Acs0610 Carotid Angioplasty And Stenting


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Acs0610 Carotid Angioplasty And Stenting

  1. 1. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 1 10 CAROTID ANGIOPLASTY AND STENTING Timothy M. Sullivan, M.D., F.A.C.S. Choice between Carotid Endarterectomy and Carotid who were determined to be good operative risks on the basis of Angioplasty and Stenting reasonable life expectancy (so that they would be available for fol- low-up) and the absence of other potential causes of stroke (e.g., OPERATIVE RISK AND PATIENT SELECTION atrial fibrillation) [see Table 1]. For most patients with high-grade carotid lesions, whether A number of investigators have examined the results of CEA symptomatic or asymptomatic, surgical endarterectomy [see 6:9 in high-risk subsets, albeit using several different ways of defining Treatment Carotid Artery], rather than best medical therapy (i.e., high risk [see Tables 2 and 3].4-8 In one study, involving 776 con- risk factor reduction and antiplatelet agents), is the treatment of secutive CEAs performed at the Mayo Clinic in Rochester, choice for stroke prophylaxis, as the North American Sympto- Minnesota, procedures were categorized as high risk according to matic Carotid Endarterectomy Trial (NASCET) and the Asymp- the patient inclusion and exclusion criteria employed by the tomatic Carotid Atherosclerosis Study (ACAS) have shown.1,2 In Stenting and Angioplasty with Protection in Patients at HIgh Risk NASCET, the risk of disabling stroke or death after carotid for Endarterectomy (SAPPHIRE) trial of CAS with cerebral endarterectomy (CEA) was 1.9%, and the risk of minor stroke embolic protection.4 Of the 776 CEAs, 323 (42%) were consid- was 3.9%. In ACAS, the risk of major stroke or death was 0.6% ered high risk. The clinical presentation was similar in the high- when the 1.2% risk of stroke caused by diagnostic arteriography risk and low-risk groups.The overall postoperative stroke rate was was excluded.These findings have led to the performance of CEA 1.4% (symptomatic, 2.9%; asymptomatic, 0.9%), and there was in increasing numbers of patients. no statistical difference in stroke rate between the high-risk group In the past few years, however, despite the proven efficacy of and the low-risk group. Patients at significantly increased stroke CEA in preventing ischemic stroke, great interest has been gener- risk were those who underwent cervical radiation therapy, those ated in carotid angioplasty and stenting (CAS) as an alternative to who had class III or IV angina, those who were symptomatic at surgical therapy, and CEA has again come under attack.3 Pro- presentation, and those who were 60 years of age or younger. ponents of CAS have suggested that the results of NASCET and Overall mortality was 0.3% (symptomatic, 0.5%; asymptomatic, ACAS are not achievable in general practice outside selected cen- 0.2%), and there was no significant difference between the high- ters of excellence. Both ACAS and NASCET included patients risk group (0.6%) and the low-risk group (0.0%). Myocardial Table 1 High-Risk Carotid Endarterectomy as Defined by Major Exclusion Criteria for NASCET and ACAS Exclusion Criteria Study Comorbidities Age History Anatomic Criteria Cardiac Pulmonary Renal Other Unstable angina Uncontrolled hypertension Contralateral CEA < 4 mo Atrial fibrillation Previous ipsilateral CEA Renal Uncontrolled diabetes Major surgical procedure Valvular heart mellitus Tandem lesion > target > 79 yr Pulmonary failure failure NASCET1 < 1 mo disease stenosis Hepatic failure Stroke in evolution Symptomatic CHF Cancer with < 50% chance of 5-yr survival MI < 6 mo > 180 mm Hg systolic BP, > 115 mm Hg Unstable angina diastolic BP Previous ipsilateral Atrial fibrillation Fasting glucose > 400 CEA Major surgical procedure Pulmonary failure ACAS2 > 79 yr < 1 mo Valvular heart with impact on 5-yr SCr > 3 mg/dl Tandem lesion > target disease survival mg/dl Hepatic failure stenosis Stroke in evolution Symptomatic Cancer with < 50% Cervical radiation CHF chance of 5-yr survival treatment Active ulcer disease Warfarin anticoagulation ACAS—Asymptomatic Carotid Atherosclerosis Study BP—blood pressure CEA—carotid endarterectomy CHF—chronic heart failure MI—myocardial infarction NASCET—North American Symptomatic Carotid Endarterectomy Trial SCr—serum creatinine concentration 1
  2. 2. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 2 Table 2 High-Risk Carotid Endarterectomy as Defined by Major Inclusion Criteria for Selected Studies Inclusion Criteria Study Comorbidities Age History Anatomic Criteria Cardiac Pulmonary Renal Other Ouriel (2001)5 PTCA or CABG < 6 mo Severe COPD SCr > 3 History of CHF mg/dl Coronary procedure < 1 mo Previous ipsilateral CEA CABG < 6 wk Cervical radiation treatment Jordan (2002)6 Major vascular proce- FEV1 < 1 L Angina NYHA class III or IV Contralateral carotid occlusion dure < 1 mo Home oxygen EF < 30% High cervical lesion MI < 4 wk Lesion below clavicle NYHA functional class III or IV Previous ipsilateral CEA Heart failure Canadian class Steroid dependence SCr > 3 Cervical radiation treatment Gasparis (2003)7 ≥ 80 yr mg/dl III or IV Oxygen dependence Contralateral carotid occlusion CABG < 6 mo High cervical lesion Open heart surgery < 6 wk Previous ipsilateral CEA MI < 4 wk Severe tandem lesion Angina CCS class III or IV Cervical radiation treatment SAPPHIRE8 > 80 yr CHF class III or IV Contralateral carotid occlusion EF < 30% High cervical lesion (at least C2) Abnormal cardiac stress test Lesion below clavicle result Contralateral laryngeal palsy CABG—coronary artery bypass graft CCS—Canadian Cardiovascular Society CEA—carotid endarterectomy CHF—congestive heart failure COPD—chronic obstructive pulmonary disease EF—ejection fraction FEV1—forced expiratory volume in 1 sec MI—myocardial infarction NYHA—New York Heart Association PTCA—percutaneous transluminal coronary angioplasty SAPPHIRE—Stenting and Angioplasty with Protection in Patients at High Risk for Endarterectomy SCr—serum creatinine concentration infarction (MI) was more frequent in the high-risk group (3.1%) tive pulmonary disease (COPD), or the development of renal than in the low-risk group (0.9%); notably, all of the MIs report- insufficiency [see Table 2]. For the composite end point of stroke, ed in this series were nontransmural (non-Q). A composite cluster death, and MI, the rate was 3.8% in the group as a whole (stroke, of adverse clinical events (death, stroke, and MI) was more fre- 2.1%; MI, 1.2%; death, 1.1%); however, the rate was 7.4% in quent in the symptomatic high-risk group (9.3% versus 1.6%), high-risk patients, compared with only 2.9% in low-risk patients. but not in the asymptomatic cohort. There was a trend toward a High-risk patients were further subdivided into those who under- higher incidence of major cranial nerve injuries in patients with went CEA alone and those who underwent CEA combined with local risk factors (e.g., high carotid bifurcation, reoperation, and coronary artery bypass grafting (CABG). Not surprisingly, the cervical radiation therapy) (4.6% versus 1.7%). In 121 patients incidence of the composite end point was greater in the latter sub- who were excluded on the basis of synchronous or immediate sub- group than in the former. Among patients who underwent CEA sequent operations (and who would also have been excluded from SAPPHIRE), the overall rates of stroke (1.65%), death (1.65%), and MI (0.83%) were not significantly different from those in the study population. The authors concluded that SAPPHIRE-eligi- Table 3—SAPPHIRE Exclusion Criteria for ble high-risk patients could undergo CEA with stroke and death Defining High-Risk Carotid Endarterectomy8 rates that were well within accepted standards and that patients Acute stroke (≤ 48 hr) with local risk factors were at higher risk for cranial nerve injuries Staged elective procedure (within 30 days after CEA) but not necessarily for stroke.These conclusions call into question Elective percutaneous intervention the application of CAS as an alternative to CEA, even in high-risk Contralateral CEA patients. Other elective operation Although these studies do not support the premise that opera- Synchronous operation CCA angioplasty/stenting or bypass tive risk is higher in patients who would be excluded by NASCET Cardiac operation and ACAS criteria, there may in fact be categories of patients for Noncardiac operation whom CEA is not optimal therapy.A 2001 study from the Cleveland Intracranial pathology Clinic attempted to identify a subgroup of patients who, on retro- Intracranial mass spective analysis, were at increased risk for CEA and therefore Aneurysm > 9 mm might be better served by CAS.5 A total of 3,061 carotid endar- Arteriovenous malformation terectomies were examined from a prospective database covering Ventriculoperitoneal shunt a 10-year period. A high-risk cohort was identified on the basis of CCA—common carotid artery CEA—carotid endarterectomy the presence of severe coronary artery disease, a history of conges- tive heart failure (CHF), the presence of severe chronic obstruc-
  3. 3. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 3 Reasonable criteria for patients at increased risk for CEA have Table 4—Indications for Carotid Angioplasty been developed, and these criteria may serve as a general guide for and Stenting in High-Risk Patients those embarking on a program of carotid stenting [see Table 4]. Relative contraindications to CAS have also been determined [see Severe cardiac disease Table 5]. CAS should be performed by physicians with a thorough Requirement for PTCA or CABG knowledge of the pathophysiology and natural history of carotid History of CHF disease and by those with current expertise in peripheral, cardiac, Severe chronic obstructive pulmonary disease or neurointerventional procedures. If a physician is unable to par- Requirement for home oxygen ticipate in FDA-approved trials, the procedure should be per- FEV1 < 20% of predicted Severe chronic renal insufficiency formed as part of a local Institutional Review Board (IRB)–approved Serum creatinine concentration > 3.0 mg/dl protocol with dispassionate oversight, independent preprocedural Patient currently on dialysis and postprocedural neurologic examination, and prospective case Previous CEA (restenosis) review. In addition, development of a carotid stenting program Contralateral vocal cord paralysis may facilitate cooperation among the various specialists who may Surgically inaccessible lesions desire to participate in this high-profile arena. A team of experi- Lesion at or above C2 enced personnel should be assembled (including one or two physi- Lesion inferior to clavicle cians and a technician) to ensure patient safety, maximize expo- Radiation-induced carotid stenosis sure within a small cadre of operators, and avoid duplication of Previous ipsilateral radical neck dissection effort. All patients considered for CAS should have provided in- formed consent, should receive counseling regarding the risks and benefits vis-à-vis CEA and best medical therapy, and should pos- alone, the risk of death was significantly greater in the high-risk sess a clear understanding of the investigational nature of the pro- group. Notably, although the risk of the combined end point was cedure. In addition, patients must agree to regular and careful fol- greater in the high-risk group, the difference was not statistically low-up examinations. significant. In addition, the rates of the individual end points of MI and stroke did not differ statistically between the high-risk group and the low-risk group. Anatomic Considerations These data appear to support the notion that the patients A solid grasp of the anatomy of the cerebral circulation is cru- enrolled in the multicenter trials of CEA (i.e., NASCET and cial for the planning of CAS. Several anatomic considerations are ACAS) were probably similar to the Cleveland Clinic’s low-risk particularly germane. The first important anatomic issue to ad- group and that the patients in the Clinic’s high-risk group might not dress is the configuration of the aortic arch. With advancing age, have had such stellar outcomes if they had been included in those the apex of the arch tends to become displaced further distally. multicenter endarterectomy trials. Subsequent clinical investiga- This displacement makes selective catheterization of the brachio- tions have therefore focused on medically compromised, high-risk cephalic vessels more challenging and influences the choice of a patients who may benefit from alternatives to CEA (e.g., CAS). catheter.The interventionalist should become familiar with a vari- ety of selective catheters; I prefer the Simmons II catheter, which INDICATIONS FOR CAROTID ANGIOPLASTY AND STENTING allows deep cannulation of the common carotid artery (CCA) and The basic indications for CAS are essentially the same as those thus facilitates eventual passage of a guide wire for delivery of a for standard open CEA: sheath. As the level of the vessel’s origin becomes farther from the dome of the arch, the task of obtaining guide-wire and sheath access 1. Asymptomatic lesions that fall within the high-grade steno- becomes more difficult. Cannulation of a left CCA arising from a sis (typically, 80% to 99%) range on duplex ultrasonography, common brachiocephalic trunk (bovine arch) may be particularly which correlates with a stenosis of at least 60% on angiogra- phy [see 6:2Asymptomatic Carotid Bruit]. Most clinical trials of CAS in asymptomatic patients require an angiographic stenosis of at least 80% for study inclusion. 2. Symptomatic lesions (e.g., hemispheric transient ischemic Table 5—Limitations of and Contraindications attack [TIA], amaurosis fugax, or stroke with minimal resid- to Carotid Angioplasty and Stenting ua) with a stenosis of at least 70% on angiography [see 6:1 Stroke and Transient Ischemic Attack]. Symptomatic patients Inability to obtain femoral artery access who have greater than 50% stenoses with ulceration may Unfavorable aortic arch anatomy benefit from endarterectomy; however, this finding has not Severe tortuosity of CCA or ICA yet been extrapolated to carotid intervention. Severely calcified or undilatable stenoses Lesions containing fresh thrombus At present, there is a paucity of data on the safety and efficacy Large amount of laminated thrombus at site of patch angioplasty of CAS from well-controlled studies. Accordingly, my use of CAS (previous CEA) on duplex ultrasonography to date has been limited to treatment of patients judged to be at Extensive stenoses (> 2 cm) high risk for CEA. Such patients account for 10% to 20% of all Critical (≥ 99%) stenoses Lesions adjacent to carotid artery aneurysms patients undergoing intervention (open or endovascular) for Contrast-related issues carotid disease in the Division of Vascular Surgery at the Mayo Chronic renal insufficiency Clinic. Symptomatic patients with greater than 50% angiograph- Previous life-threatening contrast reaction ic stenosis and asymptomatic patients with greater than 80% Preload-dependent states (e.g., severe aortic valvular stenosis) angiographic stenosis are considered for intervention, either surgi- cal or percutaneous.
