10 local anesthetics (1)

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Mosby items and derived items 2007 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 10

Local Anesthetics


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Local Anesthetics

History of Local Anesthetics

Local anesthetics are derivatives of cocaine

which is a derivative of the coca leaf

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Desirable Properties of Local

Anesthetics

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

Local anesthetics are divided into 2 chemicalgroups; esters and amides.

Esters have a higher rate of allergic reaction.

Local anesthetics slows or blocks depolarizationby reducing Na permeability into the nervecytoplasm, thus inhibiting the flow of K out of thecell.

Nerve impulse travels from node to node. Local

anesthetics effectively block nerve impulse travel.

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Ionization Factors

Local anesthetics are weak bases occurringequilibrated 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 theenvironment.

Once injected into local tissue, the amount of localanesthetic in the free base form increases andallows for greater tissue penetration.

If there is an infection or inflammation, the free baseform decreases and less drug penetrates the tissue.

Other factors that can affect tissue penetrationinclude inflammation, vasodilation, and dilution byfluid.

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Local Anesthetics

Absorption

Absorption of local anesthetics is dependent

on the route of injection.

Once injected, the rate of absorption isdependent 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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7/20Mosby items and derived items 2007 by Mosby, Inc., an affiliate of Elsevier Inc.

Local Anesthetics

Distribution

The local anesthetic becomes partially bound toplasma 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. 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

Metabolism and Excretion

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

Pharmacologic Effects

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, andconduction.

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Local Anesthetics

Adverse Reactions

Adverse reactions and toxicity of localanesthetics are directly related to drug plasmalevels.

The factors that influence toxicity include: Drug itself Concentration Route of administration

Rate of injection Vascularity Patients weight Rate of metabolism and excretion

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Local Anesthetics

Adverse Effects Both CNS stimulation and depression can occur.

Local anesthetics can produce myocardialdepression and cardiac arrest with peripheralvasodilation.

Local effects include physical injury caused by poorinjection technique.

Malignant hyperthermia only occurs in thosepersons with the inherited autosomal dominantgene. It is not related to amide local anesthetic use.

The incidence of allergic reactions to amide localanesthetics is very low.

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Composition of Local Anesthetics

Local anesthetic solutions are also made up

of the following:

Vasoconstrictor that delays local anesthetic

absorption, reduces systemic toxicity, andprolongs 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 solutionto between 6 and 7.

Sodium chloride makes the injectable solution

isotonic.

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Local Anesthetic Agents

Several different amide local anesthetics

are available for use.

Choice of local anesthetic should be based

upon onset of action, duration of localanesthesia required and side effects.

Presence or absence of a vasoconstrictor.

This is of particular importance if thepatient has hypertension.

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Local Anesthetic Agents

Anesthetic pKa Onset

Duration

(with epi) (min)

Max Dose

(with epi) (mg/kg)

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

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Vasoconstrictors

Benefits of Local Anesthetics

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

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

Dental Concerns Vasoconstrictors increase the length of anesthetic effect

which increases the chance of accidental patient self-mutilation.

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 possibleby means of the best injection technique.

Adverse Reactions

Ischemia Prolonged pain, numbness or paresthesia

Feelings of nervousness or fast heart rate

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Topical Anesthetics

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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10 local anesthetics (1)

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