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Topical local anesthetic agents are often applied in the form of a gel or cream. There are numerous combinations of anesthetic agents to use topically on wounds. TAC (tetracaine, adrenaline, and cocaine), LET (lidocaine, epinephrine, and tetracaine), and EMLA (eutectic mixtures of lidocaine and prilocaine) are three commonly applied combinations (Table 124-1). Other topical local anesthetic agents include lidocaine gel and liposomal lidocaine.
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Many Emergency Departments use TAC for topical anesthesia. It is a combination of 0.5% tetracaine, 1:2000 or 0.05% adrenalin (epinephrine), and 11.8% cocaine. TAC can be purchased or Hospital Pharmacists can compound these agents into a gel or liquid. It can be partitioned into individual use vials for easy application. The degree of anesthesia achieved with TAC is comparable to that of local infiltration with lidocaine for wounds on the face and scalp.3 The effects are less profound, however, for wounds on the trunk and extremities.
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The cocaine component in TAC is a powerful and effective local anesthetic agent as well as a vasoconstrictive agent. Unfortunately, TAC is a controlled substance. This makes its storage, utilization, and monitoring subject to stringent regulation and documentation requirements. This may limit the availability and use of TAC in some institutions.
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Dose recommendations generally call for 5 mL of TAC for lacerations smaller than 3 cm in length and 10 mL for lacerations greater than 3 cm in length.3 Apply the chosen volume of TAC liquid or gel onto a 2 × 2 gauze square or a cotton ball. Invert the gauze square or cotton ball to apply TAC within the wound margins. Allow TAC to remain for up to 15 minutes or until visible skin blanching occurs. Blanching indicates the presence of vasoconstriction and adequate analgesia. The gauze pad can be held in place by the patient's gloved hand, the patient's representative's gloved hand, or with tape.
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TAC is not free of complications. Always wear gloves when handling TAC to prevent percutaneous absorption. Do not apply TAC to tissues with an end-arteriole supply. This includes the fingers, toes, nose, ear, and penis. There is the possibility of vasoconstrictive ischemic injury to these tissues. The systemic absorption of cocaine as a result of TAC administration has been implicated in rare episodes of respiratory arrest, seizures, and death.4,5
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LET is an alternative to TAC. It is a combination of 4% lidocaine, 0.1% (1:1000) adrenaline (epinephrine), and 0.5% tetracaine. It is considered safer, similarly effective, more practical, and more cost effective to use than TAC. LET does not contain a controlled substance and has no potential for abuse. The resultant security and documentation requirements are therefore much less complicated than those required for TAC. Hospital Pharmacists can compound this agent.
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Apply liquid LET by dripping it into the wound or taping a LET soaked cotton ball or 2 × 2 gauze square over the wound. The addition of methylcellulose to liquid LET makes a gel that is more adherent and can be painted on wounds. The gel form will not drip or run. Similar to TAC, it should not be placed on the digits, ear, nose, penis, or other areas that are supplied by end arteries due to the strong vasoconstrictive effect and risk of ischemia. This dogma is currently being challenged.6
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The use of EMLA (eutectic mixture of local anesthetics) cream has gained significant popularity, particularly in pediatric patients. It is an emulsion of 2.5% lidocaine and 2.5% prilocaine in a 1:1 ratio by weight. Each gram of EMLA contains 25 mg of lidocaine and 25 mg of prilocaine. Apply EMLA only onto intact skin and not into open wounds. It is nonsterile and preservative free. It is also commercially available in prepackaged transdermal disks.
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EMLA is indicated prior to performing venipuncture, arterial punctures, accessing indwelling ports and reservoirs, lumbar puncture, minor skin procedures, or regional nerve blockade. Apply EMLA cream to intact skin and cover it with an occlusive dressing (e.g., Tegaderm) or apply a transdermal disk. Allow at least 1 hour for the EMLA to take effect. Analgesia is usually satisfactory after 1 hour, peaks at 2 hours, and persists for about 1 hour after it is removed from the skin. The prolonged time required for anesthesia to take effect limits its practical use in the Emergency Department. EMLA should not be applied to infants less than 3 months of age due to the risk of methemoglobinemia.7
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Liposomal Lidocaine (LMX-4 or LMX-5) is a 4% or 5% lidocaine cream that is a liposome-encapsulated formulation. The encapsulation of lidocaine keeps it from being rapidly metabolized. The lipid content of the liposomes allows for better drug penetration of the stratum corneum. The LMX-4 cream has several advantages over other topical anesthetic agents. This includes a more rapid onset of action and not requiring an occlusive dressing.1 The most frequent adverse reaction is local erythema. Although no serious side effects have been reported with the use of LMX-4, it is recommended that it be applied to an area less than 100 cm2 in patients who weigh less than 20 kg.8 When compared to a placebo, intravenous cannulation after the application of LMX-4 showed improved success rates on the first attempt.9 LMX-4 is as effective as EMLA.10 However, buffered lidocaine injection decreased the pain associated with intravenous catheter insertion to a greater extent than lidocaine cream.1
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Topicaine is a 4% lidocaine gel that is used for a number of procedures in the Emergency Department including placement of Foley catheters and nasogastric tubes. It is readily available and easy to use. Patients can develop a contact dermatitis, particularly if they have an allergy to amide-type local anesthetic agents. Apply Topicaine for 30 minutes prior to a procedure for maximum efficacy.