Outcome of renal stenting for renal artery coverage during endovascular aortic aneurysm repair
Article Outline
Objective
This study was conducted to determine the outcome of adjunctive renal artery stenting for renal artery coverage at the time of endovascular abdominal aortic aneurysm repair (EVAR).
Methods
Between August 2000 and August 2008, 29 patients underwent elective EVAR using bifurcated Zenith stent grafts (Cook, Indianapolis, Ind) and simultaneous renal artery stenting. Renal artery stenting during EVAR was performed with endograft “encroachment” on the renal artery ostium (n = 23) or placement of a renal stent parallel to the main body of the endograft (“snorkel,” n = 8). Follow-up included routine contrast-enhanced computed tomography (CT), multiview abdominal radiographs, and serum creatinine measurement at 1, 6, and 12 months, and then yearly thereafter.
Results
Thirty-one renal arteries were stented successfully in 29 patients. The 18 patients with planned renal artery stent placement had a proximal neck length <15 mm. Mean proximal neck length was shorter in patients who underwent the “snorkel” technique (6.9 ± 3.1 mm) compared with those with planned endograft encroachment (9.9 ± 2.6 mm). None of the patients with unplanned endograft encroachment had neck lengths <15 mm (mean length, 26.3 ± 10.2 mm). Mean proximal neck angulation was 42.8° ± 24.0° and did not differ between the groups. One patient had a type I endoleak on completion angiography, and two additional patients had a type I endoleak on the first postoperative CT scan. All type I endoleaks resolved by the 1-month postoperative CT scan. The primary assisted patency of renal artery stents was 100% at a median follow-up of 12.5 months (range, 2 days-77.4 months). In one patient near occlusion of a renal artery stent was noted on follow-up CT scan at 9 months; patency was restored by placement of an additional stent. One patient required dialysis after sustained hypotension from a right external iliac artery injury that resulted in prolonged postoperative bleeding. Mean serum creatinine was 1.1 ± 0.3 mg/dL at baseline, 1.2 ± 0.5 mg/dL at 1 month of follow-up, and 1.2 ± 0.5 mg/dL at 2 years of follow-up. There were no late type I endoleaks (>1 month postoperatively) or stent graft migrations.
Conclusions
Adjunctive renal artery stenting during endovascular AAA repair using the “encroachment” and “snorkel” techniques is safe and effective. Short- and medium-term primary patency rates are excellent, but careful follow-up is needed to determine the durability of these techniques.
Endovascular repair of infrarenal abdominal aortic aneurysms (AAAs) is safe, durable, and effective only when the arterial anatomy permits sealing and fixation at the attachment sites.1, 2, 3 A sufficient length of proximal aortic neck is a critical requirement for successful endovascular aneurysm repair (EVAR) using the current devices approved by the United States Food and Drug Administration. Many patients with short infrarenal necks have been treated successfully using pararenal stent grafts with small holes (fenestrations) to the renal arteries.4, 5, 6 However, fenestrated stent grafts remain unavailable in the United States.
In the absence of fenestrated stent grafts, other ways to preserve renal perfusion include pushing the upper margin of a pararenal stent graft downwards—the “encroachment” technique—or inwards, away from the renal artery orifice using a renal artery stent—the “snorkel” technique. Partial encroachment on a renal orifice can be remedied after the fact by placing a stent over the top of the graft (Fig 1). More extensive coverage requires the adjunctive stent to run alongside the stent graft, creating a “snorkel” or “chimney” to the renal artery (Fig 2). In this technique, transbrachial access to the renal artery is secured before stent graft deployment.7, 8
Fig 1.
“Encroachment” of an aortic stent graft on the right renal artery, with the stented right renal artery.
Fig 2.
Left renal artery “snorkel” stent running parallel to the main body component of an aortic stent graft.
This single-center report describes the technical feasibility and short-term outcome of two adjunctive techniques during EVAR, renal encroachment and renal “snorkel”. Both techniques lengthen the proximal implantation site by covering the renal arteries while maintaining flow to the kidneys through renal stents.
