Anesthesia

Published on 04/03/2015 by admin

Last modified 04/03/2015

Print this page

This article have been viewed 2408 times

3 Anesthesia

Local anesthesia is the reversible loss of sensation to a localized area achieved by the injection or topical application of anesthetic agents. Regional anesthesia involves larger areas and is achieved by nerve block and/or field blocks through the use of injectable anesthetic agents. Local anesthetics block the pain fibers of a nerve better than those fibers that carry sensations of pressure, touch, and temperature. Therefore, if the patient feels pressure or pulling but no pain, the patient should be reassured that this is not unusual. Many anesthetic agents can be injected or applied topically.

Topical Anesthetics

Common anesthetics include the following:

• EMLA (eutectic mixture of local anesthetics; 2.5% lidocaine and 2.5% prilocaine cream) (Rx) (Figure 3-1)

• LMX 4 and LMX 5 (4% liposome-encapsulated lidocaine and 5% cream) (OTC)

• Topical lidocaine (2% and 4% gel or viscous solution, 5% ointment, 10% spray)

• Tridocaine gel (20% benzocaine, 6% lidocaine, 4% tetracaine) (available from Canada).

FIGURE 3-1 (A) An assortment of topical anesthetics for skin surgery. (B) EMLA was applied to the lower eyelids prior to electrosurgery of syringomas.

Table 3-1 shows the onset of action for the most commonly used topical anesthetics in skin procedures. EMLA (eutectic mixture of local anesthetics) cream consists of 2.5% lidocaine and 2.5% prilocaine in an oil-in-water emulsion. This anesthetic cream works best if applied for 30 minutes under occlusion or 60 minutes without occlusion before anesthesia is needed. It can be useful in treating children or a patient of any age who is afraid of needles. EMLA cream is used for some laser procedures, superficial light electrodesiccation of benign growths, and curettage of molluscum contagiosum. In Figure 3-1B the EMLA has been applied to the lower eyelids before electrodesiccation of syringomas. EMLA is available in 5-g tubes with occlusive dressings or in 30-g tubes without dressings. EMLA should not be used in infants younger than 3 months because the metabolites of prilocaine form methemoglobin. In one randomized control trial, EMLA and LMX 4 provided comparable levels of anesthesia after a single 30-minute application under occlusion prior to electrodesiccation of dermatosis papulosa nigra.¹

TABLE 3-1 Topical Anesthetics

Topical lidocaine is available as a 2% and 4% gel or viscous solution, a 5% ointment, or a 10% spray. Regardless of the vehicle, topical lidocaine takes 15 to 30 minutes to produce anesthesia on mucosal surfaces. The degree of anesthesia is not comparable to injectable lidocaine, and unless it is combined with another agent such as phenylephrine, it has no vasoconstrictive action. Therefore, for mucosal surgery in the mouth, it is best to inject 1% lidocaine with epinephrine after the topical lidocaine has numbed the surface. For this purpose, it is easy to put topical lidocaine gel on a cotton swab and have the patient hold it against the mucous membrane before the subsequent injection. This technique might be used to anesthetize a mucocele or fibroma on the lip.

Tridocaine (20% benzocaine, 6% lidocaine, 4% tetracaine) is a local analgesic, anesthetic, and antipruritic. It is used to prevent pain associated with laser surgery, superficial skin surgery, needle insertion, and intravenous cannulation. It is indicated for intact skin only and anesthesia is obtained in 10 to 15 minutes. It can be purchased online from Canadian sources or be created by a compounding pharmacy.

EMLA Technique for Intact Skin

1. Use alcohol wipes to remove oil from the area of skin to be anesthetized.

2. Apply EMLA in a thick layer to skin then apply occlusive dressing (Tegaderm, OpSite, or plastic wrap) for 30 to 60 minutes. Thicker skin may take more than 1 hour to achieve anesthesia.

3. Remove EMLA with alcohol or with a tissue.

4. Perform the procedure or additional anesthesia without delay. Topical anesthesia will last only minutes after the EMLA is removed.

Topical Refrigerants/Cryoanesthesia

• Ethyl chloride spray

• Medi-Frig (tetrafluoroethane) (Ellman International, Inc.)

• Liquid nitrogen.

Topical refrigerants make the skin cold and provide some anesthesia for the removal of small superficial skin lesions such as skin tags and molluscum. These agents provide some cryoanesthesia before snip excisions of skin tags or curettage of molluscum. Cryoanesthesia does not provide adequate anesthesia for the removal of larger lesions. These agents are sprayed on the skin until a white frost develops. The numbing effect is partial at best and disappears in seconds. Refrigerant sprays may be used before injecting local anesthesia with a needle in a needle-phobic patient. While these sprays are probably less effective than EMLA, they work faster and are therefore more convenient if the patient can tolerate the subsequent procedure.