  4. 4. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 4 difficult to accomplish. Problematic arch configurations should be and more efficient, thereby serving the main goals of therapy— identified on preprocedural contrast studies or magnetic reso- patient safety and exemplary results. nance angiography (MRA) of the aorta. Especially at the begin- In all cases, a thorough history should be taken and a careful ning of an operator’s experience with these interventions, a com- physical examination performed, with close attention paid to plete study that includes the aortic arch and the origins of the bra- comorbid medical conditions and femoral pulses. A complete neu- chiocephalic trunks is essential. rologic examination should be performed by a certified neurolo- Another important issue is the presence of any tandem lesions gist. In addition to preprocedural arteriography or MRA, duplex along the course of the cerebral circulation. If a proximal CCA ultrasonography should be performed in an accredited vascular lesion is present, intervention may be required before revascular- laboratory—ideally, the same laboratory where the follow-up ization of the internal carotid artery (ICA) to provide safe access examinations will be performed. In my practice, patients receive to the ICA. A third concern is the tortuosity of the ICA. Fortun- aspirin, 325 mg daily for at least 1 week before the procedure, and ately, most ICAs are relatively straight; however, some patients do clopidogrel, 75 mg daily for at least 3 days beforehand. All patients have extremely tortuous ICAs, which may preclude safe passage of receive antibiotics (typically cefazolin, 1 g I.V.) immediately before a guide wire or protection device and thus render safe intervention the procedure. impossible. The anatomic configuration of the external carotid artery (ECA) typically is not an important consideration in carotid intervention, even when this vessel is affected by stenosis of iatro- Operative Planning genic origin or is covered with a bare stent. Regardless of the exact physical location at which the procedure Finally, the collateral circulation (or lack thereof) through the is performed, access to high-quality imaging equipment is manda- circle of Willis is an important consideration that may profound- tory; portable C-arms are less than ideal for this purpose. I per- ly influence procedural strategy. The status of the contralateral form CAS in a neuroradiology suite, which has the advantage of ICA, the vertebral-basilar system, and the intracranial collaterals offering biplane imaging. With this arrangement, duplication of may affect the type of embolic protection to be used. Anatomic effort and equipment is avoided, and the room is staffed by knowl- variations in the circle of Willis are actually the rule rather than edgeable personnel, including a certified registered nurse anes- the exception: a complete circle is present in fewer than 50% of thetist (CRNA), who monitors the patient with continuous electro- all cases. Common variations include a hypoplastic (10%) or cardiography, blood pressure, and pulse oximetry readings. Patients absent A1 segment of the anterior cerebral artery and a plexi- are placed in a supine position, and both groins are prepared.The form (10% to 33%) or duplicated (18%) anterior communicat- head is placed in a cradle and gently secured to minimize patient ing artery. Anomalies of the posterior portion of the circle of motion during critical portions of the procedure.The procedure is Willis, including a hypoplastic (33%) or absent posterior com- generally performed with the patient awake, though minimal seda- municating artery, occur in about 50% of all cases. During pre- tion is acceptable if the patient is particularly anxious. procedural angiography or intracranial MRA, careful attention should be paid to the anterior and posterior communicating arteries. Patients with limited collateral circulation may exhibit Operative Technique reversible neurologic symptoms during inflation of a protective The technique of CAS has evolved over time. In its current iter- balloon or angioplasty of the target lesion. They may also be ation, the procedure is performed in the following steps, with few at higher risk for permanent neurologic deficits because their exceptions. Although it is, of course, essential to be able to adjust limited collateral blood supply is less likely to be able to com- the technical approach to deal with unanticipated situations, it is pensate for any iatrogenic arterial occlusions that may compli- nonetheless desirable to standardize the procedure as much as cate the procedure. possible. Basic principles of endovascular surgery are discussed more fully elsewhere [see 6:8 Fundamentals of Endovascular Surgery]; the Preoperative Evaluation following steps focus primarily on the procedural elements specif- For those with limited experience in carotid intervention, ic to CAS. diagnostic angiography of the arch, the carotid arteries, and the STEP 1: SECURING OF ARTERIAL ACCESS cerebral arteries (done well in advance of the proposed interven- tion) is suggested; high-quality MRA that includes the aortic Retrograde femoral access is obtained, and a 5 French sheath is arch is an acceptable substitute. This measure allows the inter- inserted. Full heparin anticoagulation (typically 100 mg/kg body ventionalist to make a careful, unhurried evaluation of the aortic mass) is instituted after arterial access is secured and before mani- arch and the brachiocephalic origins, which is essential for pulation of catheters in the aortic arch and the brachiocephalic determining the ease or difficulty of sheath or guide access to the vessels. CCAs—an issue that has a direct bearing on the success of the STEP 2: SELECTIVE CAROTID CATHETERIZATION procedure. If the brachiocephalic trunk (i.e., the innominate AND ARTERIOGRAPHY artery) or the left CCA originates in a location more than two CCA diameters (approximately 2 cm) below the dome of the After selective catheterization of the midportion of the ipsilater- aortic arch, some difficulty with access should be anticipated. In al CCA (typically with a Simmons II catheter), a selective arterio- addition, the length of the lesion, the maximum degree of steno- gram of the carotid bifurcation is obtained, with care taken to choose sis, and the diameters of the CCA and the ICA can be deter- a view that yields minimal overlapping of the ICA and the ECA mined by placing a radiopaque marker of known diameter in the and maximal visualization of the target lesion. A complete cerebral field of view; ball bearings of progressively increasing diameter (2 arteriogram, if not previously obtained, is obtained at this point to through 7 mm) are ideal for this purpose. These measurements serve as a baseline and to identify any intracranial pathologic con- facilitate preprocedural selection of appropriately sized balloons ditions (e.g., aneurysms and arteriovenous communications) that and stents and make performance of the procedure smoother may be present.
