Regional anesthesia is required for a wide variety of urgent care procedures. Its purpose is to lessen patient distress and deaden the sensation of pain. This chapter will describe various techniques of obtaining adequate regional anesthesia using the most commonly available local anesthetic agents: lidocaine, mepivacaine, and bupivacaine.
Local anesthetic agents work by blocking the neural depolarization of pain fibers. Neural depolarization is mediated by a rapid influx of sodium ions through special channels in the nerve cell membrane. Local anesthetic agents bind to specific protein receptors within these sodium channels. Once bound, sodium influx is temporarily inhibited, thus blocking neural depolarization. Small fibers carrying pain and temperature sensations are blocked before larger fibers carrying touch, motor function, and proprioception sensations. Local anesthetic potency is determined by the lipid solubility of the agent. Highly lipid-soluble agents cross the lipoprotein nerve cell membrane more readily and are more effective at inducing a blockade. Onset of action is mediated by the pKa (the pH at which a drug exists in equal proportions of ionized and un-ionized molecules). Un-ionized molecules cross into the nerve cell more rapidly, but it is the ionized molecules that bind most successfully to the sodium channels. At physiologic tissue pH, agents with a lower pKa will have relatively more un-ionized particles available to bind to the nerve cell. If the tissue is very acidotic (i.e., in an abscess cavity), these agents have relatively more particles in the ionized state, and the agent becomes less effective. Duration of action of a local anesthetic agent is determined by the dose (or concentration) administered. Injecting a larger dose or adding a vasoconstricting agent such as epinephrine serves to keep local concentration of the agent high and prolongs the duration of action.
summarizes the properties and dosages of commonly available local anesthetic agents.
TABLE 46.1. Summary of Properties of Common Local Anesthetic Agents
What You Need
Syringe for drawing up anesthetic agent and needles for drawing up (16- or 18-gauge) and infiltrating the solution (25-, 27-, or 30- gauge)
Local anesthetic agent of choice
Betadine or similar antiseptic skin prep solution
Contraindications to Local Anesthetic Use
Allergy to anesthetic agent (RARE!)
The need to inject through infected tissues
Severe bleeding diatheses
A field block is a method of injecting anesthesia in a “wall” across the path of the nerves supplying the operative field. This provides good anesthesia for a relatively small area. A field block is ideal for small excisional procedures. Advantages of a field block include no distortion of the tissues in the operative field from “volume effect” of the local anesthetic, and a longer duration of anesthesia over simple infiltration techniques. Figure 46.1
demonstrates a simple
field block. Note that repeated injections are required until the entire border of the field is infiltrated.
FIG. 46.1. A simple field block.
As the procedures under a field block tend to be small, select lidocaine 1% or 2% with epinephrine as the anesthetic agent. Allow 5 to 10 minutes for full anesthetic effect.
Peripheral Nerve Blocks
A nerve block involves infiltration of local anesthetic near the nerve branch supplying sensation to a specific area. A nerve block provides a longer duration of anesthesia than local cutaneous infiltration. A nerve block is precisely applied and thus requires a smaller anesthetic dose than a field block or cutaneous infiltration. Tissue distortion from volume effect is also reduced, as the nerve block is applied at a distance from the surgical site.
Nerve blocks, however, require knowledge of the anatomy of peripheral nerves and strict aseptic technique for maximal success. A nerve block is appropriate for larger excisional procedures, extensive skin lacerations, tendon repair, reduction of small fractures, or any procedure that may require a prolonged period of anesthesia.
Lidocaine 1% or 2% without epinephrine is appropriate for most nerve blocks. Blocks that require an extended anesthesia effect should use a half-and-half mixture of lidocaine and mepivacaine or bupivacaine with epinephrine. Use a 25-, 27-, or 30-gauge needle for infiltration of the block.
Common nerve blocks of a digit, hand, face, ear, and foot will be demonstrated here. Oral and dental nerve blocks are discussed in Chapter 14
, “Intraoral Anesthetic Techniques and Supraperiosteal Dental Nerve Block.”
Take an appropriate history from the patient and perform a focused neurologic examination of the area to be anesthetized. Document any preexisting neurologic deficits. Obtain appropriate consent. REFER to a surgical consultant any case that you feel is beyond your expertise, or if a significant neurologic injury exists.
Identify nerve to be blocked and review appropriate anatomic relationships.
Prep skin over the site of the block with Betadine or similar antiseptic skin prep solution.
Using a small (25-, 27-, or 30-gauge) needle, infiltrate appropriate amount of anesthetic solution around (NOT INTO) the nerve to be blocked. You may want to withdraw the syringe plunger periodically to ensure there is no intravascular injection. If the patient reports a sudden paresthesia in the distribution of the nerve to be blocked, withdraw the needle 2 to 3 mm and continue the infiltration.
Allow ample time (5 to 15 minutes) for the block to take effect.