Methods
Patients
Between August 2000 and August 2008, 29 patients underwent elective EVAR using bifurcated Zenith stent grafts (Cook Inc, Bloomington Ind) and simultaneous renal artery stenting at the University of California, San Francisco (UCSF) Medical Center. Only patients whose renal artery stents were placed (1) to treat planned or inadvertent encroachment of the stent graft fabric on a renal artery orifice or (2) as part of a planned “snorkel” procedure are included in this study. Patients who underwent renal artery stenting for stenosis in the absence of renal artery coverage were excluded. The choice of technique to lengthen the aortic neck (encroachment or “snorkel”) was at the discretion of the primary surgeon.
Patient demographic information, including age, gender, aneurysm size, medical comorbidities, and serum creatinine levels, were collected prospectively. Data on medical comorbidities included the presence of diabetes mellitus, chronic obstructive pulmonary disease, hypertension, hypercholesterolemia, and cardiac disease. Cardiac disease was defined as any history of myocardial infarction, congestive heart failure, ischemic heart disease, or atrial fibrillation.
Techniques
Selective renal catheterization for subsequent stent placement may be performed before stent graft deployment, or afterwards; through the proximal stent, or alongside it; from the femoral artery, or from the brachial artery. However, the net result depends mainly on the timing of renal catheterization relative to stent graft deployment. If renal catheterization follows stent graft deployment, the renal stent tends to run transaxially, pushing the proximal margin of the encroaching graft down towards the aneurysm; hence, the term “renal encroachment.” If renal catheterization precedes stent graft deployment, the renal stent tends to run axially alongside the graft as a small vertical conduit from the source of inflow to the orifice of the renal artery; hence, the term “snorkel.”
Endograft encroachment with adjunctive renal artery stenting
The Zenith stent graft is deployed according to the usual sequence, with particular attention to the position of the proximal margin of the graft relative to the most caudal renal artery. The position of the graft is shown by radiopaque markers sutured to the fabric, approximately 1 to 2 mm from the proximal margin. The position of the renal ostium is identified by catheter angiography. The optimal obliquity depends on the anteroposterior position of the renal artery (especially the right) and the anteroposterior angulation of the aorta. The extent of renal artery coverage is a product of two competing goals, renal preservation and augmented aortic overlap, which depends on neck length, neck angulation, renal function, and the relative positions of the two renal arteries. We seldom cover the entire renal artery.
Catheter access to the renal artery is obtained through the base of one of the triangular interstices of the uncovered proximal stent. The failure of a 5F catheter to follow a guidewire into the renal artery usually indicates that the wire has found its way into the renal artery through one of the narrow interstrut apices. The only option is to remove the wire and try again.
Clearly, one cannot traverse the uncovered stent in its compressed, undeployed state, but that does not mean it has to be fully deployed. The partially deployed uncovered stent assumes a conical shape, with the top cap constraining the proximal end while the distal end expands with the rest of the stent graft. Under these circumstances, a catheter can enter the stent graft through the contralateral gate and exit through the base of the uncovered stent on its way to the renal artery. The technique resembles a step in fenestrated stent graft insertion.
The choice of femoral vs brachial access depends largely on the orientation of the aorta. Catheters tend to follow the outer curvature of any angulation, and if the neck is angulated in a coronal plane, a transfemoral catheter will track towards one renal artery and away from the other. Switching to the transbrachial route often eliminates, or even reverses, this effect. In addition, transbrachial access may be the preferred alternative in cases of extensive renal coverage because the transbrachial route to the renal artery does not bend around the proximal margin of the stent graft.
Whatever the route of access, transbrachial or transfemoral, the guidewire has to be robust enough to push the struts of the uncovered proximal stent and the proximal margin of the graft margin aside for unimpeded renal stent insertion. We use a stiff 0.035-inch guidewire. The delivery profile of the corresponding stent is larger than the delivery profile of 0.014- and 0.018-inch stent platforms, but not to a degree that has practical consequences.