  5. 5. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 5 STEP 3: ADVANCEMENT OF SHEATH AND DILATOR INTO The second technique is to use a variable-diameter wire (0.018 COMMON CAROTID ARTERY in. at the tip, enlarging to 0.035 in. more proximally) to cross the Two techniques have been employed to advance a sheath into internal carotid lesion, providing additional guide-wire support to the CCA. The first technique—which I prefer—is to place an ex- facilitate sheath advancement. This approach is a reasonable change-length guide wire into the terminal branches of the ECA. option, but it ultimately necessitates crossing the target lesion I generally use a stiff, angled glide wire (keeping in mind that twice. sheath exchange over this highly slippery wire can be tricky). The STEP 4: REMOVAL OF GUIDE WIRE AND DILATOR AND diagnostic catheter and the 5 French sheath are removed (with SELECTIVE ARTERIOGRAPHY OF CAROTID BIFURCATION constant visualization of the guide wire in the ECA maintained all the while). A long (70 to 90 cm, depending on patient body habi- Once the sheath is in place, the guide wire and the dilator are tus) 6 French sheath is then advanced, along with its dilator, into removed.The sheath sidearm may be attached to a slow, continu- the CCA. If larger (> 8 mm in diameter) stents are required, a 7 ous infusion of heparin-saline solution to keep blood from stag- French sheath may be necessary to allow contrast injection around nating in the sheath. Selective angiography of the carotid bifurca- the stent delivery system. Care must be taken to identify the tip of tion is then performed through the sheath, again demonstrating the dilator (which is not radiopaque) because it may extend a sig- the area of maximal stenosis, the extent of the lesion, and the nor- mal ICA and CCA above and below the lesion. Roadmapping, if nificant distance from the end of the sheath, depending on the available, is helpful in crossing the lesion with an embolic protec- brand of sheath used. Obviously, inadvertently advancing the dila- tion device or guide wire [see Step 6, below]. The overwhelming tor into the carotid bulb may have disastrous consequences. In majority of procedures are now performed with the aid of an patients with short CCAs or low bifurcations, the sheath can be embolic protection device, typically a filter [see Figure 1]. advanced over the dilator once the sheath edge (a radiopaque marker) is past the origin of the CCA. STEP 5: MEASUREMENT OF ACTIVATED CLOTTING TIME The second technique is to advance the long sheath into the It is advisable to measure the activated clotting time (ACT) transverse arch over a guide wire. The dilator is removed, and an before crossing the lesion and performing CAS. If balloon occlu- appropriate selective diagnostic catheter is advanced into the sion of the ICA is being used for embolic protection, an ACT CCA. This catheter must be substantially longer than the sheath, longer than 300 seconds is desirable. If a filter-type device or a typically more than 100 cm long. A stiff guide wire is then standard guide wire is being used, an ACT longer than 250 sec- advanced into the ECA. Both the wire and the catheter are pinned onds is probably sufficient. The interventional team should dis- at the groin to provide support, and the sheath (without the dila- cuss, in detail, all the subsequent steps to be performed so that all tor) is advanced into the CCA. This technique may be advanta- members are on the same page. Balloons are flushed and pre- geous in so-called hostile arches, in that the catheter and the wire pared, and special care should be taken to remove all air from the provide more support than a wire alone would; however, without system as a protective measure against the unlikely event of bal- the protection of the sheath dilator, there is a risk of “snowplow- loon rupture.The stent is opened and placed on the table, and the ing” the edge of the sheath at the junction of the aortic arch and crossing guide wire or embolic protection device is prepared. For the innominate artery or the left CCA, which can cause dissection de novo lesions, I typically administer atropine (0.5 to 1.0 mg I.V.) or distal embolization. as prophylaxis against bradycardia during balloon inflation in the The importance of gaining and maintaining sheath access to the carotid bulb; for restenoses after CEA, this measure may not be distal CCA should not be underestimated: once the 0.035 in. necessary.The monitoring nurse or CRNA should be alerted that guide wire is removed (and, eventually, exchanged for a 0.014 in. balloon inflation may cause significant hemodynamic instability. wire), support for angioplasty and stent placement is provided solely by the sheath. If the sheath backs up into the aortic arch STEP 6: ADVANCEMENT OF GUIDE WIRE ACROSS LESION during the interventional procedure, it is virtually impossible to The guide wire or embolic protection device (0.014 in.) is advance it into the CCA over a 0.014 in. guide wire or protection advanced across the lesion with the aid of roadmapping (if avail- device. Appropriate patient selection and accurate recognition of able). Care should be taken when the device is inserted through which arches to avoid are essential for success. In particularly dif- the sheath valve because the tip can be damaged at this juncture. ficult arches, deep inspiration or expiration may facilitate sheath If a protection device is being used, it should be deployed into the advancement by subtly changing the configuration of the brachio- distal extracranial ICA, just before the horizontal petrous segment. cephalic origins once guide-wire access has been obtained. Alter- For balloon-occlusion devices, absence of flow in the ICA must be natively, when a sheath cannot be advanced into the CCA, a pre- demonstrated. For filter devices, apposition of the device to the shaped guide catheter can be seated in the proximal CCA. Al- ICA must be documented, along with flow in the ICA through the though this maneuver may facilitate an otherwise impossible inter- device; these should also be documented after each subsequent vention, it should be kept in mind that guide catheters provide a step of the intervention to detect possible occlusion of the filter by less stable support