10 Local Anesthetic

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Chapter 10
Local Anesthetics
Mosby items and derived items © 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
Local Anesthetics
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History of Local Anesthetics
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Local anesthetics are derivatives of cocaine
which is a derivative of the coca leaf
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Desirable Properties of Local
Anesthetics
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Potent
Reversible
Absence of local and systemic reactions
Absence of allergic reactions
Rapid onset and satisfactory duration
Adequate tissue penetration
Low cost
Stability in solution
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Site and Mechanism of Action
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Local anesthetics are divided into 2 chemical
groups; esters and amides.
Esters have a higher rate of allergic reaction.
Local anesthetics slows or blocks depolarization
by reducing Na permeability into the nerve
cytoplasm, thus inhibiting the flow of K out of the
cell.
Nerve impulse travels from node to node. Local
anesthetics effectively block nerve impulse travel.
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Ionization Factors
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Local anesthetics are weak bases occurring
equilibrated between their 2 forms, the fat soluble,
free base and water-soluble hydrochloride salt.
The portion of drug in each form is determined by
the pKa of the local anesthetic and the pH of the
environment.
Once injected into local tissue, the amount of local
anesthetic in the free base form increases and
allows for greater tissue penetration.
If there is an infection or inflammation, the free base
form decreases and less drug penetrates the tissue.
Other factors that can affect tissue penetration
include inflammation, vasodilation, and dilution by
fluid.
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Local Anesthetics
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Absorption
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Absorption of local anesthetics is dependent
on the route of injection.
Once injected, the rate of absorption is
dependent on tissue vascularity.
Tissue vascularity is a function of inflammation,
vasodilating properties of the local anesthetic,
presence of heat, or the use of massage.
Systemic absorption of the local anesthetic is
reduced with the addition of a vasoconstrictor.
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Local Anesthetics
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Distribution
The local anesthetic becomes partially bound to
plasma and red blood cells.
The unbound drug freely diffuses to other tissue
including the CNS and across the placenta.
Distribution allows absorption to occur in 3
phases.
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The drug occurs at highly vascular tissues in the
lungs and kidneys
 It appears in less vascular muscle and fat
 The drug is metabolized
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Local Anesthetics
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Metabolism and Excretion
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Primarily inactivated in the liver
 Excretion in urine 2-5% of active drug form
 Articaine is removed faster than the other
amides
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Local Anesthetics
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Pharmacologic Effects
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The main pharmacologic effect of the local
anesthetic is to reversibly block peripheral
nerve conduction.
Local anesthetics also have a direct effect on
the cardiac muscle by blocking cardiac Na
channels and depressing abnormal cardiac
pacemaker activity, excitability, and
conduction.
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Local Anesthetics
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Adverse Reactions
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Adverse reactions and toxicity of local
anesthetics are directly related to drug plasma
levels.
The factors that influence toxicity include:
• Drug itself
• Concentration
• Route of administration
• Rate of injection
• Vascularity
• Patient’s weight
• Rate of metabolism and excretion
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10
Local Anesthetics
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Adverse Effects
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Both CNS stimulation and depression can occur.
Local anesthetics can produce myocardial
depression and cardiac arrest with peripheral
vasodilation.
Local effects include physical injury caused by poor
injection technique.
Malignant hyperthermia only occurs in those
persons with the inherited autosomal dominant
gene. It is not related to amide local anesthetic use.
The incidence of allergic reactions to amide local
anesthetics is very low.
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Composition of Local Anesthetics
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Local anesthetic solutions are also made up
of the following:
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Vasoconstrictor that delays local anesthetic
absorption, reduces systemic toxicity, and
prolongs the duration of action.
Antioxidant that delays the oxidation of
epinephrine. Some are used to prolong shelf-life.
Sodium hydroxide adjusts the pH of the solution
to between 6 and 7.
Sodium chloride makes the injectable solution
isotonic.
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Local Anesthetic Agents
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Several different amide local anesthetics
are available for use.
Choice of local anesthetic should be based
upon onset of action, duration of local
anesthesia required and side effects.
Presence or absence of a vasoconstrictor.
This is of particular importance if the
patient has hypertension.
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Local Anesthetic Agents
pKa
Onset
Duration
(with epi) (min)
Procaine
9.1
Slow
45-90
8-10
Lidocaine
7.9
Rapid
120-240
4.5-7
Bupivacaine
8.1
Slow
240-480
2.5-3
Prilocaine
7.9
Medium
90-360
5-7.5
Articaine
7.8
Rapid
140-270
4-7
Anesthetic
Max Dose
(with epi) (mg/kg)
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Vasoconstrictors
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Benefits of Local Anesthetics
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Suppress systemic absorption of anesthetic agent
Increase duration and intensify block
Localize hemostasis
Reduce toxicity
Act on alpha and beta receptors in body tissues,
causing the constriction of blood vessels
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Vasoconstrictors
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Contraindications
Heart attack in the last 6 months
Uncontrolled high blood pressure
Daily angina
Tricyclic antidepressants
Uncontrolled hyperthyroidism
Uncontrolled arrhythmias
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Vasoconstrictors
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Dental Concerns
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Vasoconstrictors increase the length of anesthetic effect
which increases the chance of accidental patient selfmutilation.
In small quantities, epinephrine acts as a vasodilator
thus having the potential to increase post-op bleeding.
Patients with cardiovascular disease, who can receive a
vasoconstrictor, should receive the lowest dose possible
by means of the best injection technique.
Adverse Reactions
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Ischemia
Prolonged pain, numbness or paresthesia
Feelings of nervousness or fast heart rate
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Topical Anesthetics
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Several different topical anesthetics are
available for use.
Choice should be based upon onset,
duration of action, and allergenic potential.
The patient should be instructed to avoid
eating for 1 hour after application so that
the gag reflex can become fully functional.
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Topical Anesthetics
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Topical Anesthetics
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