We implant short, balloon-expandable renal stents. Self-expanding stents are not robust enough. Approximately 5 mm of the renal stent is deployed into the aorta (Fig 3). This is the functioning part of the stent. Accurate deployment depends on the imaging system having the correct orientation; otherwise, the inner end of the stent appears to be further into the aorta than it actually is.
Fig 3.
A, Complete “encroachment” of an aortic stent graft is shown on the orifice of the left renal artery. B, A stent has been successfully placed into the left renal artery from the brachial approach. The arrows depict the proximal and distal extent of the renal artery stent.
Renal “snorkel”
The “snorkel” technique is only for cases of planned renal artery coverage because access to the renal arteries must precede stent graft deployment. As a result, the renal stent comes to lie entirely outside the stent graft. Because the renal stent does not traverse the uncovered stent, it pushes the proximal margin of the stent graft in, not down, and renal coverage is not limited by the maximum extent of graft deformation. In the “snorkel” technique, the only possible route of renal access is from above, through the brachial artery, because the source of arterial inflow is above the margin of the graft.
Transbrachial catheterization of the renal artery is performed (in the absence of a stent graft) much as it would be for the treatment of renal artery stenosis. We use a stiff 0.035-inch guidewire in case we have to reinstrument the renal artery after stent graft deployment, whereupon the additional stiffness would help sheaths, catheters, and balloons pass obstacles such as the uncovered stent and proximal graft margin. The stent graft is usually deployed by reference to the location of the contralateral renal artery on catheter angiograms, with the renal sheath, wire, and stent already in place (Fig 4, A).
Fig 4.
A, A balloon-expandable stent is placed over a Rosen wire from a brachial approach, while the Zenith main body component is placed through the femoral access site. B, Simultaneous inflation of balloon-expandable right renal artery stent and a Coda balloon (Cook Inc) with a Palmaz aortic stent. C, Completion angiography shows a right renal artery “snorkel” stent.
The length of the renal stent depends on the distance between the proximal margin of the graft and the renal orifice, because the functioning part of the renal stent lies within the aorta. The upper end of the renal stent is deployed at, or above, the margin of the graft, as indicated by the positions of radiopaque markers. We have used self-expanding stents, balloon-expandable stents, bare-metal stents, and covered stents. They all require the luminal support of a balloon during intra-aortic Palmaz stent (Cordis, Miami Lakes, Fla) deployment (Fig 4, B). Therefore, renal artery access is maintained until completion angiograms (Fig 4, C) show that no further aortic interventions will be needed to treat an endoleak.
Follow-up
Follow-up included routine contrast-enhanced computed tomography (CT), multiview abdominal radiographs, and serum creatinine levels postoperatively, at 1, 6, 12 months, and yearly thereafter. Two patients were monitored with noncontrast CT scans in addition to renal artery ultrasound examinations to determine the patency of the renal arteries. Data on complications and interventions were collected prospectively.
Anatomic measurements
Data on proximal neck length, diameter, and angulation were measured on preoperative CT scans by two authors (J. H. and T. C.). Neck angulation was defined as the angle between the neck of the aneurysm and the main channel of the infrarenal aortic aneurysm after 3-dimensional reconstruction of the images on an Aquarius Workstation (Terarecon, San Mateo, Calif).
Results
Patient demographics are summarized in Table I. There were 23 men and 6 women, with an average age of 74.4 ±7.6 years (range, 57.6-84.2 years). Baseline serum creatinine was 1.1 ± 0.3 mg/dL. Mean follow-up was 19.6 ± 21.5 months (median, 12.5 months; range, 2 days-6.5 years). One patient was lost to follow-up, and six of the 29 patients (20.7%) died.