platform than sheaths do and should be used debris. only if there is no other reasonable alternative. STEP 7: PREDILATION OF LESION Troubleshooting The lesion is predilated with a 5.0 mm angioplasty balloon, typ- In patients with an occluded ECA, sheath access to the CCA ically on a monorail or rapid-exchange platform.To save time, the may be difficult to obtain. Two techniques may be employed to balloon can be advanced into the distal CCA before the lesion is overcome this difficulty. The first is to place a stiff 0.035 in. wire crossed with the guide wire or protection device. In most cases, the with a preshaped J into the distal CCA, taking care to avoid the desired balloon profile can be achieved with relatively low inflation bulb and the bifurcation.The J configuration keeps the guide wire pressures (4 to 6 mm Hg). After the predilation balloon is re- from crossing the lesion. A stiff wire with a shapable tip can be moved, another bifurcation angiogram is performed through the used to the same end. sheath (unless distal balloon occlusion is employed, in which case
  6. 6. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 6 a b c d e Figure 1 Carotid angioplasty and stenting. In this example, a filter device is used for cerebral protection. (a) MRA shows aortic arch and cervical carotid and vertebral arteries. (b) MRA shows recurrent (postendarterectomy) high-grade stenosis of right ICA. (c) Selective angiogram shows right carotid bifurcation. (d) FilterWire EX (Boston Scientific, Natick, Massachusetts) is deployed within ICA; angiographic assessment is performed in two planes (left, right) to confirm adequate apposition of filter to vessel wall (arrows). (e) Shown is vessel after angioplasty and placement of self-expanding nitinol stent. Sheath is properly positioned in distal CCA (star). Spasm may be seen at site of filter (arrow). the ICA will not be visualized and the distal stent must be placed operator to miss the target lesion.To counter this tendency, I typ- according to predetermined bony landmarks and the location of ically expose and deploy two or three stent rings, then wait for 5 the CCA bifurcation). to 7 seconds, allowing the distal stent to become fully expanded, well apposed to the ICA, and attached to the vessel wall above the STEP 8: DEPLOYMENT OF STENT lesion. I can then deploy the remainder of the stent more rapidly, Once correct positioning has been confirmed, the stent is with little worry that it will migrate. deployed. Currently, I prefer to use nitinol stents, most common- The diameter of the stent is determined by the diameter of the ly extending an 8 to 10 mm × 30 mm stent from the ICA into the largest portion of the vessel, which is typically the distal CCA (not CCA and covering the origin of the ECA. Nitinol stents exhibit a the ICA). It is important that there be no unapposed stent in the tendency to “jump” distally when deployed rapidly (despite man- CCA: any part of the stent that is not apposed to the vessel wall ufacturers’ claims to the contrary), and this jump may cause the may become a nidus for thrombus formation.The diameter of the
  7. 7. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 7 unconstrained stent should be at least 10% (approximately 1 to 2 been described, but the majority of strokes are attributable to cere- mm) larger than the maximum diameter of the CCA. On occasion, bral embolization of atheromatous debris or thrombus. Even when the lesion is limited to the ICA well above the carotid bifurcation; cerebral emboli do not produce a major stroke, they may cause in such cases, a shorter stent confined to the ICA may be used. substantial cognitive impairment. A prospective study of 100 patients monitored with transcranial Doppler ultrasonography STEP 9: POSTDILATION OF LESION (TCD) during CEA found that microemboli were detected dur- If necessary, the lesion is postdilated with a 5 or 5.5 mm bal- ing 92% of procedures.9 Although most of the emboli were air loon; larger balloons are rarely necessary. As noted [see Step 7, emboli (and were not associated with adverse clinical events), above], I currently prefer to use a relatively large balloon (5 mm) there were more than 10 particulate emboli whose presence was for predilation so as to obviate postdilation if possible. A residual correlated with significant deterioration of postoperative cognitive stenosis of 10% or so is completely acceptable; the goal of the function. intervention is protection from embolic stroke, not necessarily a A subsequent study evaluated 70 patients who underwent car- perfect angiographic result. diac surgery with cardiopulmonary bypass, measuring cognitive function at 1 week, 2 months, and 6 months after operation; elder- STEP 10: COMPLETION ARTERIOGRAPHY ly patients who underwent urologic surgery served as the control A completion angiogram of the carotid bulb and bifurcation group.10 TCD detected more than 200 emboli in 40 patients, prin- and the distal extracranial ICA is obtained before wire removal to cipally during aortic clamping and release, when bypass was initi- verify that no dissection or occlusion has occurred. Severe vaso- ated, and during defibrillation. Emboli were associated with sig- spasm may be encountered (sometimes mimicking dissection); nificant memory loss. In addition, cognitive function deteriorated watchful waiting, coupled on occasion with administration of more in the study group than in the control group, though it vasodilators through the sheath (e.g., nitroglycerin in 100 µg recovered by 2 months after operation. aliquots), usually resolves this problem. Sometimes, the wire must A study using a rat model examined the effect of atheroemboli be removed before vasospasm will resolve completely, but this of varying sizes on neuronal cell death.11 The animals exposed to should be done only after dissection is excluded. After the wire is particles less than 200 µm in diameter showed no evidence of removed, completion angiography of the carotid and intracranial brain injury at 1 and 3 days, whereas those exposed to particles circulation is performed in two views. between 200 and 500 µm showed a scattered pattern of neuronal cell death at necropsy. At 7 days after embolization, however, both STEP 11: ACCESS-SITE HEMOSTASIS groups showed