• Marta Putzu, MD

I.	INTRODUCTION
II.	PRINCIPLES OF MULTISTIMULATION
III.	CLINICAL ADVANTAGES
IV.	MULTIPLE STIMULATION & THE RISK OF NERVE INJURY
V.	CLINICAL EXAMPLES
	Brachial Plexus Block
	Interscalene Brachial Plexus Block
	Axillary Brachial Plexus Block
	Midhumeral Brachial Plexus Block
	Femoral Nerve Block
	Sciatic Nerve Block
	High Sciatic Approaches
	Popliteal Approaches
VI.	SUMMARY

INTRODUCTION

Contrary to common, oversimplified representations by medical illustrators, the arrangement of peripheral nerves and plexuses is quite complex. Various tissue compartments contain connective tissue and fat; terminal branches of plexuses and nerves divide and separate frequently, and the different branches may run at some distance from one another. Accordingly, once injected at a particular site, the local anesthetic molecules have to diffuse through several barriers before reaching the nerves, the first barrier being the distance between the injection site and each branch.

In regional anesthesia, the most frequently used strategy for overcoming this “spatial dispersion” of nerve branches is to place the needle somewhere close to the nerves and then inject a volume of local anesthetic large enough to spread toward desired different branches. This practice has been traditionally taught and clinically used for decades throughout the development of regional anesthesia. However, with the advent of modern nerve stimulators and nerve-stimulating techniques, electrolocation can be utilized to augment selectivity in nerve location, increase the success rate, and minimize the amount of local anesthetic required to accomplish a nerve block.

This chapter will discuss general principles of multistimulation for the commonly used peripheral nerve block procedures. For more discussions on anatomy and techniques, the reader is referred to the respective technique and detailed description of the anatomy elsewhere in this book.

PRINCIPLES OF MULTISTIMULATION

The use of a nerve stimulator makes it possible to readily identify different muscular twitches during block placement, by simply redirecting the stimulating needle according to the anatomic topography of each nerve block considered, using the so-called multiple injection technique. The rationale of the multiple-injection technique is to specifically and separately localize and block each major nerve required for surgical anesthesia using a small volume of local anesthetic solution.

Clinical Pearls

The rationale of the multiple-injection technique is to specifically and separately localize and block each major nerve required for surgical anesthesia using a small volume of local anesthetic solution.

The needle is inserted as usual based on classical anatomic landmarks, with the nerve stimulator set at 1- to 1.5-mA intensity until the initial motor response is obtained. Thereafter, the intensity of the stimulating current is progressively reduced to less than 0.5 mA while maintaining the twitch response, at which point an injection of 5 to 7 mL of local anesthetic solution is made. When the desired component (nerve) is blocked, the current intensity of the nerve stimulator is again increased to 1 to 1.5 mA, and the needle is redirected according to the anatomic relationship among the individual components of the nerve being blocked. When the twitch of another component of the nerve is accomplished, an additional 5–7 mL of local anesthetic is injected. This maneuver is then repeated for all the main branches of the nerve required for surgery.

CLINICAL ADVANTAGES

Several clinical studies have demonstrated the advantages of the multiple-injection technique. The first report was by Lavoie and coworkers in 1992, who demonstrated that when performing an axillary block with the help of a nerve stimulator, stimulating three or four of the terminal nerves of the brachial plexus resulted in a higher success rate than a single injection of a larger volume of local anesthetic.¹ Subsequently, several authors reported on the improved quality and shorter onset time of the nerve block when using a multiple-injection rather than a single-injection technique.^2–5 And recent meta-analytic studies supported the evidence that, at least for axillary brachial plexus, the multiple injection should be always used. Similar results have also been reported with lower limb nerve blocks, including sciatic and femoral nerve blocks.^{6, 7} Of note, multiple- injection techniques also allowed a reduction (30–40%) in the volume of local anesthetic solution required to produce an effective nerve block.^{8, 9} This may help minimize the risk of systemic local anesthetic toxicity, especially when multiple nerve blocks are used, such as for lower limb surgery.