Table I. Patient demographics
| Variable | Value |
|---|---|
| Age at operation, mean ± SD (range), y | 74.4 ±7.6 (57.6-84.2) |
| Gender, No. (%) | |
| Male | 23(79.3) |
| Female | 6(20.7) |
| Creatinine, mean ± SD (range) mg/dL | |
| Preoperative | 1.1±0.3(0.6-1.7) |
| Postoperative | 1.2±0.5(0.7-3.2) |
| 2 years | 1.2±0.5(0.7-2.5) |
| Follow-up, days | |
| Mean±SD | 597±655 |
| Median(range) | 381(2-2354) |
| Aneurysm size, mean ± SD (range) mm | 60±7(44-75) |
| Medical comorbidities, No (%) | |
| Diabetes mellitus | 5(17.2) |
| Cardiac disease | 20(69.0) |
| COPD | 3(10.3) |
| Past smoker | 24(82.8) |
| Hypertension | 21(72.4) |
| Hyperlipidemia | 20(69.0) |
Unilateral renal artery stents were placed in 27 patients (93.1%), and bilateral renal artery stents were placed in two patients. Thirty-one renal arteries were successfully stented in 29 patients. Twenty-three renal arteries in 21 patients were stented for endograft encroachment. Renal stent insertion was part of the planned intervention in 12 renal arteries in 10 of these patients, and was an unplanned response to inadvertent coverage in 11 renal arteries in 11 patients. The “snorkel” technique was used to stent eight renal arteries in eight patients.
Table II summarizes the details of the proximal neck, access sites, and stents used in the renal arteries. In the 18 patients with planned renal artery stent placement, the aortic neck lengths were <15 mm. The mean proximal neck length was shorter in patients who underwent the “snorkel” technique (6.9 ± 3.1 mm) compared with those with planned endograft encroachment (9.9 ± 2.6 mm). By using the “snorkel” technique, an additional 10.4 mm (range, 4.6-25.9 mm) of aortic neck length was gained. None of the 11 patients with unplanned renal stent placement had aortic neck lengths <15 mm (mean neck length, 26.3 ± 10.2 mm). No significant difference in neck angulation was noted among the three groups.
Table II. Characteristics of aneurysm neck and stented renal arteries
| Characteristic | Value |
|---|---|
| Renal artery | |
| Stent technique, No. | |
| Encroachment | |
| Planned | 12 |
| Inadvertent | 11 |
| “Snorkel” | 8 |
| Diameter, median (range), mm | 6(5-8) |
| Access site, No | |
| Femoral | 17 |
| Right brachial | 3 |
| Left brachial | 9 |
| Neck variables, mean±SD (range) | |
| Length, mm | 15.3±10.9 (3.0-48.6) |
| “Snorkel” | 6.9±3.1 |
| Encroachment | |
| Planned | 9.9±2.6 |
| Inadvertent | 26.3±10.2 |
| Diameter, mm | 24.2±4.0 (16.1-32.9) |
| Angulation, degrees | 42.8±24.0 (0-100.2) |
| Type of stents placed, No. | |
| Palmaza | 9 |
| Biliary Expressb | 13 |
| Zilverc | 2 |
| Genesisa | 4 |
| Cordisa | 1 |
| i-Castd | 1 |
| Acculinke | 1 |
| Visi-Prof | 1 |
aCordis, Miami Lakes, Fla. |
bBoston Scientific, Natick Mass. |
cCook Inc, Indianapolis, Ind. |
dAtrium Medical Corp, Hudson, NH. |
eAbbott Vascular, Abbott Park, Ill. |
fev3 Endovascular Inc, Plymouth, Minn. |
A type I endoleak was detected intraoperatively in five of 29 patients (17.2%), four of which occurred after endograft encroachment, and one after the “snorkel” technique. Four of these five type I endoleaks resolved after placement of a Palmaz stent. The first postoperative CT scan showed a type I endoleak in three patients (10.3%). Two of these had undergone a “snorkel” procedure and one had undergone renal stent placement for partial endograft encroachment on the renal artery. All of the type I endoleaks had resolved by the time of the next CT scan, 1 month after stent graft implantation. A type II endoleak was evident on the postoperative CT scan in eight of 29 patients (27.6%). No stent graft migration or late appearing (>1 month after EVAR) type I endoleaks occurred in the study cohort.