evidence of cell death.These findings suggest that Heparin anticoagulation typically is not reversed. Access-site whereas the brain may have a substantial tolerance for small hemostasis is achieved with a percutaneous closure device. emboli in the acute setting, even particles smaller than 200 µm may cause neuronal ischemia at a later point. Postoperative Care Angioplasty and Carotid Embolization After the procedure, the patient is monitored in the recovery Various studies have assessed the embolic potential of carotid area for approximately 30 minutes, then transferred to a moni- plaques in ex vivo models. One such study, which used specimens tored floor; admission to an intensive care unit generally is not of human atherosclerotic plaque in a flow chamber to model CAS, necessary. In 2 to 3 hours, patients are allowed to ambulate if a clo- found that substantial numbers of particles were released from the sure device was used and are allowed to resume a regular diet. lesions during angioplasty and that most of these particles were Some patients experience prolonged hypotension from carotid captured by an experimental filter device.12 The filter device itself sinus stimulation; this problem can be managed with judicious produced very few particles, thanks to its low crossing profile. fluid administration, pharmacologic treatment of bradycardia, A subsequent study also addressed the embolic potential of and, occasionally, I.V. infusion of a vasopressor (e.g., dopamine). human carotid plaques during experimental angioplasty.13 An Rarely, a patient experiences prolonged hypotension that must be average of 133 emboli per angioplasty were measured, and there treated with an oral agent (e.g., phenylephrine or midodrine). was a positive correlation between lesion severity and the maxi- A duplex ultrasonogram is obtained before hospital discharge mum size of the embolic particles. A notable finding was that as a baseline study; the criteria for stenosis and restenosis change patients who had been placed on statin therapy more than 4 weeks after placement of a metal stent in the carotid artery. Subsequent before operation had significantly fewer and smaller emboli. ultrasound examinations are performed at 6 weeks, 6 months, and A clinical study that employed a distal balloon-occlusion system 1 year, then yearly thereafter. Neurologic evaluation is performed during CAS found that the median number of particles, their according to the same schedule as the duplex studies. Patients are maximum diameter, and their maximum area were all significant- treated with aspirin for life and with clopidogrel for 4 to 6 weeks. ly higher in patients with periprocedural neurologic complica- As with carotid endarterectomy, proper patient selection, pro- tions.14 Three procedural steps are associated with an increased cedural standardization, and meticulous attention to detail are risk of embolization: predilation, stent deployment, and postdila- mandatory for a successful procedure, regardless of the exact tech- tion [see Operative Technique, Steps 7, 8, and 9, above].15 nique used for CAS. Devices for Cerebral Embolic Protection The use of embolic protection in large numbers of patients has Special Considerations been described with respect to the coronary vascular bed, where interventional procedures performed on diseased aortocoronary CEREBRAL PROTECTION saphenous vein grafts carry a 20% risk of periprocedural compli- Ischemic stroke remains the most devastating complication of cations, largely related to distal embolization of atheromatous procedures directed at the extracranial carotid artery, whether debris. The Saphenous vein graft Angioplasty Free of Emboli open or endovascular. A number of different causes of stroke have Randomized (SAFER) trial was the first multicenter randomized
  8. 8. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 8 a b c d e Figure 2 Carotid angioplasty and stenting. In this example, a balloon occlusion device is used for cerebral protection. (a) MRA shows aortic arch and cervical carotid and vertebral arteries. (b) MRA shows de novo athero- sclerotic stenosis of right ICA. (c) Diagnostic arteriogram of right carotid bifurcation in a symptomatic patient shows tan- dem ulcerated stenoses. (d) Right ICA is occluded with PercuSurge GuardWire device. (e) Shown is vessel after angioplasty and place- ment of nitinol stent. study to evaluate the use of distal embolic protection during Three types of embolic protection—distal balloon occlusion, saphenous vein graft interventions.16 Of the 801 eligible patients, distal filter protection, and reversal of ICA flow—are currently 406 were randomly assigned to angioplasty and stenting with dis- available for use in the carotid artery, either as part of an FDA- tal balloon protection and 395 to angioplasty and stenting over a approved clinical trial or as an “off-label” application of a device conventional guide wire.The composite primary end point (death, approved for noncarotid use. A 1987 study was the first to report MI, emergency coronary bypass, target lesion revascularization) on the use of an angioplasty technique involving temporary occlu- was reached in 16.5% of the control group and 9.6% of the study sion of the internal carotid artery during manipulation of ulcerat- group; the differences between the two groups were largely driven ed plaques.18 The same investigators subsequently reported on the by decreases in MI and distal embolization.These dramatic results use of a triple-coaxial catheter system for angioplasty with cerebral suggest that patients with lesions of high embolic potential (e.g., protection in 13 patients19; cholesterol crystals ranging in size from those at the carotid bifurcation) may benefit from some type of 600 to 1,200 µm were aspirated at the time of intervention. The embolic protection. A 2000 study using an ex vivo model of angio- PercuSurge GuardWire (Medtronic AVE, Santa Rosa, California), plasty confirmed the findings of other investigators that emboli which is approved for aortocoronary saphenous vein graft inter- occur at several stages of carotid intervention and stressed that vention, has been used in the MAVERIC trial of carotid interven- cerebral protection techniques should be effective from the earli- tion and has been employed extensively for off-label carotid inter- est stages of the procedure.17 vention outside FDA-approved clinical trials [see Figure 2]. The
