The Aortic Arch: Markers, Imaging, and Procedure Planning for Carotid Intervention

Abstract

Among the several anatomic risk predictors for carotid stenting (e.g., extensive plaque ulceration, aneurysmal internal carotid artery, lesion length > 3 cm) (CAS), the aortic arch emerges as a key anatomic feature. Gaining access to the carotid lesion necessitates traversing the aortic arch and the proximal carotid arteries. Most technical failures in carotid stenting are related to a complex aortic arch whose role in CAS outcome rises as a crucial element of patient selection. The arch markers for selecting patients for carotid interventions include arch elongation, arch vessel origin configuration, arch calcification, and arch vessel origin stenosis. These markers get significantly unfavorable with increasing age. Cautious pre-interventional imaging is paramount in indentifying potential arch complexity and direct the interventional strategy. CAS practitioners would be advised to start their experience in younger patients with predominantly noncalcified type 1 arches.

Introduction

Despite increasing experience in carotid angioplasty and stenting (CAS), optimal patient and anatomy selection remain the most important considerations for a successful outcome. Technical limitations and complications still exist, denoting that several issues still remain to be resolved. Contrary to carotid endarterectomy (CEA) being non-favorable for patients with severe medical comorbidities, CAS is usually not indicated on the basis of anatomical limitations, not only of the carotids, but of the entire pathway, from the puncture site to the cerebral arteries.¹ Among the several anatomic risk predictors (e.g., extensive plaque ulceration, aneurysmal internal carotid artery, lesion length > 3 cm), the aortic arch emerges as a key anatomic feature for CAS success (or failure).^1,2 Regardless of the complexity of the internal carotid lesion or the degree of stenosis, accessing those lesions with the appropriate filter and stent is usually technically feasible. The arch, however, is a different challenge. Initially, its role had been underestimated and was not considered a major procedural limitation. Recently, the aortic arch has become a crucial element in patient selection for CAS, with arch markers being used to guide the operator for case selection and pre-interventional planning.^2,3 The arch markers for selecting patients for carotid interventions include arch elongation, arch vessel origin configuration, arch calcification, and arch vessel origin stenosis. This review focuses on aortic arch features, anatomy assessment, patient selection, and planning and execution without compromising patient safety.

Arch Markers

Arch elongation and arch vessel configuration. It is important to recognize the type of aortic arch and the configuration of the great vessels in each patient, since these anatomic features influence procedure complexity. The aortic arch elongation classification was conceived to picture an increasing procedural difficulty in vessel cannulation, which was also helpful in designing catheter configurations, allowing easier access to the great vessels off the arch. There are three types of aortic arches based on the relationship of the innominate artery to the aortic arch or on the parallel planes perpendicular to the greater (outer) curvature and lesser (inner) curvature of the arch.^4–6 Some authors have also suggested a type IV arch.² Alternatively, arch complexity can be assessed by drawing a line horizontally across the upper inner aspect of the arch. When the origin of the target artery is above the horizontal line and to the patients’ right, catheterization presents a moderate degree of challenge. The closer the arch branch origin is to the horizontal line, the more challenging it becomes^.7

• Type I arch. The arch vessels arise from the outer curvature of the arch in the same horizontal plane (no angulation). The vertical distance from the origin of the innominate artery to the top of the arch is 2 left CCA diameters.

• Type IV arch. The arch vessels arise with severe angulation, accompanied by increased length and transverse diameters of the arch. This arch type is associated with redundancy of the CCA. It is more frequently noted on the right, where tortuosity and the redundant loop exist in the proximal segment of the CCA.