Three complications occurred in these patients. In one patient who underwent the “snorkel” procedure, the shoulder of the balloon caused a dissection in a stented renal artery; this was successfully treated by placement of a self-expanding nitinol stent. One patient required dialysis after sustained hypotension from a right external iliac artery injury that resulted in prolonged postoperative bleeding, colon ischemia, reoperation, and death. One patient in the encroachment group had CT findings suggestive of partial renal artery occlusion on a follow-up study at 9 months. A subsequent angiography demonstrated a high-grade stenosis in the region of the renal artery stent, which was successfully restented.
Preoperative imaging showed evidence of stenosis in three of 31 renal arteries (9.7%), all of which underwent successful stent placement. No restenosis occurred during the follow-up period in these patients. Primary assisted patency was 100% at a median follow-up of 12.5 months.
Discussion
Secure hemostatic proximal stent graft implantation is an absolute requirement for successful EVAR.1, 2, 3 Under ideal circumstances, the proximal end of the stent graft conforms to the shape of the neck, resulting in a long zone of overlap, secure hemostatic implantation, and durable aneurysm exclusion.9 If, on the other hand, the neck is short and more than half of the proximal stent resides within the aneurysm, the two may not assume the same shape, and the overlap will be neither stable nor hemostatic. In the case of the Zenith stent graft, the sole device in this series, the proximal stent is 15 mm long; therefore, any neck shorter than 10 mm will likely jeopardize infrarenal implantation. Other stent grafts have shorter proximal stents and some have been used to treat short necks, but these stent grafts have problems such as migration.10, 11, 12
The other approach is to change the aortic side of the equation by implanting the stent graft at a more proximal level and simultaneously maintaining flow through, over, or around the stent graft using renal stents. Fenestration has the best track record of durable success, but the necessary stent grafts are not available in the United States.5, 6 In addition, fenestrated stent grafts take a long time to prepare and cannot be used in symptomatic patients for whom long manufacturing delays may be fatal.
The technique referred to here as “encroachment” was first used as a way to rescue inadvertent renal artery coverage. The barbs of the Zenith stent graft permit very little caudal movement once the proximal stent has been deployed. Traction on a balloon or cross-femoral wire is ineffective and potentially dangerous. When the degree of encroachment is small, it is rarely difficult to obtain access to the renal artery, using either a transfemoral or transbrachial approach, as evidenced by our high success rate. The most difficult aspect may be making the diagnosis, because the partially occlusive graft wall may create no filling defect or apparent delay in renal perfusion.
More extensive renal coverage does impede renal catheterization. Planned coverage of the entire renal artery calls for preemptive renal access through a brachial approach, as in the “snorkel” technique.7 We have not performed bilateral “snorkel” procedures. We consider the proximity of the aneurysm to both renal arteries a relative contraindication to the procedure because we are hesitant to risk the perfusion to both kidneys.
Our limited experience with this approach has yielded a high rate of success under difficult circumstances. Others have reported similar technical success with this approach, also known as the “chimney” technique, in the pararenal aorta and the aortic arch.8, 13, 14 The long-term durability of this technique has yet to be determined, however.
The ideal characteristics of the renal artery stent (self-expanding vs balloon-expandable, covered vs uncovered) for use in the “snorkel” procedure have also not yet been clarified. We have used all of the above in our small series of patients. Only one covered stent was used in our series because these tend to be bulkier, stiffer, and more difficult to insert than uncovered stents. We are not entirely convinced that the covered nature of the stent confers any advantage in renal patency or any protection against a type I endoleak. Our current preference is to use a balloon-expandable uncovered stent in the renal artery during “snorkel” procedures because these stents have good trackability, deploy precisely, and provide excellent radial force.
Because our primary indication for the elective combination of renal stents and aortic stent grafts was the presence of a short proximal aortic neck, our data on neck length warrant scrutiny (Table II). Inadvertent renal coverage was the result of technical error. Some of the necks were short, but most were quite long, certainly long enough to meet the standard selection criteria for EVAR.
The same cannot be said of planned encroachment, where the mean proximal neck length was <10 mm. However, the neck lengths for the cases of planned encroachment were not as short as the necks treated using the “snorkel” technique. The need to obtain access to the already covered renal artery limited the degree of encroachment, which limited the additional neck length to be gained.
