Download - Pharmacology of Local Anesthetics I
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Pharmacology Of Local Anaesthesia
By: Dr. Alshaimaa Ahmed Lecturer of Oral & Maxillofacial
surgery
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What is local anesthetics?
Local anesthetics are drugs that block nerve conduction when applied to a nerve fiber in appropriate concentrations.
Transient completely reversible
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Characteristics of an ideal local anesthetic
Reversible anesthesia. Effective. Rapid onset and sufficient duration. Stable in solution. Stable for sterilization. Easy metabolism and excretion. Not allergic Should not have any systemic side effect. Should not be expensive.
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Where do Local anesthesia work?
Nerve Membrane is the site at which local anaesthetics exert its pharmacological action
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Remember >>>>> Nerve cell
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Remember >>>>> Cell Membrane
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Remember >>>>> Nerve Conduction
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How Local anesthetics work???
Theories of action of local anaesthesia: Acetyl choline theory
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Theories of action of local anaesthesia:
Calcium displacement theory
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Theories of action of local anaesthesia:
Surface Charge theory
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Theories of action of local anaesthesia:
Membrane expansion theory
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Theories of action of local anaesthesia:
Specific receptor theory
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Chemical structure of local anesthetics
-------------------O
R
RAromatic portion
Ester Or
AmideAmine portion
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Chemical structure of local anesthetics
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Classification of local anaesthetics:
The classification is based on the chemical structure of the intermediate chain. Commonly used amides include: lidocaine,
Mepivacaine. Commonly used esters include: tetracaine,
procaine, cocaine. There are third group know as Quinoline.
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Chemical structure of local anesthetics
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Chemical structure of local anesthetics
In Laboratory >>>>> local anaesthetics is poorly soluble in water and unstable on exposure to air >>>>>in this form it is on no clinical value.
For injection >>>>> local anaesthetics are dispensed as acid salts most commonly as hydrochloride salts dissolved in sterile water or saline
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Dissociation of Local Anesthetics
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Dissociation of Local Anesthetics & PH
The proportion of each ionic form in the sol varies with the PH of the sol or surrounding tissue
In the presence of high conc. Of H ion (low PH), the equilibrium shifts to the left and most of anesthetic sol exists in cationic form
RNH+> RN+H+
As hydrogen ion decreases ( higher pH) the equilibrium shifts to the free base form
RNH+ < RN +H+
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Action on Nerve Membrane
Outside the cellRNH RN + H+ +
H + RN RNHInside the cell
Sodium channel
Lipid bilayer
PH7.4
750 250
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Action on Nerve Membrane
Outside the cellRNH RN + H+ +
H + RN RNHInside the cell
Sodium channel
Lipid bilayer
PH 6
990 10
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Onset of action
It is the time required for the local anesthetics to develop adequate anesthesia.It is affected by:
The pH of the tissues and pKa of the agent.
Diffusion to the site ( anatomical barrier).
Concentration.
When will it start?? “onset”
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Dissociation constant “pKa ”
It measures the affinity of a molecule for hydrogen ions
A drug with lower pKa will have a rapid onset than that of higher pKa
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Diffusion
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Keep in Mind:
Nerve Fiber…….single nerve cell
Endoneurium ……covers each nerve cell
Fasculi……..bundles of 500 to 1000 nerve
Perinurium……….. Cover fasculi
Perilemma….. Inner most layer of perinurium
Epineurium …… alveolar connective tissue supporting fasculi and carrying nutrient vessels
Epinural sheath ……. Outer most layer of epinerium
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How deep my anaesthesia “ potency”?
The potency of a local anesthesia is its ability to provide complete analgesia under almost all circumstances.
The potency of a local anesthetic depend on:
1. Lipid solubility
2. Concentration.
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How long will it stay? “ Duration”
The duration of action of local anesthetics depends primarily on the redistribution of the drug away from the site of action.
Redistribution can be altered by:
1. Protein binding.
2. Vasodilator activity
3. Individual variation in response to the drug.
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Duration of action
Short acting L.A (5-40 min.): Without Vasoconstrictors:
Lidocaine 2%.
Prilocaine 4%.
Mepivacaine 3%.