Digital Nerve Block of a Toe or Finger
A digit has two dorsal and two volar nerves. Anesthetic solution needs to be infiltrated around all four of these nerves to provide an effective block. Refer to Fig. 46.2
for specific techniques. We recommend using 1% or 2% lidocaine without
epinephrine and infiltrating with a 0.5-inch 25- or 27-gauge needle. Insert the needle into the base of the digit in the web space to begin. For the finger, inject 1 mL of anesthetic into each lateral aspect, then 1 mL each across the dorsal and the volar surfaces to complete the block. For the toe, use 2 mL of anesthetic into each lateral aspect and 1 mL each across the volar and dorsal surfaces. If you contact bone during the infiltration, pull the needle back 2 mm.
FIG. 46.2. Performing a digital nerve block.
Nerve Blocks of the Hand
Three nerves supply sensation to the hand. They are the ulnar, median, and radial nerves. See Fig. 46.3
for a review of the portions of the hand supplied by each nerve.
FIG. 46.3. Schematic representation of sensory innervation to the hand.
Ulnar Nerve Block
We recommend using a lidocaine and mepivacaine/bupivacaine mixture with epinephrine; a total of 3 to 4 mL of this mixture should be plenty. Using a 25- or 27-gauge 0.5-inch needle, infiltrate the anesthetic around the ulnar nerve as it passes between the medial epicondyle of the humerus and the olecranon (Fig. 46.4
FIG. 46.4. Performing an ulnar nerve block.
Median Nerve Block
We recommend using 2 to 5 mL of 1% or 2% lidocaine without
epinephrine. Infiltrate the anesthetic at a depth of 5 to 7 mm between the flexor carpi radialis and palmaris longus tendons at the flexor crease (Fig. 46.5
). It is often useful to have the patient flex the wrist against some counterpressure to make these tendons stand out and aid in identification of the anatomy.
FIG. 46.5. Performing a median nerve block.
Radial Nerve Block
As there are multiple divisions of the radial nerve, a larger dose of anesthetic agent will be required to successfully perform this block. Lidocaine, or a mixture
of lidocaine and mepivacaine/bupivacaine without
epinephrine is recommended. Use 10 mL in total. Infiltrate 3 mL of anesthetic 2 cm distal to the lateral aspect of the radial styloid process, lateral to the radial artery. Then infiltrate the rest of the anesthetic in a ring pattern dorsally over wrist, and volarly to the lateral border of the anatomic snuffbox (Fig. 46.6
). The nerve is quite superficial, lying just under the superficial fascia.
FIG. 46.6. Performing a radial nerve block.
Nerve Blocks of the Face
Sensory innervation to the face may be interrupted by selective blocks of the supraorbital and supratrochlear nerves (forehead block), infraorbital nerve (midface block), and mental nerve (jaw block).
illustrates the anatomic relationships of these nerves and the clinical techniques.
FIG. 46.7. Anatomic relationships of the supraorbital, supratrochlear, infraorbital, and mental nerves, with infiltration techniques.
For a forehead block, both the supraorbital and supratrochlear nerves must be infiltrated. Use 4 to 5 mL of lidocaine, or a mixture of lidocaine and
epinephrine. Infiltrate just above the bone beneath the entire medial two-thirds of the eyebrow.
The midface block is quite useful for repair of larger midfacial lacera-tions, upper-lip lacerations, nasal lacerations, or nasal manipulation. Use 2 to 3 mL of lidocaine with epinephrine. Palpate the infraorbital notch; the infraorbital foramen lies just 1 cm below this landmark. Infiltrate directly over this area.
The mental nerve may be blocked to provide anesthesia over the lower half of the ipsilateral lip. This nerve exits the mandible inferior to the lower second bicuspid, 2 to 3 cm lateral to the midline of the lower jaw. Use 2 mL of lidocaine without epinephrine and inject the anesthetic after entering the gingival buccal margin just inferior to the second bicuspid.
There are multiple cutaneous nerves providing sensation to the ear. See Chapter 8
, “Lacerations of the Ear,” for a demonstration of a field block technique for the ear.
Foot blocks are useful to limit patient discomfort during brief surgical procedures. The sole of the foot is exquisitely sensitive, and direct anesthetic infiltration of the plantar surface is quite painful.
The plantar surface of the foot receives sensory innervation from the sural nerve and the tibial nerve. Figure 46.8
demonstrates the distribution of these two nerves.
FIG. 46.8. Sensory innervation of the plantar surface of the foot.
Sural Nerve Block
Infiltrate anesthetic in a fan-shaped pattern by inserting the needle lateral to the Achilles tendon, 1 to 2 cm proximal to the level of the distal tip of the lateral malleolus (Fig. 46.9
). Depending on the magnitude of the procedure, you may use lidocaine, or a mixture of lidocaine and a longer-acting agent with or without
FIG. 46.9. Performing a sural nerve block.
Tibial Nerve Block
Because of the close proximity of the tibial artery to the tibial nerve, palpate, identify, and make a mental note of where the tibial artery is. The nerve is just posterior to the artery. Infiltrate just posterior to the artery with 5 mL of 1% or 2% lidocaine, or with a mixture of lidocaine and mepivacaine/bupivacaine without epinephrine (Fig. 46.10
). Aspirate periodically during infiltration to make sure that you are not injecting intraarterially.
FIG. 46.10. Performing a tibial nerve block. Infiltrate behind the artery!