CHAPTER 8 Peripheral Nerve Blocks and Field Blocks
Many ambulatory procedures lend themselves well to local anesthesia with a field block or a peripheral nerve block. A field block is a method of providing anesthesia to a relatively small area by injecting a “wall” of anesthetic solution across the path of the nerves supplying the operative field (Fig. 8-1). Instead of the injection being made directly into the area of the procedure, it is made into the soft tissue some distance away, where the nerves are situated. Advantages include longer duration of anesthesia and no distortion of the operative field.
Figure 8-1 Field block technique. This method of injecting around the lesion prevents distortion of the anatomy and allows any deeper central lesion to remain palpable. The red lines indicate the path of the needle (four separate insertions). The black line indicates the incision line.
A nerve block is the infiltration of a local anesthetic near the nerve branch supplying sensation to a particular area. Blocking a nerve provides longer duration of anesthesia than that obtained with local cutaneous infiltration. Knowledge of the anatomy of peripheral nerves and a scrupulous sterile technique are important for successful peripheral nerve blocks. Use of this technique may reduce the amount of anesthetic needed, reduce distortion of tissues, and allow palpation of pathology to be excised.
In some sites (e.g., the breast) a nerve block cannot be obtained, and thus the field block is the only reasonable alternative. However, where possible, the nerve block may be the procedure of choice.
There are few complications with field and nerve blocks. The benefits of the blocks versus the alternatives (e.g., general anesthesia, no anesthesia, and Bier block) may be explained. Depending on the agent used, the duration of anesthesia may be prolonged, and the patient should be informed of the expected length of action. In rare instances a nerve could be traumatized, but long-term consequences are rare. Any precautionary advice, such as avoidance of heat or cold after the procedure, should be given to the patient. The possibility of paresthesia during the injection should be explained.
The technique of administering a field block is similar to the technique discussed for local anesthetics (see Chapter 4, Local Anesthesia). In this instance, however, the area to be incised is spared from the injection. Rather, the area around the site is injected (see Fig. 8-1). Repeat injections are made until the entire border of the field has been infiltrated. Allowing 5 to 10 minutes for the block to take effect improves the resulting anesthesia.
Figure 8-2 Anatomy and injection technique for digital nerve block. A, The four digital nerves. The bone is used as a landmark to find the proper plane of the dorsal digital nerve. B, Site of injection in web space. When removing a toenail, an additional 1 mL of anesthetic can be placed just proximal to the nail. C and D, Digital nerve block of the finger. The sites of the nerves are injected bilaterally. Insert the needle and, after touching bone, withdraw slightly and then inject 0.5 mL of anesthetic. E, Digital nerve block of the toe, showing an alternative method of injection from the dorsal aspect. This is followed by a ventral injection in the same manner. F, Nail wing block.