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Duration of action
Medium –Acting (45- 90 min.)
Lidocaine 2% with epinephrine.
Mepivacaine 2% with epinephrine
Prilocaine 4% with epinephrine.
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Duration of action
Long –Acting (> 90 min.)
Bupivacaine 0.5% with epinephrine.
Etidocaine 1.5% with epinephrine.
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Recovery from local anaesthesia By Diffusion>>>> in slower rate than onset
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Readministration of Local anesthetics
Recurrence of immediate profound anesthesia
Difficulty reachieving profound anesthesia
(Tachyphylaxis …….increased tolerance to the drug that administrated readetly)
Tachyphylaxis caused by: edema localized hemorrhage – clot formation – decreased ph of the tissue)
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the local anaesthetic >>>> absorbed from the site of administration into circulation.
The presence of a local anaesthetic drug in the circulatory system means that drug will be transported to every part of the body.
What does body do to the drug ?What the drug do to the body?
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Pharmacokinetics
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UptakeLocal Action:
All local anesthetics posses a degree of vasoactivity, mostly vasodilatation except Cocaine which causes vasoconstriction.
Oral Route: All local anesthetics are poorly absorbed from the GIT
except cocaine Most LA undergo hepatic first-pass after
administration where 72% of the drug is transformed into inactive metabolites
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Topical Route
Application to mucous membranes the tracheal mucosa Intact skin
Injection
The rate of uptake of local anesthetic after parental administration (subcutaneous, intramuscular or iv) is related to vascularity of the site and vasoactivity of the drug
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Distribution
Local anesthetic absorbed and distributed to all tissues in the body.
Intravascular injection results in a sequential distribution first to the lung then rapidly distributed to other organs with large blood supplies, especially the brain, heart, liver, kidneys, spleen and then to muscle and fat.
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Biotransformation
Esters
Metabolized In plasma by the enzyme pseudocholine esterase
Amides
Metabolism In the liver by microsomal enzymes
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Excretion
Kidneys are excretory organs for ester & amide anesthetics and their metabolites
Patients with renal dysfunction may be unable to eliminate local anesthetics
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Pharmacodynamics of Local anesthetics
LA are chemicals that reversibly block action potentials in all excitable membranes
Most of the systemic actions are related to their blood or plasma level, the higher the level the greater will be the clinical action
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Central nervous system
All local anesthetics cross the blood-brain barrier At low therapeutic ,non toxic doses no CNS effects
of any clinical significance is manifested At higher toxic overdose levels the primary
clinical manifestations is generalized tonic-clonic convulsions
Between these two extremes there are pre- convulsive signs and symptoms
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Preconvulsive stageSigns(objectively observable)
• Slurred speech
• Shivering
• Muscle twitches
• Visual disturbance
• Drowsiness
• Disorientation
Symptoms (Subjectively felt)
• Numbness of tongue and circumoral region
• Warm flushed feeling of skin
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Convulsive stage
Further elevation of LA blood levels leads to signs and symptoms of Tonic conic convulsive episode
further increase will lead to depression
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pre- convulsive & convulsive stages
Dose releated
Mechanism
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Inhibitory impulses
Faclitatory impulses
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Central nervous system
Analgesia Mode elevations
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Cardiovascular system
Action on myocardium : local anesthetics produce a myocardial
depression that is related to drug blood level
Decrease cardiac excitability Decrease conduction rate Decrease force of contraction Used for treatment of cardiac dysrhythmias
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Cardiovascular systemDirect action on peripheral vasculature: All Local anaesthetics cause vasodilatation
except cocaine which is a vasoconstrictor all except cocaine cause hypotension
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Respiratory System
Has dual effect on respirations At non overdose level they have a direct
relaxant action on bronchial smooth muscle As a result of CNS depression respiratory
arrest may occur
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Vasodilatation effect of local anesthesia
Increase rate of absorbtion Higher plasma levels of local anaesthesia Decrease in both depth and duration of local
anesthesia Increased bleeding at the site of treatment
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Vasoconstrictors
vasoconstrictors constrict blood vessels
It counteract vasodilating effect of the local anesthetics.
