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Role of Shunting During Carotid Endarterectomy
Temporary clamping of the internal carotid during carotid endarterectomy (CEA) interrupts antegrade blood flow to the brain in the distribution of the ipsilateral middle and anterior cerebral artery. The consequence of clamping the internal carotid artery is of critical importance to the patient and has been the subject of debate for more than 50 years. In a 1998 editorial, Denton Cooley wrote, “No consensus concerning the proper conduct of this procedure exists, or probably ever will.” In describing their first carotid endarterectomy, Cooley and colleagues fabricated a shunt from polyvinyl tubing with large-bore needles at each end. The same patient underwent adjunctive cerebral protection measures including hypercarbia, cooling the head in ice, and barbiturate use. Interestingly, for most of his career, Cooley argued for a no-shunt approach.
Few modern surgeons argue that no patients should undergo shunt placement to reestablish antegrade flow during endarterectomy. Therefore, the question is not if to shunt but when to shunt: selectively or routinely? Because little level I evidence exists, a Cochrane report summarized insufficient evidence is available to support the superiority of either method. Therefore, this chapter discusses the techniques and the proposed advantages and disadvantages of both approaches.
Selective Shunting
The impetus for selective shunting originates from the agreed observation that a majority of patients tolerate carotid clamping, and therefore shunting offers no benefit to most patients. Also, routine carotid shunting has several disadvantages, further weakening the argument for universal shunt placement. A selective approach offers the potential advantages of a shorter operative time and unobstructed inspection of the distal endpoint, and it avoids any risks associated with shunt placement for a majority of patients. Identification of the minority of patients who benefit from shunting remains the challenge. In particular, patients with history of stroke or contralateral carotid occlusion have been reported as high-risk patients. However, the use of preoperative risk factors alone has not proved reliable in identifying patients at risk for ischemic stroke following carotid clamping. The traditional intraoperative techniques for identifying patients for shunt insertion—awake CEA, carotid back pressure measurement, and electroencephalographic (EEG) monitoring are relevant. Transcranial Doppler and cerebral oximetry are newer and less-validated methods to assess the need for shunting.
Awake Carotid Endarterectomy
Most agree that awake CEA is the gold standard for neurologic monitoring during carotid clamping. The technique involves intraarterial blood pressure monitoring, frequent neurologic checks, and cervical block for analgesia. Patients who develop neurologic changes after test clamping are shunted.
Interestingly, many authors report immediate reversal of neurologic symptoms after the establishment of a working shunt, suggesting the value of shunting. Awake CEA has low rates of shunt insertion, with rates around 5%, and reported stroke rates of 1%.
An important disadvantage of awake CEA remains: Not all surgeons are trained to perform the operation under cervical block anesthesia. From a metabolic standpoint, awake CEA has been reported to increase the metabolic demands of the brain when compared with general anesthesia. Therefore, patients undergoing an awake CEA may be at greater risk of ischemic injury when compared with those receiving general anesthesia. Another disadvantage is not all patients tolerate the operation for reasons including claustrophobia, pain, cervical arthritis, and redo neck surgery. Without another method for selective shunting, the surgeon may elect to shunt patients routinely.
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