Topical Anesthetics

Topical anesthetics such as tetracaine (0.5 percent), adrenaline (0.05 percent, 1:2000), and cocaine (4 percent to 11.8 percent) (TAC); lidocaine (4 percent), epinephrine (0.1 percent, 1:1000), and tetracaine (0.5 percent) (LET); and EMLA can be used for a number of different painful procedures and may eliminate or decrease the need for painful infiltrative injection. Compared to infiltrative anesthesia, TAC and LET provide good hemostasis, do not distort wound edges, and can be applied painlessly. Until recently, TAC was the most frequently used topical anesthetic in the United States. Concerns over the safety of TAC stem from the reported cases of seizures, respiratory arrest, and even death in children from the inadvertent systemic absorption of cocaine from its improper application. TAC is a controlled substance, making it less readily available and creating the potential for abuse. In addition, TAC has been estimated to cost from 10 to 17 times more than LET, depending on the concentration of cocaine. LET has been found to be just as effective as TAC in providing local anesthesia for laceration repair. For these reasons, LET is a safer, more practical, and more cost-effective choice.

LET should be made available in single-dose individual vials with a maximum volume of 5 mL. Several drops of the solution can be dripped into the wound with a syringe. Alternatively, a gel can be made by adding methylcellulose powder to each vial, allowing the LET to be "painted" in the wound with a sterile, cotton-tipped swab, with less chance for dripping the solution into mucous membranes. The remainder of the solution/gel should be placed on a cotton ball or sterile gauze roughly the size of the wound and either taped to the wound or held by the patient or caregiver wearing a glove. It should be left in place at least 20 min for the maximum anesthetic effect. LET should be avoided on mucous membranes, the pinna of the ear, the nose, penis, and fingers and toes. The gel form may be used cautiously near the eyes, nose, or mouth.

Lidocaine may be used as a topical anesthetic for a variety of painful conditions and procedures. It is marketed in solution, jelly, and ointment forms in concetrations from 2 percent to 10 percent. Viscous lidocaine solutions (2 percent) can be used for temporary relief of inflamed or irritated mucous membranes of the mouth and pharynx (viral stomatitis). A sterile preparation of the viscous solution can be used for insertion of Foley catheters and gastrostomy tubes. Strict instructions to the patient and caregivers about sparing use of the viscous oral solution must be given. Only a limited amount (about 2 ml) of the solution should be applied. Topical benzocaine anesthetic solutions such as Auralgan (antipyrene/benzocaine) may be effective in temporarily alleviating pain due to acute otitis media or external otitis media.

EMLA is an emulsion in which the oil phase is a eutectic (melting point below room temperature) mixture of lidocaine and prilocaine in a ratio of 1:1 by weight (lidocaine 2.5 percent and prilocaine 2.5 percent). Each gram of cream contains 25 mg lidocaine and 25 mg prilocaine, carboxypolymethylene (thickening agent), sodium hydroxide to adjust to a pH of 9, and purified water to 1 gram. EMLA is for use on intact skin. It contains no preservative, is not sterile, and is not recommended for anesthesia of open wounds, although this is being further investigated. Onset, depth, and duration of dermal analgesia depend primarily on the duration of the application. EMLA cream should be applied under an occlusive dressing. There are also new EMLA prepackaged transdermal adhesive disks available. Satisfactory analgesia is achieved after 1 h, peaks at 2 h, and persists for 1 h after removal; both delivery systems have similar pharmacokinetics. The indications are to provide topical anesthesia for venipuncture, arterial puncture, vascular cannulation, drug reservoir or port access, lumbar puncture, sites for needle insertion for regional blocks, and superficial minor skin procedures. However, the time required for adequate anesthesia limits its use for emergent procedures in the ED. Higher effectiveness observed in children over 7 years and adults suggests the importance of emotional and psychological support of younger children undergoing procedures.

Absorption is more rapid on diseased, facial, or Caucasian skin, when compared with nondiseased intact skin, the extremities, or African American skin. Although the incidence of systemic adverse reactions is very low, caution should be exercised, particularly with application to large areas or for longer than 2 h ( Table,, 32-1.0). Local reactions of blanching or erythema are common. In infants <3 mos there is a theoretical risk of methemoglobinemia because of the low levels of methemoglobin reductase at this age. Toxicity as for lidocaine and prilocaine may be encountered, but is unlikely when EMLA is used as recommended; it is not contraindicated in labor and delivery.2 27

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