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Vasoconstrictors
Advantages:
Increase the concentration of drug at the site:
Prolong anesthetic duration.
Produces more profound anesthesia.
Decrease the blood level of the drug, thus reduce the risk of toxicity.
Decrease bleeding at site.
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Vasoconstrictors
Systemic effects of vasoconstrictors:
Vasoconstrictors are sympathomimetic i.e. they mimic the action of norepinephrine on sympathetic effector organs where they bind to specific receptors called adrenergic receptors.
These receptors are divided into alpha (α) and beta (β ) receptors.
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Vasoconstrictors
Alpha (α) receptors
Alpha 1 (α1)
Alpha 2 (α2)
Smooth muscles of the BVs
vasoconstriction
Inhibitory receptors
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Vasoconstrictors
Beta (β ) receptors
Beta 1 (β 1)
Beta 2 (β 2)
In the heart increase rate and
force of contraction
In bronchi bronchodilationIn coronaries vasodilation.
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Vasoconstrictors
Nor-epinephrine excites mainly alpha receptors and to slight extent beta receptors.
Epinephrine excites both receptors with slight predominant effect on beta ones.
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Vasoconstrictors
Mode of action:
In the small amounts commonly used in dentistry,
vasoconstrictors stimulate alpha receptors located
in the walls of the arterioles in the area of injection
causing vasoconstriction which in turn slowing the
removal of the anesthetic
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Vasoconstrictors
Disadvantages:
Local anesthetic molecules are relatively stable
and degrade very slowly. As a result, the shelf
life of local anesthetic depends mostly on the
stability of the vasoconstrictor.
For this reason, sodium metabisulphite is used a
s a preservative or stabilizer for the
vasoconstrictor molecule.
Systemic effect
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Vasoconstrictors
Dilution of vasoconstrictors:
The dilution of vasoconstrictors is commonly
referred to as a ratio (e.g. 1:1000) this ratio
can be interpreted and converted as the
following:
1:1000 means that there is one gram
(1000mg) of the drug in 1000 ml (1 litre) of
solution, or 1.0 mg/ml of solution.
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What is meaning of 1:100.000 epinephrines ?
1: 1000 = 1 mg / ml
1: 50.000 = 0.02 mg / ml
1: 100.000 = 0.01 mg / ml
1: 20.000 = 0.05 mg / ml
Content of carpule contain 1: 100.000 epinephrine.
0.01 mg x 1.8 ml
----------------------- = 0.018 mg / carpule.
11:100.000 = 0.01
mg / ml 1 ml
? 1.8 ml
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Vasoconstrictors
Epinephrine:
Nature:
Known as adrenaline.
Most commonly used due to its strong action.
Stability:
Unstable, undergo oxidation by heat.
Sodium bisulphite preservative used to delay oxidation
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Epinephrine (adrenaline):
Systemic effect:
Increase blood pressure.
Increase heart rate.
Increase cardiac output.
Increase myocardial oxygen consumption.
Vasculature
Hemostasis: vasoconstriction followed by vasodilatation
Termination : reuptake – inactivated in blood by COMT & MAO.
Available concentration: 1:50.000- 1: 100.000 (in Canada) and 1:80.000- 1:300.000 in other countries
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Maximum permeable dose:
For healthy patient 0.2mg in appointment:
10 ml of 1:50,000 dilution (5 crtridges).
20 ml of 1:100,000 dilution (11 cartridges).
40 ml of 1:200,000 dilution (22 cartridges).
For cardiac patient 0.04 mg/appointment:
2ml of 1:50,000 dilution (1 cartridge).
4ml of 1:100,000 dilution (2 cartridges).
8ml of 1:200,000 dilution (4 cartridges).
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Vasoconstrictors Norepinephrine (levarterenol):
Nature:
Known as noradrenaline .
Stability:
Undergo deterioration on exposure to light.
Systemic effect:
Increase blood pressure.
Decrease heart rate.
Decrease cardiac output.
It is 1/8 as effective in raising the blood sugar as epinephrine.
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The norepinephrine act almost on receptors . It is one fourth as potent as epinephrine.
