Embolic Protection Devices in RAS
The use of a distal embolic protection device can significantly improve results after stent revascularization for patients with ischemic nephropathy.
The common indications for endovascular revascularization of renal artery stenosis (RAS) are renovascular hypertension and ischemic nephropathy. Patients with a critical RAS, normal renal function, and hypertension refractory to optimum medical therapy are rewarding to treat with endovascular techniques for two reasons: (1) satisfactory results reported in the literature,1,2 with approximately 60% of all patients experiencing improvement in blood pressure control after endovascular stent revascularization; and (2) several noninvasive investigations that can more accurately predict candidates who will benefit from endovascular stent revascularization. These investigations include captopril scintigraphy and renovascular Doppler assessment of intrarenal vascular resistance.3,4
Conversely, patients with critical renal artery stenoses and impaired renal function (ischemic nephropathy) are more challenging to treat. Many studies have reported a low incidence (15% to 25%) of improved renal function after revascularization.1,5 There are no reliable noninvasive predictors of patients who will maintain or improve renal function after revascularization, although assessment of intrarenal vascular resistance may have a role.4 Even more concerning is a procedure-related acute decline in renal function after stent revascularization, which has a reported incidence of 10% to 20%1,5,6 but may actually be much higher. The combination of poor reported results and the potential for the procedure to exacerbate the patient’s condition has significantly restricted the use of stent revascularization for ischemic nephropathy.
Cholesterol atheroembolization is the embolization of atheromatous fragments and cholesterol crystals. A number of studies have documented emboli occurring during the endovascular treatment of carotid bifurcation stenoses,7 usually with transcranial Doppler. Whereas guidewire and catheter traversal of stenoses may produce emboli, the most embologenic event during carotid stent revascularization is angioplasty.7 Experience has shown that the use of cerebral protection devices can reduce cerebral damage.8
It is likely that the renal vasculature is even more susceptible than the cerebral circulation to cholesterol atheroembolization during stent revascularization. Most reported cases of atheroembolization have occurred as a result of catheter or surgical manipulation of a severely atheromatous aorta.9 Because most atheromatous stenoses occurring at the renal artery ostium involve atheroma of the aortic wall, the risk of embolization during endovascular manipulation is likely to be high. We believe cholesterol atheroembolization is the underlying etiology in many cases of procedure-related decline in renal function and may account for the poor reported results after stent revascularization for ischemic nephropathy.
EXPERIENCE WITH RENAL ANGIOPLASTY AND STENTING WITH DISTAL PROTECTION IN ISCHEMIC NEPHROPATHY
We have recently reported our experience using stent revascularization and distal protection in patients with ischemic nephropathy.10 Prospective analysis was performed on consecutive patients who presented with chronic renal impairment and a decline in renal function during the preceding 6 to 12 months. All patients had critical (>80% diameter loss) stenoses involving the main renal artery ostium with diagnosis made on contrast-enhanced MRA in the majority of cases (Figure 1A).
In 95% of patients, renal function was stabilized or improved after stent revascularization with distal protection, as measured by creatinine clearance. This response was maintained during a mean follow-up period of 12.5 months. The remaining 5% of patients experienced an unchanged decline in renal function but no patients had acute procedure-related deterioration. Interestingly, the distal protection filters contained atheroembolic material in 65% of cases.
These results compared favorably with a historical group of ischemic nephropathy patients treated with stent revascularization but without distal protection. The results also compare favorably with many reported series in the literature.
TECHNIQUES FOR RENAL STENTING WITH A DISTAL PROTECTION DEVICE
The first reported experience in renal artery stenting with protection involved a distal temporary occlusive balloon.11 However, proximal or distal occlusive protection systems have the potential to cause permanent nephron loss because renal arterial anatomy lacks the excellent collateral supply of the cerebral circulation. For this reason, we believe the ideal renal protection system should be a distal filter. Most of our experience has been gained with use of the AngioGuard emboli capture guidewire system (Cordis Corporation, a Johnson & Johnson company, Miami, FL).
A further difference between stenting carotid and renal arteries is the relationship of the target artery to the access artery. In carotid stenting, the target artery (internal carotid artery) is approximately parallel to the access artery (common carotid artery). Conversely, in renal artery stenting, the target artery (main renal artery) is approximately perpendicular to the access artery (abdominal aorta). This orientation puts much greater demand on the profile and support strength of the distal filter-guidewire system. We initially experienced major difficulties in passing a filter-guidewire through a stenosed main renal artery ostium. We therefore used a “Dotter dilatation” technique with a guide catheter and central dilator passed through the stenosis after initially passing an 0.018-inch guidewire. Occasionally, we used a “buddy wire” for extra support strength. More recently, primary passage of the distal filter-guidewire system has become a standard technique for two main reasons. First, the profile of filter-guidewire systems has been reduced. Second, we have found that contrast-enhanced MRA and preliminary digital subtraction (Figure 1B) allows selection of an appropriately shaped guide catheter (eg, multipurpose or renal double curve) to provide a stable position at the renal artery ostium (Figure 2).
The filter (mounted on a 0.014-inch guidewire) is passed through the stenosis (Figure 3A), positioned in the distal main renal artery, and deployed (Figure 3B). A high radial force balloon-expandable stent mounted on a low-profile angioplasty balloon is introduced and optimally positioned at the renal artery ostium using angiography via the guide catheter. The stent is deployed (Figure 4), and the filter is recaptured. Most of our experience is with a femoral approach, although we have performed these procedures via a brachial approach.
THE IDEAL FILTER-BALLOON-STENT SYSTEM FOR RENAL ARTERIES
The features of an ideal filter-balloon-stent system are listed in Table 1. This system has not yet been developed. The issues of filter-guidewire profile and support strength have been discussed. Short guidewire lengths and rapid exchange balloon systems reduce procedure times. The overall length of the filter-balloon-stent is important in renal arteries. The main renal artery is usually approximately 4 cm long, although early branching occurs in approximately 10% of kidneys.12 If the total length of the filter-balloon-stent is >4 cm (as is the case with many of the systems now available), the filter will be deployed in a lobar artery, leaving some of the kidney unprotected.
IN WHICH ISCHEMIC NEPHROPATHY PATIENTS IS STENT REVASCULARIZATION INDICATED?
In our experience, the best results were achieved with stent revascularization in patients with moderate renal impairment (creatinine clearance, 15 to 40 mL/min).10 These patients have significant renal insufficiency but still have a reasonable amount of salvageable renal parenchyma. Natural history studies of RAS have shown that at least 50% of significant renal artery stenoses will progress, with progression more marked in more severe stenoses and bilateral disease.13 Based on these data, we select candidates for stent revascularization who have critical atherosclerotic ostial renal artery stenoses, mild-to-moderate chronic renal impairment, and evidence of a decline in renal function during the preceding 6 to 12 months.
The use of a distal filter device during stent revascularization of renal artery stenoses can significantly improve results in the ischemic nephropathy patient group. The most attractive patient group has critical atherosclerotic ostial renal artery stenoses, mild-to-moderate chronic renal impairment, and evidence of a recent decline in renal function. Advances in filter and balloon-stent technology allow primary passage of the filter and primary stenting in most patients. However, the ideal filter-balloon-stent system has not yet been developed.
Andrew Holden, MBChB, FRANZCR, is Director of Interventional Radiology, Auckland Hospital, Auckland, New Zealand. He holds no financial interest in any product or manufacturer mentioned herein. Dr. Holden may be reached at +64-9-3797440; firstname.lastname@example.org.
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