General Medical Officer (GMO) Manual: Clinical Section

Local Anesthetics

Department of the Navy
Bureau of Medicine and Surgery

Introduction	Distribution and Elimination	Epinephrine Effects
Chemistry	Toxicity	Systemic Epinephrine Absorption
Classification	Treatment of Systemic Toxicity	Contraindications to Epinephrine
		Epinephrine Solution Preparation

Introduction

Local anesthetics are drugs that block the generation and propagation of impulses in excitable tissues, most notably the spinal cord, spinal nerve roots, and peripheral nerves, as well as skeletal muscle, cardiac muscle, and the brain. This section will briefly cover the pharmacology of commonly available local anesthetics, review the maximum recommended doses, and discuss the potentially catastrophic allergic and toxic responses.

Chemistry

Classification of Local Anesthetics

	Type	Clinical Uses	Usual Concentration (%)	Usual Duration (hour)	Max. Dose (mg) **
2-Chloroprocaine	Ester	Infiltration PNB	1 2	0.5-1.0 0.5-1.0	1000 with EPI 1000 with EPI
Tetracaine	Ester	Topical	2	0.5-1.0	80
Lidocaine	Amide	Topical Infiltration PNB	4 0.5-1.0 1.0-1.5	0.5-1.0 1.0-2.0 1.0-2.0	500 with EPI 500 with EPI 500 with EPI
Mepivacaine	Amide	PNB	1.0-1.5	2.0-3.0	500 with EPI
Bupivacaine	Amide	PNB	0.25-0.5	4.0-12	200 with EPI

** Dose for 70 kg male. Use only as a general guide. PNB = Peripheral Nerve Block, EPI = Epinephrine

• A common problem faced by the clinician is the report by patients that they are allergic to local anesthetics. Unfortunately, most of these patients are then subjected to a lifetime of inconvenience because this diagnosis is often incorrectly established. It is estimated that less than 1 percent of all adverse reactions to local anesthetics are actually caused by a true allergic reaction. The remaining balance of reactions occur because of the rapid rise in circulating local anesthetics or the absorption of epinephrine.
• Aminoesters are more allergenic than aminoamides because of their relationship to p-aminobenzoic acid (PABA), a metabolic by-product. Parabens are present in multidose local anesthetic solutions, other drugs, cosmetics, and foods. Prior exposure to parabens may sensitize patients to subsequent administration of local anesthetic solutions containing these materials, resulting in an allergic reaction unrelated to the local anesthetic. Using preservative free local aminoamides will eliminate the risk of paraben sensitivity.

Treatment of Systemic Toxicity

Prevention is the best solution for avoiding systemic toxicity with local anesthetics. Meticulous attention to technique and to recognition of intravascular injections with appropriate monitoring is indicated. Signs and symptoms of local anesthetic toxicity include tinnitus, perioral numbness, metallic taste in mouth, slurring of speech, and mental status changes. Oxygen should be administered at the first sign of toxicity. Should symptoms progress, maintain the patient's airway and follow advanced cardiac life support (ACLS) guidelines. Be prepared to treat for seizures. (Bretylium being used in preference to lidocaine.)

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