  9. 9. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 9 advantages of this device include its low profile (which allows it to The evidence accumulated to date suggests that most, if not cross the target lesion easily), its procedural simplicity, and its abil- all, CAS procedures should be performed with a cerebral embol- ity to aspirate large debris particles after intervention. The disad- ic protection device. In time, more such devices will be approved vantages include the potential for incomplete occlusion of the ICA by the FDA and will be available outside the setting of clinical in patients with particularly large arteries (because the device can trials. be inflated only from 3 mm to 6 mm in diameter) and the device’s inability to aspirate exceptionally large particles into the suction catheter after intervention.14 In addition, as flow is diverted into Outcome Evaluation the ECA, emboli may travel by this route and into the ipsilateral The short-term results of CAS depend largely on the presence middle cerebral artery via periorbital and ophthalmic artery col- or absence of cerebral embolization. It is therefore not surprising laterals.20 Finally, a small percentage of patients with an incom- that the addition of cerebral protection to CAS appears to have plete circle of Willis and an isolated cerebral hemisphere may be reduced the stroke risk associated with the procedure. Admittedly, intolerant of even temporary ICA occlusion. A 2002 study that however, ongoing technological developments have created some- evaluated the PercuSurge device in 75 CAS procedures found that thing of a moving target, making evaluation of the results of CAS four patients (5%) experienced transient neurologic symptoms difficult at best. Nevertheless, the available literature is sufficient during balloon occlusion that resolved with deflation and restora- tion of internal carotid flow.21 Embolic material was aspirated from the ICA in all patients; no patients had suffered major or minor stroke at 30 days after the intervention. Table 6 Results of Carotid Angioplasty and Stenting Filter protection devices have been the object of a great deal of Combined interest in the past few years. Such devices are designed to be able Study Arteries (N) Symptomatic Cerebral Stroke-Death to trap large particulate debris while maintaining flow in the ICA, Vessels (%) Protection Rate (%) thereby not only providing continued cerebral perfusion during intervention but also allowing angiography of the target vessel Diethrich (1996)28 117 28 No 7.3 during various phases of the procedure. All of the currently avail- Yadav (1997)29 126 59 No 7.9 able filters have pores larger than 100 µm, which means that they Henry (1998)30 174 35 Mixed 2.9 allow passage of smaller particles that may not cause clinically sig- Teitelbaum (1998)31 25 68 No 27.3 nificant neurologic events but may nonetheless cause silent neu- Bergeron (1999)32 99 44 No 2 ronal injury. These filters also have higher physical profiles than Shawl (2000)33 192 61 No 2.9 balloon occlusion devices and thus may be difficult to pass across Malek (2000)34 28 100 No 3.6 particularly severe stenoses. Filters that are not completely Roubin (2001)26 604 52 Mixed 7.4 apposed to the vessel wall may allow emboli to pass around the Ahmadi (2001)35 298 38 Mixed 3.0 device and into the distal cerebral circulation. A 2001 study CAVATAS (2001)36 251 96 No 10 described CAS performed in 86 lesions with three different filter Brooks (2001)37 53 100 No 0 devices.22 In three cases, the filter could not be advanced across d’Audiffret (2001)38 68 30 Mixed 5.8 Chakhtoura (2001)39 50 39 No 2.2 the lesion. However, more than half of the successfully deployed Leger (2001)40 8 38 No 0 filters contained macroscopic embolic material, and only one Dietz (2001)41 43 100 Yes 5 patient (1.2%) experienced a neurologic complication (a minor Baudier (2001)42 50 98 Mixed 6 stroke). Reimers (2001)22 88 36 Yes 2.3 Finally, a device has been developed that induces reversal of Pappada (2001)43 27 93 Mixed 3.7 flow in the ICA through balloon occlusion of the CCA and the Paniagua (2001)44 69 16 No 5.6 ECA and creation of a temporary arteriovenous shunt between Criado (2002)27 135 40 Mixed 2 the ICA and the femoral vein.23 This device, though potentially Guimaraens (2002)45 194 92 Yes 2.6 more cumbersome than other protection devices, may produce Al-Mubarak (2002)46 164 48 Yes 2 virtually complete protection by preventing any emboli from trav- Bonaldi (2002)47 71 100 Mixed 5.6 eling to the intracranial circulation. Kao (2002)48 118 75 No 4.2 A 2004 report summarized one group of operators’ experience Whitlow (2002)21 75 56 Yes 0 with 1,202 CAS procedures between 1994 and 2002.24 In 33% of Qureshi (2002)49 73 37 Mixed 4.1 the patients studied, a cerebral protection device was used during Macdonald (2002)50 50 84 Yes 6 the procedure.The overall stroke rate was 4.4%; the risk of stroke Stankovic (2002)51 102 37 Mixed 0 reached its height (9.1%) during the period extending from Kastrup (2003)52 100 63 Mixed 5 September 1996 to September 1997 and reached its nadir (0.6%) Cremonesi (2003)53 442 57 Yes 1.1 during the final year of the study. The authors concluded that Terada (2003)54 87 80 Yes 2.3 improvements in technique, equipment, and pharmacotherapy, as Bowser (2003)55 52 60 No 5.7 well as the use of neuroprotection, contributed significantly to Wholey (2003)56 12,392 53 Mixed 4.75 improvements in results. In an earlier series, 46 patients were Becquemin (2003)57 114 33 Mixed 7.0 Dabrowski (2003)58 73 Not stated Mixed 5.5 treated with CAS, of whom 25 received cerebral protection.25 Two Cernetti (2003)59 104 26 Yes 4 neurologic events (9.5%) occurred in the unprotected cohort; no Bush (2003)60 51 29 No 2 events occurred in the protected group. Although the small num- Gable (2003)61 31 69 No 3 bers of patients prevented meaningful statistical analysis, the Lal (2003)62 122 45 Mixed 3.3 authors concluded that cerebral protection was technically feasi- SAPPHIRE (2004)8 167 30 Yes 5.5 ble and effective in preventing neurologic complications during Total 17,087 4.7 CAS.