Brachiocephalic vessel configuration (branches’ number and position) are equally important to the arch elongation type. In the usual configuration, the innominate artery, the left CCA, and the left subclavian artery have separate origins.³ Aortic arch anomalies are not infrequent in the population. The most common anomaly of the aortic arch is the bovine variety, which occurs in about 27% of the population: the innominate artery and the left CCA have a common origin (20%), or the left CCA is a separate branch of the innominate artery (7%). Less frequent variations include a common origin of left CCA and left subclavian artery (1%) or a left vertebral artery originating from the arch (0.5%).⁸

Arch calcification. The higher reported embolization rates during CAS, compared with surgery, have been attributed to emboli dislodged to the brain during the passage of the aorta.^9,10 Aortic arch calcification is a predictor of neurologic events in non-surgical patients¹¹ and in patients undergoing coronary artery bypass grafting.^12,13 It seems that a similar risk exists in carotid angioplasty and stenting. Severe aortic calcification is not only a marker of cerebral arteriosclerosis, which is less tolerant to hypoperfusion and hypoxia, but mainly provides a treacherous intravascular operating field, with a fragile intraluminal surface prone to ulceration and disruption by catheter and guidewire manipulations.^3,4 Extensive maneuvers can produce emboli or disrupt aortic plaques with subsequent delayed dislodgment and, therefore, thrombus can form and dissections can occur.³ When carotid angioplasty and stenting are considered, aortic arch calcification can be categorized as favorable if there is no calcium shadowing or if there is a trace, and unfavorable if there is luminal irregularity or diffuse calcification.⁵ In the latter case, the vascular interventionalist should be cautious with catheter manipulation.

Arch vessel origin stenosis. When carotid angioplasty and stenting are considered, the aortic arch vessel origin stenosis 50% as unfavorable.⁵ When a severe proximal CCA lesion is present, stenting (under filter protection, if possible) is required before further continuation of the procedure.³ The aortic arch of elderly people. Patients in their 80s and 90s who are fit enough to be managed interventionally upon indication are common. Arch characteristics tend to change over time with age and prolonged hypertension. Also, aortic calcification, arch vessel origin stenosis, arch vessel tortuosity, and aortic arch elongation and distortion emerge.^4,5,11–15 The ascending aorta and transverse arch elongate, pushing the aortic valve and the origins of innominate and left CCA inferiorly. The locations of the origins of these major arch branches become harder to reach. These factors make carotid angioplasty and stenting technically difficult, leading to a higher risk of thromboembolic complications. Thus, carotid angioplasty and stenting in elderly patients (> 80 years old) have been associated with higher rates of stroke and death.^16,17

Imaging of the aortic arch. While Doppler ultrasound is usually efficient when carotid endarterectomy is considered, it is definitely not enough for CAS. Several anatomic considerations are particularly crucial for CAS planning. Arch anatomy and lesions, vessel rigidity, the presence of ostial plaques, and underestimated tortuosities can be a source of unexpected technical challenge and thus, should be meticulously evaluated, before intervention. A complete study that includes the aortic arch and the origins of the brachiocephalic trunks is essential. Imaging should be performed with arteriography, magnetic resonance angiography (MRA), or computerized tomographic angiography (CTA). However, while MRA and CTA have been validated to assess severity of stenosis, neither has yet been validated to assess disease of the arch or great vessel origins. For most patients, arch angiography remains the gold standard and can be achieved by a left anterior oblique angle visualization to open up the arch to the exact amount varying on the patient’s anatomy, but commonly at least 30 degrees. A 15 to 20 X 30 rate of injection (power injector), meaning a rate of 15 mL/s to 20 mL/s for a total volume of 30 mL is usually enough.¹⁸ The most important aortic arch issue to address is its configuration, followed by the extent of calcification and the originating vessel’s stenosis.³ Digital subtraction, CTA, or MRA will allow careful evaluation of the aortic arch and brachiocephalic origins, which is imperative in determining the ease or difficulty of CCA access, an absolute key to procedural success. Such information will the influence the choice of catheters and the interventional strategy. Aortic calcification and/or branch stenosis can be visualized angiographically (digital subtraction angiography, CT angiography). In many cases, MR angiography is not sufficient for the evaluation of the presence of calcification. Transoesophageal echocardiography may be more accurate but is of less (or no) clinical significance. Plain chest radiography has been recently advocated as a useful, widely available, and inexpensive diagnostic tool in detecting aortic calcification.¹⁹ Intravascular ultrasound could be utilized for both quantitative and qualitative measurements of aortic plaque, though not of arch vessel origin during angiography.³

