Cervical Plexus Block for Carotid Endarterectomy - Anatomy

Cervical Plexus Block for Carotid Endarterectomy

Anatomy

Adequate anesthesia for surgery on the carotid artery may be obtained by blocking cervical nerves 2, 3 and 4. The distribution of these nerves covers the area from the back of the head to the clavicle. There is, however, some overlap of unanesthetized dermatomes onto the surgical field, ipsilaterally, as well as contralaterally. On the ipsilateral side, overlap may occur at the level of the clavicle from the second thoracic nerve. This is the next dermatome after C4, as C5 to T1 form the brachial plexus. The third division of the trigeminal nerve provides sensation down to the edge of the mandible. Some overlap of nerve fibers from this distribution may result in patient discomfort at the upper pole of the incision. Overlap of dermatomes from the contralateral side may also occur because the incision line is close to the midline of the neck. This overlap may result in areas of the surgical field that need supplemental local anesthetic administered by the surgeon. In addition, structures inside the carotid sheath receive innervation from cranial nerves IX and X. This may cause additional patient discomfort as the carotid sheath is entered, which can be eliminated by topical administration of local anesthetic to the area.

When performing a block of the cervical plexus, it is important to keep in mind the relationship of palpable structures and landmarks to the transverse processes of the cervical vertebrae. There are two aspects of this block. The first involves blocking the roots of C2 to C4. The second part of the block, or superficial block, involves anesthetizing the nerves of the superficial cervical plexus as they emerge from the posterior border of the sternocleidomastoid muscle.

Deep Cervical Plexus Block

The roots of cervical nerves 1 to 7 emerge superior to the transverse process of each cervical vertebra. In an adult, the larynx overlies cervical vertebrae 4 to 6 (1). The upper border of the larynx (C4) is palpable as the notch in the thyroid cartilage. The lower border of the larynx (C6) is the cricoid cartilage. Another landmark which can frequently help delineate the level of C6 is the point at which the external jugular vein crosses the posterior border of the sternocleidomastoid muscle. With the patients' head turned to the contralateral side, a line can be drawn from the cricoid cartilage to the point where the external jugular vein crosses the posterior border of the sternocleidomastoid. This will be at the level of C6. A line from the thyroid notch can be drawn parallel to the line for C6 to determine the level of C4. To determine the position of the transverse process of C4, the mastoid process is located, and a mark is made 1cm posterior to it. A line is then drawn from this mark to intersect at right angles to the line drawn back from the thyroid cartilage for C4. Where these two lines cross is the location of the C4 transverse process. The location of C3 and C2 can be located quite easily once the position of C4 is known. This is done by following the line drawn from the mastoid process to C4 back up from C4 towards the mastoid, half of the distance between the C4 line and the C6 line to determine the position of C3 and the same distance again for C2.

After drawing the positions of the transverse processes on the patient's skin, the neck is prepped and draped. Using a 22 gauge 1.5 inch needle, the transverse processes are located by entering perpendicular to the skin. The depth of the transverse process varies with the body habitus of the patient. Once bone is contacted, the needle is pulled back slightly, and directed cephalad. This is repeated until the needle is walked off the bone. Moving cephalad will help assure that the needle is close to the desired nerve root. It is important to only walk off the bone a millimeter or two, because going any further can result in entering the subarachnoid space or the vertebral artery. After negative aspiration, 5cc of local anesthetic is injected slowly. This is repeated at the other two transverse processes. During injection, continuous communication with the patient is essential in order to detect a change in mental status that could occur with intravascular injection. (Video of block being performed.)

Superficial Cervical Plexus Block

The superficial block is performed by injecting local anesthetic along the posterior border of the sternocleidomastoid. First, a subcutaneous injection is made along the entire border using 5 to 7 cc of local. Then, another 4 cc is injected at the midpoint of the posterior border. This is the point where the bulk of the superficial cervical plexus exits the posterior border of the sternocleidomastoid. This final injection is done by entering to a depth of half the needle length, while aspirating, then injecting 1cc of local while withdrawing. This is repeated 3 more times, each time turning the needle 90 degrees.

Local Anesthetic Choice

A variety of local anesthetics can be used for cervical plexus block. The potential toxicity of a particular drug and concentration must be weighed against the benefits of using that drug. The length of the surgery must also be taken into account, as different local anesthetics have different durations of action. A volume of approximately 30cc of local anesthetic is necessary for this block. The total dose of drug to be given can be determined by multiplying this volume by the number of milligrams per cc in the concentration of drug chosen. Since the neck is a relatively vascular area, there may be a good deal of systemic absorption of local anesthetic with a cervical plexus block (2). This may increase the risk of local anesthetic toxicity. Absorption can be reduced by adding epinephrine in a concentration of 1:200,000 to the local anesthetic, which will cause local vasoconstriction. We have found that mepivacaine, in a concentration of 1.5% gives an adequate block for carotid endarterectomy, with a duration of approximately four hours. At this concentration, the likelihood of reaching toxic levels is low for an average size adult. In a particularly small patient, either the volume or the concentration of the drug must be reduced.

