Carotid endarterectomy (CEA) is a surgical procedure which is performed to prevent the incidence of stroke in patients with significant carotid artery stenosis.These patients can be asymptomatic or have already suffered from Transient Ischaemic Attack (TIA) or minor stroke.The CEA should preferably be performed within 2 weaks of transient ischaemic attack as the risk of stroke is maximum within first few days of minor stroke or TIA.1
Prospective studies in Europe and North America demonstrated that in symptomatic patients with more than 70% carotid artery stenosis surgical treatment is beneficial as compared to medical management only.As a result of these studies practice is changing and the recent recent European Carotid Surgery Trial suggests a three- to five fold increase in the number of surgeries performed. But still the issue choice of anaesthetic technique is debatable.
After careful surgical exposure common carotid artery, external carotid artery, and internal, carotid artery are cross-clamped. Carotid bifurcation is isolated from the circulation. The affected segment of artery is commonly opened by longitudinal incision. The atheromatous plaque is dissected. The artery can be closed by primary closure or patch angioplasty to reduce the incidence of re-stenosis. The surgical procedure of eversion CEA is preferred by some surgeons, in which to remove the plaque, the internal carotid artery is transacted and turned inside out. Whichever technique is used but the debris should be removed completely from the intimal surface of the artery to avoid postoperative emboli occurring.
During the procedure of CEA collateral blood flow via the circle of Willis should be adequate otherwise it can lead to cerebral ischemia. To maintain cerebral perfusion in such patients a surgically inserted shunt is used to bypass the isolated section of carotid artery. After clamping the common and external carotid arteries, the pressure measured in the internal carotid artery reflects the perfusion pressure which is transmitted around the Circle of Willis. This is called the stump pressure. A number of thresholds for the stump pressure, ranging between 25 and 70 mm of mercury, have been proposed below which shunting would be appropriate.2
There are different types of carotid shunts available to maintain blood flow from the common carotid to the internal carotid artery during the surgery. Although in patients with contralateral carotid stenosis or a compromised Circle of Willis, carotid shunts seem to be useful, but it is not a totally benign intervention. Acute complications which may be associated with shunt insertion are plaque embolization, air embolization, carotid dissection and intimal tears. Other associated complications may include haematoma formation, infection, nerve injury and late carotid restenosis. Practice varies widely between surgeons; from insertion of shunts in all the patients as a routine to complete avoidance of their use.
It is essential to avoid cerebral ischemia. To monitor cerebral perfusion various techniques are available. Assessing sensory, motor and higher mental function in an awake patient is the gold standard monitoring of cerebral function. Monitoring options in patients receiving general anaesthesia include carotid artery stump pressure, middle cerebral artery blood flow trans-cranial Doppler analysis, electroencephalography (EEG), somatosensory evoked potentials and near infrared spectroscopy. None of these techniques can be completely reliable. These electromechanical monitors can be used to determine the requirement for temporary shunt although their reliability varies.3
There are risks associated with the procedure of CEA. Patients only benefit from CEA when perioperative risks are low. The North American Symptomatic Carotid Endarterectomy Trial (NASCET) and the European Carotid Surgery Trial (ECST) are large randomized class 1 studies which have defined current indications for carotid endarterectomy.4,5 NASCET found that for symptomatic patients with a 70 ' 99% stenosis, for every six patients treated with CEA, one major stroke would be prevented at two years (i.e. a 'number needed to treat' (NNT) of six). Symptomatic patients with 50 ' 69% carotid occlusion had a lesser benefit, with a NNT of 22 at five years.6 Patients with multiple co- morbidities have a higher post-operative mortality and therefore get less benefit from the procedure. The European asymptomatic carotid surgery trial (ACST) found that asymptomatic patients with a greater than 80% stenosis in men (unclear in women) may also benefit from the procedure.7 For best outcome patients should be operated at the earliest possible preferably within a month after a TIA or stroke. Preoperative optimisation should be done for any co-morbidities such as ischemic heart disease, hypertension, diabetes mellitus and chronic obstructive pulmonary disease. When the CEA is to be performed as emergency procedure, it is more challenging for the medical, surgical and anaesthetic team due to inability to optimise the co-morbidities.
