Tony P. Smith, MD, is chief of vascular and interventional radiology at Duke University Medical Center, Durham, NC. In the future, Smith has predicted, carotid stenting may be available to all patients. Smith presented Carotid Artery Angioplasty and Stenting: Who Do I Do and Why? at the 29th annual meeting of the Society of Interventional Radiology (held in Phoenix on March 25-30, 2004). At present, he noted, carotid stenting no longer appears to be an experimental procedure. Instead, it should be considered a compassionate procedure that spares patients the risk of a surgical procedure.
In choosing appropriate patients for carotid stenting, the first criterion should be the patient’s best interest. The second criterion is self-protection in today’s medicolegal environment. While there is not yet a standard of care for carotid stenting, patient selection should be based on some predefined protocol that can be defended later, if necessary. Smith suggests that the results of published studies, tempered by the physician’s personal judgment, constitute the best basis for deciding which procedure to use to treat each individual. Patient selection also depends on a thorough understanding of the health care environment, the people who will be involved in providing the procedure, and the local patient population.
STUDY CRITERIA
The research on which patient selection can be based has varied from single case studies to replicated clinical trials. For the most part, however, published reports have covered case series, database analyses based mainly on multicenter registries of selected (nonrandomized) patients, or controlled clinical trials. Generally speaking, patients should meet the inclusion criteria for angioplasty and stenting that were used in the nonrandomized research. They should also meet the exclusion criteria for surgery, and should not meet the angioplasty and stenting exclusion criteria. It can be helpful to know how patients would be categorized and treated if they were part of a study, even if they will never be enrolled in one.
The most important patient-selection criteria are those of exclusion. Patients should be adults less than 80 years old; individual circumstances may warrant stenting in older patients, but complication rates are likely to be higher. Stenting is called for in treating symptomatic lesions showing 50% stenosis or more, but in asymptomatic lesions only if the narrowing is greater than 80%. Smith issued a strong warning against guessing at the degree of stenosis of the vessel to be treated. Electronic calipers should be used because appearances can be deceptive. Symptomatic patients will be more likely to experience complications, but are also more likely to understand that complications are an inevitable risk. The individual who has experienced a transient ischemic attack (TIA), for example, will probably find the risk of complications more acceptable than the person without symptoms might.
RISK ASSESSMENT
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The medical literature has focused on patients who have anatomical and constitutional factors that increase the risk of surgery. Anatomical high risk may consist of an inaccessible lesion, radiation therapy, prior carotid endarterectomy, or (more rarely) inability to flex the neck due to ankylosing spondylitis or similar conditions. The constitutional factors that make patients poor surgical candidates are mainly heart and lung disorders. Can the patient tolerate general anesthesia or undergo an open surgical procedure? If not, the patient may or may not be an angioplasty candidate. Angioplasty is probably not indicated for patients still undergoing an evolving cerebrovascular accident (CVA); Smith prefers to wait until the patient is neurologically stable.
Patients who cannot take antiplatelet agents and heparin are excluded from studies and should be accepted for stenting only with caution; bleeding times and platelet counts should be as normal as possible before the procedure.
Anatomical difficulties can sometimes be overcome using special tools, but may be sufficient cause for exclusion. Patients without femoral-artery access are not accepted in studies, and calcification may reach a point that prevents stenting. As an absolute contraindication, Smith named thrombus within the vessel. Intracranial stenoses are also reason for exclusion from studies because it will be impossible to tell whether a future CVA is due to stenting or stenosis. If this risk is understood, however, stenting outside a study can proceed.
TIPS AND TECHNIQUES
Tools of the Trade for Carotid Artery Stent Placement was presented by Michael Wholey, MD, MBA, associate professor, Cardiovascular and Interventional Radiology, University of Texas Health Sciences Center, San Antonio. Carotid stenting equipment, he explained, is based on the angiography catheter, with the addition of stent-placement tools and, more recently, distal protection against thrombosis in the form of filters designed to catch any loose material that could cause TIAs or CVAs. The critical step is not placing the filter past the lesion, however, but getting the sheath into the common carotid artery. At that point, Wholey noted, the procedure is 85% to 90% complete.
If a user is new to distal protection, Wholey suggests employing it initially during superficial femoral artery angioplasty in order to become familiar with its use in a more forgiving vessel than the carotid artery. Deploying a stent involves precise placement, so it may be necessary to begin to open it three times before withdrawing; otherwise, the stent can jump to a position 1 to 2 mm away from the target.
Linear stent expansion may also prove troublesome if the stent is too strong; it may straighten the vessel and create a compensatory kink elsewhere. If the stent’s cells are too wide, plaque prolapse may occur, possibly causing TIAs or small CVAs within the first few days following stenting. If the stent’s struts protrude, the sheath may become snagged on them during filter withdrawal.
The major advances in stenting equipment in recent years have been decreased size and increased flexibility. Roughly 60% of teams performing carotid stenting use sheaths, with the remainder using guiding catheters, Wholey estimated. Very flexible small sheaths are available; proponents of guiding catheters, however, appreciate the ability to turn the catheter in order to bring it away from the vessel wall on tight bends.
A thorough neurological evaluation and four-vessel angiography should be conducted 2 weeks before carotid stenting, since there is a risk that the diagnostic procedure will cause complications. In addition, angiography can help the team choose the ideal catheter for stenting. Clopidogrel must be taken by the patient for at least 3 to 5 days prior to the study; aspirin therapy and hydration are also needed, along with the withdrawal of some antihypertensive agents.
Wholey recommended staying with one’s chosen system and procedural methods for carotid stenting because the familiarity of the entire team with the protocol is directly related to the ability to complete carotid stenting within the recommended 15 to 20 minutes.
Deploying the filter at least 2 cm beyond the lesion prevents the stent from bumping it. The filter should not be permitted to move; a team member should be assigned to report any movement immediately. If the filter comes into contact with the lesion, it will have to be collapsed and redeployed, and too much filter movement can cause spasm. A current trend favors omitting predilation, but Wholey reported that it allows the stent to pass more easily and makes it less likely to jump out of the desired position during deployment. Using stents that are slightly longer than is strictly necessary can prevent the need for a second stent (which Wholey called a serious mistake) if such a jump occurs.
On occasion, the filter, once pulled back into its recovery sheath, will not pass the stent for removal. Wholey advised the team not to panic, but to rotate the patient’s head back and forth, lift the jaw, hyperextend the neck, move the arms downward, and/or instruct the patient to cough. If these maneuvers are no help, the guiding catheter (if used) can be turned. The filter can also be pulled into a balloon system.
If a spasm should occur, it will generally dissipate as the sheath is withdrawn; if not, 150 µg of nitroglycerin can be considered. Wholey noted that the patient should be sent to surgery if there is any doubt that carotid stenting will be successful. As he put it, a technical failure is acceptable, but a technical failure with a complication is not.
A final angiogram should be performed to ensure that there are no problems (and so that an image will be available if complications arise shortly after stenting). Wholey’s patients stay overnight in the intensive care unit for observation following stenting, and clopidogrel therapy is continued for at least a month.
Kris Kyes is technical editor of Decisions in Axis Imaging News.