  10. 10. © 2005 WebMD, Inc. All rights reserved. ACS Surgery: Principles and Practice 6 VASCULAR SYSTEM 10 CAROTID ANGIOPLASTY AND STENTING — 10 to provide a reasonably accurate picture of the current status of adverse events (stroke, death, and MI). Inclusion and exclusion the procedure [see Table 6].8,21,22,26-62 criteria for the study are listed more fully elsewhere [see Tables 2 In a study from the University of Alabama at Birmingham and and 3]. Patients were independently evaluated by a certified neu- Lenox Hill Hospital in New York,26 a total of 604 arteries were rologist before and after the procedure. The majority of the ran- treated in 528 consecutive patients over a 5-year period. CAS was domized patients were asymptomatic: in the CAS group, only performed both with balloon-expandable stents and with self- 30% were symptomatic, and in the CEA group, only 28% were expanding stents and both with and without cerebral protection symptomatic. A key point to keep in mind is that this trial was devices.The overall 30-day combined stroke-death rate was 8.1% designed to test the idea that CAS was not inferior to CEA, not to (minor stroke, 5.5%; major stroke, 1.6%; nonneurologic death, establish that CAS was superior to CEA or CEA to CAS. At 30 1%). On a year-by-year basis, the risk of stroke and death reached days, the risk of stroke was not significantly different in the two its maximum (12.5%) in the period ending in September 1997 groups (CAS, 3.6%; CEA 3.1%).Two patients in the CAS group and fell to its minimum (3.2%) the following year.This rather dra- (1.2%) and four patients in the CEA group (2.5%) died within 30 matic change in results from one year to the next probably is attri- days; this difference did not reach statistical significance (P = butable to technical advances (e.g., in protection devices, stents, 0.39). More patients experienced periprocedural MI in the CEA and guide wires), as well as to improvements in the investigators’ group (6.1%) than in the CAS group (2.4%), a difference that did ability to select appropriate patients for intervention. not reach statistical significance on the basis of intent to treat. A subsequent report describes the authors’ experience with Notably, most of the MIs were non-Q MIs, identified by routine CAS in a vascular surgery practice.27 During a 40-month period postprocedural laboratory studies.The incidence of the combined from 1997 to 2001, 135 procedures were performed in 132 pa- end point (stroke-death-MI) was higher in the CEA group (9.8%) tients, most (60%) of whom were asymptomatic.The rate of com- than in the CAS group (4.8%), but this difference was statistical- plications was acceptable (2%), and only one patient had a signif- ly insignificant as well. These results have been used to support icant restenosis at follow-up (mean follow-up interval, 16 months). FDA approval of the stent and filter protection device used in this Perhaps more important, these 132 patients represented 41% of important study. all patients being treated for carotid disease in this practice. This Lesions of the proximal CCA are relatively uncommon in percentage seems extraordinarily high, but it may simply be a bell- comparison with lesions of the carotid bifurcation, but they can wether of things to come in the practice of vascular surgery, and it also be effectively treated with CAS. In my experience, most are underscores the importance of multispecialty involvement in this treated with common carotid cutdown, retrograde angioplasty, new technology. and placement of a balloon-expandable stent. Of 14 consecutive The results of the SAPPHIRE trial of CAS in high-risk patients CCA interventions performed at the Cleveland Clinic,63 one were published in 2004.8 To date, this trial, which included 334 was converted to carotid-subclavian transposition after iatro- patients randomly assigned to either CAS (N = 167) or CEA genic dissection, and two resulted in stroke secondary to ICA (N = 167), is the only industry-sponsored FDA-approved trial thrombosis. In both of the interventions resulting in stroke, that was actually randomized. A separate registry of nonrandom- which were performed in conjunction with redo bifurcation ized patients was compiled that included those who were felt to endarterectomies, the CCA was patent at the time of surgical be at unacceptably high risk for CEA and therefore underwent reexploration and ICA thrombectomy. Although the carotid CAS (N = 406), as well as those who were not suitable candidates stent procedure is unlikely to have played a role in causing these for CAS and therefore underwent CEA (N = 7). The goal of the strokes, it is nonetheless advisable to exercise caution when per- SAPPHIRE trial was to evaluate the combined end point of major forming these combined procedures. References 1. North American Symptomatic Carotid Endarterec- 7. Gasparis AP, Ricotta L, Cuadra SA, et al: High-risk 13. Bicknell CD, Cowling MG, Clark MW, et al: Carotid tomy Trial Collaborators: Beneficial effect of carotid carotid endarterectomy: fact or fiction. J Vasc Surg angioplasty in a pulsatile flow model: factors affecting endarterectomy in symptomatic patients with high- 37:40, 2003 embolic potential. Eur J Vasc Endovasc Surg 26:22, grade carotid stenosis. N Engl J Med 325:445, 1991 8. Yadav JS,Wholey MH, Kuntz RE, et al: Stenting and 2003 2. Executive Committee for the Asymptomatic Carotid Angioplasty with Protection in Patients at High Risk 14. Tübler T, Schlüter M, Dirsch O, et al: Balloon-pro- Atherosclerosis Study: Endarterectomy for asympto- for Endarterectomy Investigators. Protected carotid- tected carotid artery stenting: relationship of peripro- matic carotid artery stenosis. JAMA 273:1421, 1995 artery stenting versus endarterectomy in high-risk cedural neurological complications with the size of patients. N Engl J Med 351:1493, 2004 particulate debris. Circulation 104:2791, 2001 3. Menzoian JO: Presidential address: carotid end- arterectomy, under attack again! J Vasc Surg 37:1137, 9. Gaunt ME, Martin PJ, Smith JL, et al: Clinical rele- 15. Al-Mubarak N, Roubin GS, Vitek JJ, et al: Effect of 2003 vance of intraoperative embolization detected by the distal-balloon protection system on macroem- transcranial Doppler ultrasonography during carotid bolization during carotid stenting. Circulation 104: 4. Mozes G, Sullivan T, Torres-Russotto D, et al: endarterectomy: a prospective study of 100 patients. 1999, 2001 Carotid endarterectomy in SAPPHIRE-eligible Br J Surg 81:1435, 1994 16. Baim DS,Wahr D, George B, et al: Randomized trial ‘high-risk’ patients: implications for selecting patients 10. Fearn SJ, Pole R,Wesnes K, et al: Cardiopulmonary of a distal embolic protection device during percuta- for carotid angioplasty and stenting. J Vasc Surg support and physiology. J Thorac Cardiovasc Surg neous intervention of saphenous vein aorto-coronary 39:958, 2004 121:1150, 2001 bypass grafts. Circulation 105:1285, 2002 5. Ouriel K, Hertzer NR, Beven EG, et al: Preproce- 11. Rapp JH, Pan XM, Sharp FR, et al: Atheroemboli to 17. Coggia M, Goëau-Brissonière, Duval J-L, et al: Em- dural risk stratification: identifying an appropriate the brain: size threshold for causing acute neuronal bolic risk of the different stages of carotid bifurcation population for carotid stenting. 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