General Medical Officer (GMO) Manual: Clinical Section: Local Anesthetics

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 Concent. (%)	Usual Duration (hour)	Max. Dose (mg) **
2-Chloro- procaine	Ester	Infilt. 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 Infilt. 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

Distribution and Elimination

Although there are many local anesthetics available, the clinician should remember that lidocaine is frequently the safest overall choice and the standard by which all other local anesthetics are compared. When in doubt, use lidocaine.

Aminoamides are cleared from the plasma by hepatic metabolism. Active metabolites of lidocaine can contribute to toxicity even when plasma levels of lidocaine are in a therapeutic range.
Aminoesters are rapidly cleared from the plasma by plasma and liver cholinesterases. Plasma levels of these local anesthetics may be elevated in patients with deficient or atypical cholinesterase enzyme.

Toxicity of Local Anesthetics

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.)

Epinephrine Effects in Local Anesthetics

Prolongs duration of anesthesia

Increases the intensity of blockade

Reduces systemic absorption

Reduces surgical bleeding

Manifestations of Systemic Epinephrine Absorption

Increased heart rate.
Increased cardiac output.
Decreased systemic vascular resistance.

Contraindications to the Addition of Epinephrine to Local Anesthetics

Unstable angina.
Cardiac dysrhythmias.
Uncontrolled hypertension.
Treatment with monoamine oxidase (MAO) inhibitors or tricyclic antidepressants.
Uteroplacental insufficiency.
Peripheral nerve blocks in areas that may lack collateral blood flow (penis, digits).
Intravenous (IV) regional anesthesia.

Epinephrine Solution Preparation

Solutions of epinephrine containing 5 mcg/ml (1:200,000) appear to be optimal for the reduction of surgical bleeding and systemic absorption of local anesthetics. Solutions of local anesthetics may be adjusted to contain epinephrine for vasoconstriction by manually adding 5 mcg of epinephrine to 1 ml of anesthetic solution to produce a concentration of 1:200,000.

Reviewed by CAPT C.G. Bush, MC, USN, Anesthesiology Department, Naval Hospital Groton, CT. (1999).

Preface · Administrative Section · Clinical Section

The General Medical Officer Manual , NAVMEDPUB 5134, January 1, 2000
Bureau of Medicine and Surgery, Department of the Navy, 2300 E Street NW, Washington, D.C., 20372-5300

This web version of The General Medical Officer Manual, NAVMEDPUB 5134 is provided by The Brookside Associates Medical Education Division. It contains original contents from the official US Navy version, but has been reformatted for web access and includes advertising and links that were not present in the original version. This web version has not been approved by the Department of the Navy or the Department of Defense. The presence of any advertising on these pages does not constitute an endorsement of that product or service by either the Department of Defense or the Brookside Associates. The Brookside Associates is a private organization, not affiliated with the United States Department of Defense. All material in this version is unclassified. This formatting © 2006 Medical Education Division, Brookside Associates, Ltd. All rights reserved.

Home · Textbooks and Manuals · Videos · Lectures · Distance Learning · Training · Operational Safety · Search

This website is dedicated to the development and dissemination of medical information that may be useful to those who practice Operational Medicine. This website is privately-held and not connected to any governmental agency. The views expressed here are those of the authors, and unless otherwise noted, do not necessarily reflect the views of the Brookside Associates, Ltd., any governmental or private organizations. All writings, discussions, and publications on this website are unclassified.

Contact Us