In the “snorkel” technique, access to the lower renal artery was obtained before stent graft implantation. The upper margin of the stent graft was deployed at the inferior margin of the contralateral renal artery. The resultant increase in the length of the implantation site allowed the inclusion of shorter necks, some as short as 3 mm.
Our technical success rate and overall small number of patients in this series deprived us of the data we need to determine the minimum neck length required for each technique. In our series, there were no cases in which too much renal artery was covered for subsequent catheterization and stenting. In addition, there were very few cases of type I endoleak and none of persistent type I endoleak.
We recognize that although adjunctive renal stenting may permit successful EVAR in the presence of a short neck, it will not work when there is no neck. Some neck is necessary to create a seal below the renal arteries, because the presence of a renal stent disrupts contact between the pararenal stent graft and the pararenal aorta. In some cases the infrarenal seal needs a little help in the form of a Palmaz stent. This can be a problem when the inflation of a balloon within the aorta threatens the patency of an adjacent renal stent. Our current approach avoids the issue by deploying the renal stent at the same time as the routine planned deployment of an aortic Palmaz stent in cases where the “snorkel” technique is used.
Long-term data will be needed to assess whether a hyperplastic response to the presence of the renal stent will ultimately threaten renal perfusion. This does not appear to be a common outcome in the short to medium term. The sole case of intraoperative renal artery injury was the result of a technical error: the balloon was inflated beyond the orifice of the stent. Only one patient in this series had a late occurring renal artery occlusion. This observation is consistent with data from reports on stent graft fenestration, which resemble our own series in that most stented renal arteries were free of intrinsic disease.5, 6
The Zenith stent graft has two particular advantages in patients with a short proximal aortic neck. First, the two-stage deployment allows precise positioning, especially when one accounts for the downward displacement of the renal arteries that sometimes accompanies sheath withdrawal in patients with iliac artery disease. Second, the uncovered proximal stent provides secure suprarenal fixation, which is especially important because the short neck provides little infrarenal fixation, and even small amounts of migration would cause type I endoleak, aneurysm pressurization, and rupture. Although the uncovered proximal stent has the advantage of additional fixation, it has the potential disadvantage of complicating renal catheterization in cases of encroachment. Nevertheless, the gaps between stent apices are generally wide enough to accommodate a catheter and stent.
Although the renal encroachment technique does not require any unique variation in the sizing or deployment of the aortic stent graft, the “snorkel” procedure does. When we use the “snorkel” technique, we oversize the main body component by an extra 2 to 4 mm to create a gutter for the renal stent. For example, for a 28-mm neck, we would use a 36-mm device instead of a 32-mm device. The additional oversizing allows the fabric of the main body to fold around the renal stent (Fig 5).
Fig 5.
In this fly-through view from a postoperative computed tomography scan in a patient who underwent placement of renal artery “snorkel,” the white arrow designates the renal artery “snorkel” stent; the block yellow arrow, the aortic stent graft with the Palmaz stent; and the thin yellow arrow, the superior mesenteric artery orifice.
Conclusions
Adjunctive renal artery stents can be combined with currently available, off-the-shelf stent grafts to increase the length of the proximal implantation site, allowing successful EVAR in patients who have short proximal aortic necks. These techniques may be particularly useful for patients at high risk for open surgical repair who do not have access to enrollment in a fenestrated or branched-graft protocol. These techniques may also be used in the emergency setting or as part of a recovery maneuver when a renal artery is inadvertently covered. Careful follow-up is needed to determine the long-term stability of the stent graft and the patency of the renal stent.
Author contributions
References
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This publication was supported by NIH/NCRR/OD UCSF-CTSI Grant Number KL2 RR024130. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Institutes of Health.
Competition of interest: Dr. Chuter has licensed patents to Cook Inc, the manufacturer of the Zenith stent graft, and receives research and travel support from Cook Inc.
PII: S0741-5214(08)01994-0
doi:10.1016/j.jvs.2008.11.060
© 2009 Society for Vascular Surgery. All rights reserved.