Selective Carotid Catheterization by Arch Type

The selection of equipment for CAS is most dependent on the anatomy of the aortic arch and of the CCA proximal to the target lesion. A retrograde femoral artery approach to access the CCA is usually preferred, while a right brachial (or radial) access is required in complex arches or in cases where transfemoral access is not possible due to severe inflow occlusive disease. The choice of the technique to access the CCA is operator-dependent, though there are several anatomic factors that might favor one technique over another. When treating patients with a simple arch and carotid anatomy, access to the CCA can be achieved using either an 8-Fr guiding catheter or a 6-Fr long interventional sheath. Guiding catheters have a softer tip that might prevent dissection and are less likely to kink. Moreover, they have torque and steerable features. Their multiple configurations permit the choice of the most suitable catheter for the specific arch anatomy.² Interventional sheaths, however, offer better wire-catheter support. Whichever technique is employed, careful placement of the tip of the guiding catheter or interventional sheath will help prevent spasm, thrombosis, or dissection. The tip is usually positioned in the distal CCA, while in cases requiring a more aggressive guiding catheter shape, the tip of the guide is usually positioned in the proximal segment of the CCA, although this generally provides less support for the procedure.²⁰ As we move from arch type I to type III, cannulation of the CCA becomes more challenging and necessitates choosing a correctly shaped catheter. The operator should become familiar with a variety of selective catheters (Simmons 1 and 2, H1, JB1 and JB2, JR, AR2, AL3, etc, [Cook, Bloomington, Indiana]). When deep cannulation of the CCA or CCA is not possible, one can still attempt CAS by stable engagement of the origin of the arch branch with an appropriately shaped 8-Fr or 9-Fr guiding catheter (H1, AL1, etc.). Maneuvers performed by the patient, such as turning the head sharply towards one side or deep inspiration or expiration, which may facilitate sheath advancement by subtly changing the configuration of the brachiocephalic origins, may be helpful.

Some authors² recommend, especially for type IV aortic arches in patients with an absolute surgical contraindication, the two-wire technique to maintain stability. In this technique, one uses a 7 Fr sheath (instead of 6 Fr) to better accommodate multiple wires side-by-side. A 0.014-inch wire is positioned in the external carotid, and the filter wire is obviously positioned in the internal carotid beyond the target lesion, allowing deployment of the stent. After the stent is deployed, the supplementary wire in the external carotid can be removed. In bovine arches, when the target artery for CAS is the left, initial CCA catheterization and subsequent passage of the sheath is around an acute turn from the arch into the innominate and then back toward the patient’s left into the CCA, it becomes too challenging. A Simmons 2 diagnostic catheter is essentially the best option and a Vitek catheter (Cook) can be a good alternative. For a very narrow arch with left CCA angles, a very stiff guidewire, creating a better rail, facilitates sheath anchoring. In difficult bovine cases, however, as well as in hostile arches, different access sites, such as the brachial or radial artery or even the cervical CCA, should be considered as first choices.^{1, 21, 22}

Discussion

It is evident now that during CAS, sources of peri-operative escape of atheroemboli include arterial sites other than the carotid lesion itself. Gaining access to the carotid lesion necessitates traversing the aortic arch and the proximal carotid arteries. Manipulation in a difficult aortic arch anatomy with significant atheromatous disease can lead to embolization of either cerebral territory prior to internal carotid artery filter placement. While initially underestimated, it seems that most technical and clinical failures in CAS are related to a complex aortic arch. In current literature, increasing evidence justifies the key role of the aortic arch to CAS outcomes, particularly in the elderly. Preliminary reports from the large, randomized, controlled Carotid Revascularization Endarterectomy versus Stenting Trial (CREST) revealed a significantly higher proportion of cerebrovascular events during carotid angioplasty and stenting in octogenarians compared to younger patients.¹⁶ Although, at that time, no specific predictors were evaluated, it is now clearly indicated by other studies that the aortic arch’s altered anatomy and atherosclerosis may have been among the major causes^.5,23 Setacci et al, in over 1200 cases, confirmed the increased prevalence of arch type III, calcification, and tortuosities in the elderly.²⁴ Despite the lack of statistical significance in stroke and death outcomes, rates indicated a trend towards higher risk for octogenarians related to unfavorable arch markers. Similarly Lam et al, in 135 CAS cases, confirmed that all three arch markers (elongation, calcification, and vessel origin stenosis) have a significantly higher incidence in octogenarians, possibly associated with higher rates of neurologic events.⁵ Kastrup et al, in 62 CAS cases, recently reported that arch calcification is significantly increased in symptomatic octogenarians. More interestingly, arch calcification was proven to be a significant risk factor for imaging-detected embolic events outside of target carotid artery distribution.^{23 \}

