoral artery cut-down. Maintaining sympathetic tone in hypotensive patients could be a potential advantage of avoiding general anaesthesia.18,23,24
Use of an aortic occlusion balloon is strongly recommended in haemodynamically unstable patients. The femoral approach, although not necessary, is more feasible for placing the balloon. Maintaining the appropriate position of the balloon in the aortic neck is essential. This could be achieved either by advancing the sheath up to the neck or applying continuous forward force on the balloon catheter.18 The balloon has to be deflated and withdrawn just before deployment of the main body, but should be re-inserted in haemodynamically unstable patients.
Use of a particular stent-graft type is usually decided according to the aneurysm anatomy and endovascular access available. Additionally, in the context of the patient’s haemodynamic status, all of the previously described techniques with regard to complex anatomy and endoleaks have to be considered to achieve maximum success in emergency EVAR.
Medically Unfit Patients
There are some concerns that endovascular treatment might still be too risky compared with conservative management in a subset of patients with suitable anatomy. This question was first raised in the EVAR 2 trial in which a peri-operative mortality rate of 9% was identified in a population of AAA patients with large aneurysms who were unfit for open surgery and were treated with EVAR.24 However, operator discretion was one of the criteria for identifying unfit patients in this trial. which makes it rather less definitive. Egorove et al.25 recently published a retrospective analysis of 66,943 Medicare patients treated with EVAR in the US between 2000 and 2006. The overall 30-day mortality in their patient population was 1.6%. They also developed a risk model for peri-procedural mortality of EVAR based on their data set, and listed the clinical risk factors for EVAR mortality in descending order of importance as: renal failure with dialysis, lower-extremity ischaemia, age ≥85 years, liver disease, chronic heart failure (CHF), renal failure without dialysis, female gender, a neurological disorder, chronic obstructive pulmonary disease and low hospital volume and operator experience with EVAR.
Renal failure is at the top of the list of risk factors for early mortality after EVAR based on the above study and other similar studies.26,27 Pre-procedural hydration is particularly important in this group of patients. Endovascular techniques such as administration of fenoldopam into the renovascular system have also shown to be beneficial in minimising the adverse effects of using contrast on renal function.13,27
Conclusion
A variety of new techniques and modifications to pre-existing techniques may be used to successfully treat the ‘non-ideal’ patients described above. Continuous advances in endovascular devices and technologies will make EVAR a feasible approach for more AAA patients who need treatment.
The future of EVAR as the potential gold standard for aortic aneurysm therapy is highly dependent on the vision and creativity of both interventionists and technology innovators. The next generation of aortic endografts with more flexibility and a lower profile as well as the application of new techniques such as electromagnetic (EM) tracking systems, 3D navigation, realtime magnetic resonance imaging (MRI) guidance and robotic technology could facilitate visualisation, improve accuracy, reduce contrast requirements and simplify follow-up, bringing EVAR even closer to the ideal AAA treatment.