Clinical Pearls

Multiple-injection techniques typically allow a reduction in the volume of local anesthetic solution required to produce an effective nerve block

One disadvantage and possible problem with the multiple-injection technique is the increase in discomfort to the patient and in time for block placement. Several reports indicated that seeking multiple twitches resulted in a prolongation of the time required to place the block. However, the latency of the nerve block is also shorter, which helps offset this delay; consequently, the readiness for surgery of the multiple- and single-injection techniques remains similar.^4–7

MULTIPLE STIMULATION & THE RISK OF NERVE INJURY

The use of multiple injections theoretically may increase the risk of nerve injury, because the additional needle manipulation and injections are made after some volume of local anesthetic has already been injected. In other words, the needle can be inserted into an anesthetized nerve, and the pain or paresthesia symptoms of intraneural injection may be missed when present. However, a certain time is required to achieve anesthesia with peripheral nerve blockade. This time typically ranges from 10 to 30 min according to the type, volume, and concentration of local anesthetic used; longer times may be required after injection of small volumes (5–7 mL) of local anesthetic. In contrast, the time required to electrolocalize two or three neural components of a nerve or plexus is typically a few minutes in experienced hands. For these reasons, the multiple-stimulation technique should be considered an advanced technique, with adequate training and experience with single-injection blocks required for effective and safe use. Regardless, in a large observational investigation involving nearly 4000 peripheral nerve blocks performed with the multiple-injection technique, the withdrawal and redirection of the stimulating needle was not associated with an increased incidence of neurologic complications. An incidence of 2.5/1000 of persistent nerve injury (resolving after more than 6 months), was reported, similar to that reported using a conventional immobile needle technique.^{10, 11} When electrolocating components of the nerve or plexus, increasing the intensity of the stimulating current to 1.5 to 2 mA may be helpful in increasing sensitivity and recruiting partially blocked or inner mantle nerve fibers. In the near future, objective monitoring of injection pressure to avoid intraneural injection may make these techniques safer even in the hands of those with less experience in multiinjection techniques.

CLINICAL EXAMPLES

Brachial Plexus Block

The brachial plexus can be blocked at several different levels from the interscalene groove to the axilla, or even more distally. Consequently, a number of approaches can be used to produce a block of the brachial plexus. However, because of the specific anatomic characteristics of the brachial plexus, the multiple-injection technique is not recommended for all approaches. For instance, the multiple-injection technique is suitable in the interscalene, axillary, and midhumeral approaches, but is not recommended for a supraclavicular brachial plexus block. This is due to the increased risk for complications related to the greater number of needle passes in the proximity of the pleural cavity and large blood vessels in this area. Multistimulation with the infraclavicular brachial plexus has been reported,^{12, 13} but it is the preference of this author that this technique should be reserved for those with significant experience. In this chapter, we will discuss the use of the multiple-injection technique for the interscalene, axillary, and midhumeral approaches to the brachial plexus block.

Interscalene Brachial Plexus Block

To perform an interscalene block using a multiplestimulation technique, three different muscular twitches should be considered:

1. Contractions of the deltoid muscle, induced by stimulation of C4 and C5 roots.

2. Contractions of the biceps muscle with flexion of the forearm, induced by stimulation of the C6 root.

3. Contractions of the triceps muscle with extension of the forearm, induced by stimulation of the C7 root.

The patient is placed in a supine position with the head slightly turned away from the side to be blocked, similar to the single-injection technique. The interscalene groove formed by the anterior and middle scalene muscles is palpated at the level of the cricoid cartilage (C6). The nerve stimulator is set at 1 mA, 2 Hz, and 0.1 ms, and a 22- to 25-gauge, 3.75- to 5-cm needle is introduced at a 45-degree angle, with a slight caudal and posterior direction. The needle is advanced slowly until obtaining a specific motor response. The first motor response usually observed is contraction of the deltoid muscle. The position of the needle is then adjusted to maintain the same motor response with a current of 0.2 to 0.5 mA. After negative blood aspiration to rule out an intravascular needle placement, 8 mL of local anesthetic solution is slowly injected. Next, the insulated needle is withdrawn to the level of the skin, and the intensity of the current is increased to 1 to 1.5 mA. The needle is then introduced through the same skin insertion site in a more caudal direction on the same plane, identifying the projection of the brachial plexus between the two scalene muscles and running toward the midpoint of the clavicle and the sulcus between the pectoralis major and deltoid muscles, in order to elicit the second motor response (contraction of the biceps muscle with flexion of the forearm). After obtaining the biceps muscle twitch at 0.2 to 0.5 mA, 6–8 mL of local anesthetic is injected after a careful aspiration test. Afterward, the insulated needle is withdrawn again to the level of the skin with the intensity of the current set at 1 to 1.5 mA and then introduced in a more caudal direction on the same plane identifying the pathway of the brachial plexus to elicit contractions of the triceps muscle with extension of the forearm. After the same twitch is elicited with a current <0.5 mA, another 6 mL of local anesthetic is injected slowly after a careful aspiration test.

Clinical Pearls

The first motor response determines how the needle must be reoriented to look for the next motor response.

Occasionally, it may be difficult to feel the interscalene groove at the C6 level; in these cases the groove can be more easily palpated at the level of the clavicle, where it is larger; from there, the groove can be followed cranially to the C6 level.