WP Chung_OT in Head and Neck Reconstructive Surgery_97819751

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Chapter 1 Local Anesthesia of the Face

■■ The great auricular and lesser occipital nerves emerge from Erb point in the posterior cervical triangle, located approximately at the midpoint of the posterior border of the sternocleidomastoid muscle. ■■ The greater occipital (C2) and third occipital (C3) nerves supply the posterior scalp and neck. MECHANISMS OF ACTION ■■ All local anesthetics share a similar molecular structure composed of a hydrophilic amine and a lipophilic aromatic ring linked together by an intermediate chain that classifies the agent as an ester or amide. ■■ Both esters and amides cause reversible inhibition of volt- age-gated sodium channels within affected nerves, thereby blocking depolarization and inhibiting action potential propagation and ultimately sensation. 1 ■■ Esters, such as tetracaine, procaine, and chloroprocaine, are hydrolyzed by plasma cholinesterases and excreted by the kidneys (Table 1). ■■ In contrast, amides such as lidocaine, mepivacaine, and bupivacaine are metabolized by microsomal enzymes in the liver. 2 ■■ The structural components of the local anesthetic agent determine various parameters for clinical efficacy. ■■ The lipid solubility of the aromatic ring influences the diffusion and potency of the product, with more lipid- soluble molecules possessing greater diffusion through the nerve cell membrane resulting in higher potency. ■■ Agents that are highly protein-bound have a longer dura- tion of action as they possess a stronger affinity for the sodium-gated ion channels. ■■ Speed of onset is determined by the drug dissociation constant (pKa), defined as the pH at which the ionized and nonionized forms of the drug are present in equal amounts. ●● All local anesthetics have a pKa higher than physiologic pH. ■■ The closer the pKa is to physiologic pH, the more nonion- ized form of the medication is present, and the more rap- idly the drug penetrates the nerve cell membrane creating its clinical effect.

■ Ropivacaine is a newer amide anesthetic that has been shown to have a rapid onset and longer duration of action compared to lidocaine.

NATURAL HISTORY

■ The Inca populations were the first to discover the pain- reducing property of the Erythroxylum coca plant. 2 ■■ In the late 1880s, cocaine was isolated from the plant and used initially in ophthalmologic surgeries. ■■ Safer alternatives were developed early in the 20th century followed by widespread acceptance within the medical community. ■■ Lidocaine was the first amide anesthetic to undergo clinical testing in the 1940s, with a faster onset, longer duration, and lower toxicity compared to the ester formulations avail- able at the time. ■■ Eliciting relevant patient history and performing a physical exam prior to the use of any anesthetic agent are crucial to ensure patient safety and minimize complications. ■■ Patient history ■■ Determine medication allergies and prior exposure to injectable or topical anesthetics. ■■ Obtain an up-to-date list of the patient’s medications including over-the-counter herbs and agents to avoid potential interactions, particularly with drugs metabo- lized through the liver. ■■ A detailed medical history should be elicited, with par- ticular attention to cardiac, renal, liver, and neurologic diseases. ■■ Relative contraindications ■■ Hypersensitivity to the agent (particularly esters) ■■ Application of a topical agent to the skin with impaired barrier function ■■ G6PD (glucose-6-phosphate dehydrogenase) enzyme defi- ciency with the use of EMLA (eutectic mixture of local anesthetics) ■■ Significant cardiac or hepatic disease ■■ Use of class I antiarrhythmic (tocainide, mexiletine) PATIENT HISTORY AND PHYSICAL FINDINGS

Table 1 Anesthetics Used for Local Infiltration

Duration (min) Max Recommended Dose (Adults) Without Epinephrine With Epinephrine Without Epinephrine With Epinephrine

Anesthetic

Onset (min)

Amides

Articaine

2–4

30–120 120–240

60–240 240–480 240–360 60–400 60–400 60–400

5.0 mg/kg or 350 mg 2.5 mg/kg or 175 mg 4.5 mg/kg or 300 mg 4.5 mg/kg or 300 mg 6.0 mg/kg or 400 mg 7.0 mg/kg or 400 mg

7.0 mg/kg or 500 mg 3.0 mg/kg or 225 mg 6.5 mg/kg or 400 mg 7.0 mg/kg or 500 mg 7.0 mg/kg or 550 mg 10.0 mg/kg or 600 mg

Bupivacaine Etidocaine

2–10

3–5

200

Lidocaine Copyright © 2019 Wolters Kluwer, Inc. Un uthorized reproduction of the cont nt is prohibited. <1

30–120 30–120 30–120

Mepivacaine

3–20

Prilocaine

5–6

Esters

Chloroprocaine

5–6

30–60 15–90

N/A

11.0 mg/kg or 800 mg 14.0 mg/kg or 1000 mg

Procaine Tetracaine

5 7

30–180 240–480

10.0 mg/kg 2.0 mg/kg

14.0 mg/kg

120–240 2.0 mg/kg Reprinted from Kouba DJ, LoPiccolo MC, Alam M, et al. Guidelines for the use of local anesthesia in office-based dermatologic surgery. J Am Acad Dermatol. 2016;74: 1201-1219; copyright © 2016, with permission from Elsevier.

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