Finally, in the work of Faggioli et al, in 298 cases, the aortic arch elongation was a significant predictor of technical failure. Not surprisingly, a proximal tortuosity index, defined as the sum of all angles diverging from the ideal straight axis measured from the arch to the ICA stenosis, > 150º was an independent predictor of both neurologic complications and technical failure.²⁵ The same team compared CAS outcomes between cases with arch anomalies to cases with normal arch anatomy (type I, II and III). Technical failure and neurological complications occurred more frequently in the arch anomaly group. The arch type was the only variable independently associated with neurological complications.²⁶ Unfortunately, the currently available randomized controlled trials, Stenting and Angioplasty with Protection in Patients at High Risk for Endarterectomy (SAPPHIRE), Endarterectomy versus Patient with Severe Symptomatic Carotid Stenosis (EVA3s), Stent-Protected Angioplasty verus Carotid Endarterectomy (SPACE),^27–29 and large scale clinical trials and registries, ACCULINK for Revascularization of Carotids in High-Risk Patients (ARCHeR), Carotid ACCULINK/ACCUNET Post-Approval Trial to Uncover Rare Events (CAPTURE), and Boston Scientific EPI: A Carotid Stenting Trial for Risk Surgical Patients (BEACH) have not included aortic arch-related outcomes in their results.^30–32 We now know that an arch-adjusted analysis could potentially indicate evidence-based patient selection and subsequently improved CAS outcomes.

Similarly, consensus documents for carotid stenting have not included guidelines for CAS patient inclusion or exclusion oriented by arch anatomy.³³ Current knowledge suggests that the key for favorable CAS outcomes is knowing which anatomy to avoid in any given situation and which anatomy to match up to the treatment options. Treatments must be individualized to the given patient. Accordingly, CAS practitioners would be advised to start their experience in younger patients with predominantly type 1, not severely calcified arches. But even experienced CAS practitioners should continue to remember that those anatomies make the procedure more challenging or even hazardous. In complex arches, and if carotid endarterectomy is not indicated, direct exposure of the CCA as well as the use of the brachial or radial route can be a safe alternative.^34,35 A technical failure is acceptable, but a technical failure with a stroke is not. Considering these facts, the European Society for Vascular Surgery Guidelines Committee in its recently released guidelines has given the following recommendation “Invasive treatment recommendation 5: CAS is not advisable in patients with extensive aortic and supraortic vessel plaques, calcification and tortuosity or calcification, unless performed in high volume centers with documented low periprocedural stroke and death rate.”³⁶

Conclusion

The aortic arch markers should guide the operator for case selection and preplanning for carotid angioplasty and stenting. Unfavorable arch markers significantly encountered in the elderly represent potential hazards and causes for shower emboli in the process of gaining access, which can result in either contralateral or ipsilateral hemispheric strokes during the procedure. It seems imperative to have a thorough working knowledge of the arch anatomy, arch calcification, and arch vessel ostial stenosis, and how this can alter the risk of the procedure. Familiarization with the use of alternative approaches can minimize procedural risk.

Correspondence: Christos D. Liapis, MD, University Hospital Attikon, Department of Vascular Surgery, 1 Rimini Street, Chaidari, Athens 12462, Greece. E-mail: liapis@med.uoa.gr.

Manuscript submitted September 15, 2008, provisional acceptance given, November 5, 2008, accepted November 13, 2008.

Disclosure: The authors report no financial relationships or conflicts of interest regarding the content herein.