Complications

Several complications may occur during the performance of cervical plexus block, and they can be minimized by proper knowledge of the physiology of the block and the toxicity of local anesthetics. Intravascular injection of local anesthetic may occur either into a vein or artery. Systemic toxicity of local anesthetics results in either central nervous or cardiac effects (3,4). The CNS effects vary from sedation to tinnitus to seizures, depending on the blood level. Cardiac effects generally occur at higher blood levels than CNS effects (5). Conduction blockade and myocardial depression progressing to refractory arrhythmias and cardiac arrest at higher levels may occur.

The vertebral artery is very close to the site of desired injection. It lies in the vertebral canal, which is only about 0.5 cm below the tip of the transverse process. Since the vertebral artery has direct supply to the brain, only a small amount of local anesthetic needs to be injected to cause CNS effects. Therefore, it is important to remain in constant communication with the patient during injection to help detect early signs of CNS toxicity such as peioral numbness, disorientation or tinnitis. Aspiration should be done frequently. If injection is performed slowly, and patient contact is maintained, progression of toxicity from CNS to cardiac is very unlikely.

Toxicity may be reduced in the CNS by giving the patient intravenous benzodiazepine prior to performing the block. This may raise the seizure threshold (6,7). Caution must be exercised in the use of benzodiazepines, because oversedation may make the patient uncooperative and the sedation caused may be difficult to differentiate from early CNS toxicity of local anesthetics. If CNS toxicity does occur, the patient should be given oxygen by mask and observed. If progression to seizure activity occurs, this can be treated with thiopental in a dose of 2 mg/kg. The patient may need to have tracheal intubation to maintain adequate oxygenation. This can be accomplished with succinylcholine, in addition to thiopental. The decision as to whether to proceed with the surgery under these conditions must be made by both the surgeon and the anesthesiologist. Besides intravascular injection, subdural injection may also occur. If the needle is placed too far, a dural sleeve around the nerve root may be entered. Local anesthetic injected here may cause subarachnoid block. This may be seen as unconsciousness and hypotension. Pressure support and endotracheal intubation may be necessary. This will resolve when the local anesthetic is metabolized from the CNS.

An additional complication of cervical block is hematoma formation. This may occur if the needle enters a large blood vessel. Usually, local compression will alleviate the problem, but occasionally, the hematoma will progress, and rarely airway compromise may result.

Because of the fact that the phrenic nerve is composed of cervical nerves 3, 4 and 5, unilateral phrenic nerve palsy is possible with this block. This should not present a problem unless the patient has severe pulmonary disease and is dependent on diaphragmatic function for adequate respiration (8). Cervical plexus block should probably be avoided in these patients.

Sedation

Management of sedation is important during cervical block. The patient must be awake enough to respond to commands, thereby assuring adequate cerebral blood flow. On the other hand, some sedation may be needed in order to allow the patient to tolerate the surgery. Small amounts of benzodiazepines and/or narcotics may be titrated to achieve the desired effect. Occasionally, especially in elderly patients, benzodiazepines will cause disinhibition, resulting in restlessness and confusion. This can be differentiated from inadequate cerebral blood flow by reversing the benzodiazepine with flumazenil. Oversedation with narcotics can be reversed with small doses of naloxone. Caution must be exercised with naloxone, because rapid administration may result in hypertension and congestive heart failure.

References

1. Anderson JE. Grants' Atlas of Anatomy, 7th Ed. Baltimore, Williams and Wilkins, 1978.

2. Tucker GT, Moore DC, Bridenbaugh PO, et al. Systemic absorption of mepivicaine in commonly used regional block procedures in anesthesiology. Anesthesiology 1972;37:277.

3. Wagman IH, deJong RH, Price DA. Effects of Lidocaine on the central nervous system. Anestehesiology 1967;28:155.

4. Block A, Covino B. Effect of local agents on cardiac conduction and contractility. Reg Anesth 1981;6:55.

5. Liu PL, Feldman HS, Giasi R, et al. Comparitive CNS toxicity of lidocaine, etidocaine, bupivicaine and tetracaine in awake dogs following rapid IV administration. Anesth Analg 1983;62:375.

6. DeJong RH, Bonin JD. Benzodiazepines protect mice from local anesthetic convulsions and death. Anesth Analg 1981;60:385.

7. Moore DC, Balfour RI, Fitzgibbons D. Convulsive arterial plasma levels of bupivicaine and the response to diazepam therapy. Anesthesiology 1979;50:454.

8. Urmey WF, Talts KH, Sharrock NE. One hundred percent incidence of hemidiphragmatic paresis associated with interscalene brachial plexus anesthesia as diagnosed by ultrasonography. Anesth Analg 1991;72:498