That will lead to severe vasoconstriction in the peripheral circulation (ischemia in the palate )
Available consentration 1:300.000
Norepinephrine:
Maximum permeable dose:
For healthy patient 0.34 mg/appointment.
For cardiac patient 0.14 mg/appointment.
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Vasoconstrictors
Levonordefrin:
Nature:
It is synthetic vasoconstrictor. It has direct action on the receptors(75%) & it is 15% as potent as epinephrine
Stability:
Unstable, undergo oxidation and deterioration.
Sodium bisulphite used to delay oxidation.
Systemic effect:
Less effective in contrating blood vessels and in raising blood pressure than epinephrine.
It is 1/10 as active as epinephrine in increasing blood sugar.
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Levonordefrin:
Concentrations in L.A:
1:20,000.
Maximum permeable dose:
Maximum dose is 1 mg/appointment (11 cartridges).
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Vasoconstrictors
Phenylephrin:
Nature:
Synthetic. It has direct action on the receptors(95%) & it is 5% as potent as epinephrine
Stability:
Most stable vasoconstrictor used in dentistry.
Concentrations in L.A:
Used in dental practice with 4% procaine in 1:2500 dilution.
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Phenylephrin:
Maximum permeable dose:
For healthy patient 4 mg/appointment.
Far cardiac patient 1.6 mg/appointment.
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Vasoconstrictors
Felypressin:
Nature:
Synthetic analogue of the antidiuretic hormone vasopressin. Act directly on the smooth muscles in the wall of the blood vesseles
It is non sympathomimetic amine, so it has no effect on adrenergic nerve transmission safe for hyperthyroid patients.
Systemic effect:
It has oxytoxic action so it is contraindicated in pregnant women.
Concentration in L.A:
Used in dental practice with 3% prilocaine in a 0.03 IU/ml dilution.
Maximum permeable dose:
For cardiac patients is 0.27 IU
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What we will find in the market????
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Ester Types Local Anesthesia
Procaine (Novocain) Propoxycaine Tetracaine (topical anaesthesia)
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Procaine ( Novocain)
Nature → ester types L.A.
Metabolism → in plasma by pseudocholinestrases enzymes.
Excretion → Via kidney.
Onset: 6-10 min
Dental concentration → 2% or 4% conc.
2% solution gives from 12-15 minutes of anesthesia
The addition of 1:100.000 adrenaline prolong the duration to 30-45 minutes
The addition of 1:50.000 adrenaline prolong the duration to 60-90 minutes.
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Propoxycaine HCL
Nature → ester types L.A.
Metabolism → in plasma by pseudocholinestrases enzymes.
Excretion → Via kidney.
Onset: rapid 2-3 min
Dental concentration → 0.4% conc.
It is combined with procaine to provide more rapid onset and a more profound anaesthesia
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Amides Local Anesthesia
Lidocaine. Mepivacaine. Articaine HCl. Bupivacaine HCL Prilocaine HCL
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Lidocaine(Lingospan, octocaine, Xylocaine)
Amide.
Metabolism: liver.
Excretions: kidneys.
Duration of action: without V.C. 5 mins.
Pregnancy classification: B
Onset of action :2-3 min.
Effective dental concentration: 2%
Maximum dose → 300 mg without V.C & 500 mg with V.C
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Mepivacaine HCl( carbocaine, Isocaine, polocaine,
scandanest) Amide.
Metabolism: liver.
Excreation: kidneys.
Vasodilating properties: produce slight V.D.
Duration of action: without V.C. 20-40 min.
Onset of action: rapid 1.5-2 min.
Effective dental concentration: 3% without V.C.; 2% with a V.C.
Pregnancy classification: C
Concentrations of V.C.: 2% with levonordefrin (1:20,000), 2% with epinephrine (1:100,000).
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Articaine HCl( Articadent, orbbloc, septocaine)
Amide.
Metabolism: it is the only amide-type local anesthetic that contains an ester group, so biotransformation occurs in both the plasma and liver.
Excretion: kidneys.
Onset of action: 1-3 min.
Effective dental concentration: 4% with 1:100,000 or 1:200,000 epinephrine.
Clinically it is claimed that maxillary buccal infiltration, on occasion, provides palatal soft- tissue anesthesia.