Anesthetic goals for CEA:
Chief anaesthetic management goals for carotid endarterectomy include - to maintain airway control, oxygenation, avoiding ischemic injury to the heart and brain, control of haemodynamics, good analgesia without any stress responses, providing good operating conditions for surgical procedure and cerebral monitoring.
Local versus general anaesthetic technique for CEA.
Carotid endartrectomy can be performed under both general as well as regional anesthesia.8,9 Both general and regional anaesthetic techniques have their own advantages.
Advantages of an 'awake' technique under regional block are:
- Gold standard cerebral function monitoring
- Earlier detection and management of complications.
- Better haemodynamic stability
- Reduced shunt insertion rate
- Intact cerebral autoregulation
- Reduced re-exploration rate
- Better postoperative pain relief.
- Useful when general anaesthesia is contraindicated.
- Shorter stay in the hospital.
Disadvantages of an 'awake' technique under regional block:
- increased risk of myocardial ischemia due to patient stress or pain while siting block.
-Problems in accessing the airway intraoperatively.
- Requires patient's co-operation.
- May need to be converted to GA during surgery.
Advantages of General Anaesthetic technique include:
- Controlled ventilation and CO2
- Potential for neuro protection
- Attenuated stress response.
Disadvantages of General Anaesthetic technique include:
- Neurological monitoring not possible.
- Intraoperative hypotension.
- Postoperative hypertension.
-Increased rate of shunt use.
- Delayed recovery can mask the neurological complications.
Local Anaesthetic Techniques:
Minimum cervical dermatomes required to be blocked for CEA surgery are second (C2), third (3) and fourth (C4). Blockade of branches of the trigeminal nerve supplying the submandibular area may also be required to reduce the discomfort due to retraction applied during surgery. Local infiltration of the carotid sheath is also required sometimes, as this has a cranial nerve supply. Various local anesthetic techniques which can be used for CEA are; cervical epidural, superficial and deep cervical plexus blocks (alone or in combination) and local infiltration.
Cervical epidural for carotid endartrectomy is rarely selected as technique of choice although it one of the alternative techniques.10-12 CE anaesthesia is usually performed with the patient in sitting position. Using a midline approach the epidural space is located at C6'7 or C7'T1. The epidural space is confirmed using the 'hanging drop' technique. An epidural catheter is inserted to a depth of 4'5 cm and placement is verified by aspiration. After administration of a test dose, 10 '15ml of 0.5% bupivacaine with or without fentanyl 50 ' 100mcg is given through the epidural catheter. This amount causes adequqte sensory blockade between C2 and T4'6 without the motor blockade of intercostals. Epidural top-up can be given as required.
Cervicl epidural is not a preferred technique as the problems associated with this are hypotension, bradycardia and alterations in respiratory function.
Superficial cervical plexus block
Technically superficial cervical plexus block is much easier to perform. Subcutaneous infiltration of 5-10ml of local anaesthetic is done along the posterior border of Sternocleidomastoid muscle around its midpoint.13 Complications associated with this are are rare but there can be potential damage to the superficial nerves, local haematoma formation and direct venous injection of local anaesthetic drug.
Patients are placed in supine position with slight head-up tilt. The head is turned to the opposite side. A small intradermal wheal is raised with 1% lidocaine at the posterior border of the sternocleidomastoid at the mid-level of the neck and needle is to be inserted alongside the posterior border of the sternocleidomastoid muscle and injection is given along the entire length of posterior border of the sternocleidomastoid muscle, caudally and rostrally, after confirming absence of blood during aspiration.
Modified or intermediate cervical plexus block:
For the intermediate block, the needle is to be inserted in a perpendicular plane at the midpoint of the posterior border of the sternocleidomastoid muscle until a 'loss of resistance'or 'pop' is felt as the needle passing the investing layer of the cervical fascia (at 1'2 cm depth).14,15 Injection in this plane is given with a fixed needle position.
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