Field Block Anesthesia

E. J. Mayeaux, Jr, MD, DABFP, FAAFP
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Subject: Field Block Anesthesia

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Introduction

Field block anesthesia describes the infiltration of local anesthetic in a circumferential pattern around, and often under, a surgical site. Like nerve blocks, field blocks are used to anesthetize large areas of skin. Field blocks differ from nerve blocks in that more than one nerve experiences interruption of the nerve transmission. The technique permits large areas to be anesthetized, and it is useful for large dermatologic procedures. The field block does not disrupt the architecture of the surgical site and often is administered for facial or cosmetic repairs. However, it does require more time to work than intradermal blocks. It is not unusual for a block to require more than 5 minutes to reach full effect. 
Infected tissues such as areas of cellulitis or abscesses can prove difficult to anesthetize because the acidic environment of an abscess can hydrolyze the anesthetic and render it ineffective. Field block provides adequate anesthesia around an abscess by working in the normal surrounding tissue. Localized structures are often amenable to the field block technique, and it is particularly well suited for facial (e.g., cheek, eyelid, nose, pinnae) and genital structures (e.g., penis, perineum). The administration of anesthetic into distensible skin surrounding taut skin (e.g., tissues surrounding the nose or ear) permits more comfortable injections for the patient. 
Epinephrine can be added to lidocaine for some field blocks if the vasoconstrictive or anesthetic-prolonging action of epinephrine is desired. Epinephrine permits safe use of larger amounts of lidocaine because it prevents clearance of the anesthetic from the tissue. Epinephrine should be avoided in areas where vascular compromise could prove problematic, especially in individuals with vasculitis or vasoconstrictive disorders such as Raynaud’s phenomenon. Many authorities discourage the addition of epinephrine for field blocks on digits, around the ear, on the nasal tip, or surrounding the penis. 

Equipment

  • Syringes (TB, 5 mL, or 10 mL), anesthetic solutions, and needles (18 or 20 gauge, 1 in long for drawing up anesthetic; 25 or 27 gauge, 1.25 in long for delivering anesthetic) can be ordered from surgical supply houses or pharmacies. A suggested anesthesia tray that can be used for this procedure is listed in Appendix F.

Indications

  • Surrounding large lesions that would provide a large area of anesthesia

  • Around infected cysts or abscesses

  • To prevent distortion of skin landmarks from administration of local anesthesia

  • Around facial structures (e.g., nose, pinnae, forehead, cheek, eyelids, upper lip)

  • Digital blocks (see Digital Nerve Block Anesthesia)

  • Surrounding localized structures (e.g., penis, perineum)

Contraindications

  • Allergy to anesthetics

  • Cellulitis in the injection area (relative)

The Procedure

Step 1

The field block can be performed in a square- or diamond-shaped pattern around a wound. Only two skin punctures are required. After prepping with alcohol, the needle passes along one side of the proposed excision under the dermis, and anesthetic is administered as the needle is withdrawn without exiting the skin. 
  • PITFALL: Make sure to anesthetize enough area to allow for undermining.

Step 2

The needle is then redirected to the other side of the proposed excision, and anesthetic is administered as the needle is withdrawn without coming out of the initial puncture site. 

Step 3

For large lesions and subcutaneous lesions (such as cysts and abscesses), the needle may also be redirected below the lesion in case deep dissection is required. This entire technique is repeated on the opposite site of the wound. 
  • PITFALL: Be very careful not to inject anesthetic into cystic lesions because this may cause them to rupture, either below the skin or upward toward the provider.

Step 4

A field block of the ear is performed around the entire pinna. To avoid motor paralysis of the facial nerve anterior to the pinna, the needle should pass in a superficial plane (i.e., subdermally in front of the ear). 

Step 5

Separate injections may be needed for the concha and external auditory canal. 

Step 6

For nose blocks, triangulated injections provide adequate circumferential anesthesia. 

Step 7

Additional lidocaine (usually without epinephrine) must be administered to the tip of the nose to anesthetize the external nasal nerve, which arises from the deep tissues. This nerve is usually not blocked by the circumferential injections. 

Step 8

Administration of anesthetic in a linear pattern through both eyebrows produces anesthesia of the supraorbital and supratrochlear nerves on each side. A long (1.5-inch) needle should be used to provide near-complete anesthesia of the entire forehead to the scalp. 

Step 9

A dorsal penile nerve block may be accomplished by tenting the skin at the base of the penis and injecting 0.2 to 0.4 mL of 1% lidocaine (without epinephrine) into the subcutaneous tissue on both sides at the dorsal base of the penis through a single skin penetration. 
  • PITFALL: To avoid inadvertent intravascular injection, apply negative pressure to the syringe immediately before injection to check for a backflow of blood.

Step 10

A subcutaneous ring block also produces anesthesia for penile procedures. Two skin wheals can be administered near the internal inguinal rings. The long needle is placed subdermally to encircle the base of the penis with lidocaine (usually without epinephrine). 
Step 10
Step 10

Complications

  • Bleeding and hematoma formation.

  • Allergic reaction is rare. Patients who believe they are allergic to lidocaine are more likely allergic to the preservative methylparaben. Preservative-free lidocaine is available, usually in single-use vials.

  • Infection.

  • Palpitations or feelings of warmth (due to epinephrine component).

  • Temporary weakness or paralysis when large nerves are involved.

  • If a large volume (10 to 20 mL) of local anesthetic is injected into a vein, it may produce convulsions, arrhythmias, or cardiac arrest. The plasma levels are usually 3 to 5 mcg/mL with regional nerve blocks. Toxicities may be observed at 6 mcg/mL but are more common at levels >10 mcg/mL.

Pediatric Considerations

Children older than 6 years are dosed like adults except that the maximal dose is based on weight. The recommended maximal dose for lidocaine in children is 3 to 5 mg/kg, and 7 mg/kg when combined with epinephrine. Remember that 1% lidocaine is 10 mg/mL. Children 6 months to 3 years have the same volume of distribution and elimination half-life as adults. Neonates have an increased volume of distribution, decreased hepatic clearance, and doubled terminal elimination half-life (3.2 hours). 

Postprocedure Instructions

Have the patient report any postprocedure local rashes or blistering that may indicate an adverse reaction or infection. 

Coding Information and Supply Sources

Anesthesia codes (00100 to 01999) are usually limited to anesthesiologists providing patient services for surgical procedures. Local anesthesia is not reported in addition to the surgical procedure. Some insurance providers permit billing of regional or general anesthesia by the physician or surgeon performing the procedure. If reporting additional anesthesia services, the -47 modifier is attached to the surgical code. It is unlikely that additional reimbursement will be provided for field blocks; the service is considered part of the reporting of the surgical procedure. 

Bibliography

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