It is also claimed that articaine can provide pulpal and lingual anesthesia when administered by infiltration in the adult mandible
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Prilocaine (Citanest)
Nature → Amides Local Anesthesia.
Metabolism → In liver, kidney & lung so it undergo biotransformation more rapidly than other amide.
Excretion → Via kidney faster than other amide. (most safe)
Dental concentration → 4% with or without V.C.
V.C used with it is 1:200.000 Epinephrines.
Onset of action → Its onset of action is slightly slower than Lidocaine
Maximum dose → 6 mg/ kg body weight or 400 mg
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Bupivacaine (Marcaine)
Nature → Amides Local Anesthesia + long acting L.A.
Metabolism → In liver.
Excretion → Via kidney.
Dental concentration → 0.25% – 0.5% conc. With 1:200.000 Epinephrine .
Onset of action → Rapid onset of action equal to lidocaine.
Maximum dose → 2 mg/ kg body weight for adult- 90 mg as maximum dose .
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What is meaning of 2% xylocaine ?
2% xylocaine = 20 mg / ml.
Content of 2% xylocaine in one carpule = 20 mg x 1.8 ml = 36 mg/carpule.
20 mg / ml
1 ml
? 1.8 ml
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Maximum doses of Local Anesthesia
The doses of local anesthetic drugs are presented in terms of milligrams of drug per unit of body weight.
The administration of a maximum dose based on body weight produces a local anesthesia blood level just below the threshold for an overdose (toxic) reaction.
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Maximum doses of Local Anesthesia
Local anesthetic
mg/Kg MRD (mg)
Articaine
With V.C.7.0 500
LidocaineNo V.C.
With V.C.4.44.4
300300
MepivacaineNo V.C.
With V.C.
4.44.4
300300
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Maximum doses of Local Anesthesia
Local Anesthetic
Percent concentration mg/ml X 1.8 ml = mg/
cartridge
Articaine 4 40 72
Lidocaine 2 20 36
Mepivicaine23
2030
3654
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Calculation of the max dose Patient : 22 years old, Healthy , 50 kg Local Anaesthesia >>>> Lidocaine 2% epinephrine
1:100.000 Lidocaine 2% = 36mg/ cartilage Patient max dose= 50* 7 mg/kg = 350 mg (MRD) Number of cartilage = 350/ 36 = 9 cartilage
What if the patient is cardiac ?
What if the dentist use another local anaesthetic drug ?
Total dose of Both local anaesthetics should not exceed the lower of the two calculated dose.
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What is maximum dose of V.C. in normal and cardiac patients?
Type of V.C. Maximum dose in
normal patient
Maximum dose in cardiac
patient
Epinephrine1:100.000 0.2 mg 0.04 mg
Norepinephrine
1 :30.0000.34 mg 0.14 mg
Levonordefrine
1:20.0001 mg 0.2 mg
Phenylephrin
1:25004 mg 1.6 mg
Felypressin
0.03 IU0.27 IU,
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Factor in selection of a local anesthesia for a patient
Length of time pain control is necessary Potential need for posttreatment pain control Possibility of self mutilation in the post operative
period Requirment for hemostasis Presence of any contraindication to local anaesthetic
solution selected for administration
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Topical anesthesia
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Topical Anesthesia
Produce painless needle injection.
Used for some procedures as removal of very loose primary teeth or suture removal or before gingival curettage.
Reduce patient apprehension.
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Topical Anesthesia
Characteristics:
Can not penetrate intact skin.
Its concentration is higher than injectable one to
facilitate its diffusion through mucous membrane.
Higher concentration increase its risk of toxicity
because the topical anesthesia has no V.C.
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Topical Anesthesia
Characteristics:
The anesthesia is effective only on 2.3 mm depth of
the tissues on which it is applied.
Available in the form of spray or gel, the gel form is
more preferred because it can be dispensed in a
premeasured doses.
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Topical Anesthesia
Types of local anesthesia:
Benzocaine.
Lidocaine HCl.
Lidocaine base.
Tetracaine HCL.
Cocaine HCl.
EMLA (Eutectic Mixture of Local Anesthesia).
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