Local
Anesthetics
Introduction
·
Chemistry
· Classification
·
Distribution and
Elimination ·
Toxicity
·
Treatment of
Systemic Toxicity ·
Epinephrine Effects
·
Systemic Epinephrine Absorption
·
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
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
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