Ethyl chloride is the only one of the refrigerants listed that is flammable, so it must not be used before electrosurgery. Medi-Frig can be safely used before electrosurgery for small skin tags and molluscum contagiosum. The manufacturers of Medi-Frig claim that it is nontoxic and ozone safe, whereas the label on ethyl chloride warns against inhaling too much of the product.

Liquid nitrogen can freeze the skin to cause a numbing effect; however, its use causes immediate pain and little real anesthesia. Rather than using liquid nitrogen for local anesthesia for skin tag excision, it is easier and less painful to use the liquid nitrogen with a Cryo Tweezer to treat the skin tags directly (see Chapter 15, Cryosurgery). Also, liquid nitrogen can be sprayed or directly applied to skin tags or molluscum rather than using it as anesthesia. This also avoids the need to deal with blood and hemostasis issues.

Contact cooling devices and assistive external devices are frequently used in laser treatments and are covered in Chapter 20, Anesthesia for Cosmetic Procedures.

Local Anesthesia by Injection

Lidocaine is the most widely used local anesthetic for injection in skin surgery. Bupivacaine (Marcaine) is used occasionally when long-lasting anesthesia is desired. Lidocaine has the advantage of having a more rapid onset (within 1 minute) and shorter duration of anesthesia than bupivacaine. This shorter duration is sufficient and preferable for most skin surgery. When lidocaine is mixed with epinephrine, it lasts almost as long as bupivacaine and hurts less.² See Table 3-2 for a comparison of amide local anesthetics.

TABLE 3-2 Local Anesthetics for Injection

Epinephrine

Epinephrine is a vasoconstrictor, whereas lidocaine alone is a vasodilator. Epinephrine with lidocaine decreases bleeding during surgery. Also, the vasoconstriction of epinephrine keeps the lidocaine in the area where it was injected, thereby decreasing immediate systemic absorption and toxicity while increasing the duration of anesthesia. This allows greater amounts of lidocaine to be used safely for local anesthesia by reducing systemic absorption and increasing the local action of the lidocaine.

1% Lidocaine with Epinephrine

For most skin surgery procedures, the recommended anesthetic is 1% lidocaine with epinephrine. Its advantages include almost immediate anesthesia, adequate duration of anesthesia, and decreased bleeding because of the epinephrine. The incidence of true allergies is negligible. Lidocaine without epinephrine has a duration of 1.5 to 2 hours, whereas lidocaine with epinephrine has a duration of 2 to 6 hours.³ The most common commercial preparation contains 1% lidocaine (10 mg/mL) with 1 : 100,000 of epinephrine. In a study that measured the effect of subdermal injection of lidocaine combined with epinephrine on cutaneous blood flow, laser Doppler imaging demonstrated an immediate decrease in cutaneous blood flow, which was maximal at 10 minutes in the forearm and 8 minutes in the face.⁴

Note that the initial skin blanching with injection is due to hydrostatic pressure along with the effect of epinephrine. When hemostasis is critical to the procedure, it is best to wait 8 to 10 minutes for the epinephrine to produce maximal vasoconstriction. Waiting is less important for shave biopsies of lesions that are not very vascular in patients who are not anticoagulated (Figure 3-2). Injecting enough anesthetic so that the skin is firm can limit the amount of bleeding because small blood vessels become compressed by the anesthetic fluid (Figure 3-3).

FIGURE 3-2 Local anesthetic injected under and around a seborrheic keratosis prior to shave biopsy to confirm that this highly pigmented lesion is benign. The elevation of the tissue makes the shave easier to perform.

FIGURE 3-3 Local anesthesia before excising a basal cell carcinoma on the forehead. Note how the tissue distends in this location to improve hemostasis as well as ensure adequate anesthesia.

Maximal Doses

Maximal safe doses of 1% lidocaine with or without epinephrine are found in Table 3-3. Above these doses the risk of neurotoxicity and seizures is increased. In most adults (over 50 kg) up to 35 mL of 1% lidocaine with epinephrine is safe. Use of 1% lidocaine is preferable to 2% in most cases because a larger volume of anesthesia produces greater hemostasis and makes most cutaneous surgery easier to perform. Two percent lidocaine without epinephrine is useful in various nerve blocks including digital blocks when less volume is needed. Some clinicians prefer 2% lidocaine with epinephrine when they want to avoid tissue distortion from too much anesthetic volume. In this case, toxicity is not an issue because less than 5 mL of anesthesia are being used.

TABLE 3-3 Maximum Doses of 1% Lidocaine (10 mg/mL)