Local Anesthesia Flashcards by sophie rosen

Desirable Properties for Local Anesthesia

1) No Irritation to tissue
2) No permanent alteration
3) Low systemic toxicity
4) Effective if injected into tissue or mucous membranes
5) Short onset
6) Long of enough duration of action for the procedure
7) Potent yet not harmfully concentrated
8) Not elicit allergic reaction
9) Readily undergo biotransformation
10) Sterile or capable of being sterilized by heat without deterioration

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Spectrum of Pain and Anxiety
Control in Dentistry
(6)

Local Anesthesia
Oral sedation
I.M. sedation
Nitrous oxide sedation
I.V. Sedation
General anesthesia

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Local Anesthesia

• Prevent

generation and /
or conduction of a nerve
impulse

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pain pathways (4)

pain receptors
sensory nerve fiber
nerve pathways
response modulators

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Pain Threshold:

– The least experience of pain which a subject can recognize

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Pain Tolerance:

– The greatest level of pain which a subject is prepared to tolerate

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Where do local anesthetics work ?

• Nerve membrane is where local anesthesia exert their pharmacological actions

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Different theories tried to explain

– Acetylcholine theory
– Calcium displacement
– Surface charge
– Membrane expansion…….. But failed

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SPECIFIC RECEPTOR theory

– Local anesthesia bind to specific receptor on the Na channel

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Myelin sheath insulates axons (2)

electrically

and pharmacologically

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• Nodes of Ranvier

Sodium channel is

abundance

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• Ensure effective anesthesia

– 2 or 3 nodes needs to be blocked

– 8-10 mm length needed

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How Local Anesthesia Work?

• Decrease permeability of ion channels to Na
• Nerve block by local anesthesia is a
Non-depolarizing block

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Active Forms of Local Anesthetics

• Majority are

Tertiary amine

– Except prilocaine and hexylcaine ( 2nd amine)

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Active Forms of Local Anesthetics

• All local anesthetics are

amphipathic except…benzocaine

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• (2) have weak

local anesthetic properties

Antihistamine and anticholinergics

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• 2 Types of local anesthetics

– Amide
• Resist hydrolysis, excrete unchanged in urine
– Ester
• Readily hydrolyzed in aqueous solution

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Esters

– Procaine
– Propoxycaine
– Tetracaine
– Cocaine
– Benzocaine
– Dyclonine

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Amides

– Lidocaine
– Etidocaine
– Mepivacaine
– Bupivacaine
– Prilocaine
– Articaine

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Benzocaine doesn’t have hydrophilic group

Therefore,

its not suitable for injection but nice topical !!

Local Anesthesia are weak

basic compounds

– They combine with acids to form local anesthetic salt (HCl)

RNH+ ↔ RN + H+

RNH+ : Cation RN : Uncharged Molecule ( free base)

RNH+ ← RN + H+

Excess H+ (low pH), equilibrium shifts to the left RNH+

RNH+ → RN + H+

Decrease of H+ (high pH), equilibrium shifts toward free base

pKa (dissociation constant)

– Relative proportion of ionic form depends on pKa

Log Base = pH - pKa | Acid

Henderson-Hasselbalch equation | When pH of solution = pKa, you have 50/50 RN/RNH+

If you flip equation around | • pH = Log Base + pKa

Acid Henderson-Hasselbalch equation – Given pH of a solution depending on amount of base/acid proportion

Two Factors involved in the action of local anesthesia | 2

– 1) Diffusion through nerve sheath | – 2) Binding at receptor site

Etidocaine @ normal tissue pH 7.4

– 25% free base (RN), 75% cation (RNH+) (RNH+ ↔ RN + H+) • Let’s say we have 1000 molecules available – 250 lipophilic RN WILL diffuse through membrane • Once at intracellular level, 250 will re-equilibrate into – 70 RN and 180 RNH+ • This process will continue until all gone

pKa influence onset of action

– ↑ pKa translate to slow onset • b/c few free base molecule available to diffuse – ↓ pKa will have faster onset • Remember, once intracellular, we need RNH+ to bind the receptor. i.e. Benzocaine isn’t all that cool for injection purpose !!

Extracellular pH determines the --- for nerve blockade

ease

--- tissue is much more difficult to get | adequate anesthesia b/c lower pH or ↑H+

Inflamed or infected

Keep low pH equates ↑

effective shelf live of local anesthesia

Most local anesthesia have pH

5.5 to 7 – Sodium bisulfite (antioxidant) is added with vasoconstrictor – Slower onset with vasoconstrictor vs “plain”

Most topical Anesthesia is prepared in more concentrated form compared to injectable local anesthesia (1% or 2 % lidocaine) – This is due to

poor buffering capacity of mucous membrane | Example: Benzocaine

Kinetics of Local Anesthesia

• Endoneurium Perineurium Epinerium

• --- is greatest barrier for diffusion

Perineurium | – Slower diffusion is dependent on the thickness

• Mantle fiber tends to innervate

proximal region (molars)

• Core fiber innervates more

distal points (incisors)

Complete conduction blockade requires | 2

– Volume and Concentration

What happen to injected drug? | 4

– Absorbed by nonneural tissue – Diluted by interstitial fluid – Removed by capillaries and lymphatic system – For Ester-type: immediate enzymatic hydrolysis

Lower pKa posses

rapid onset of action

Greater lipid solubility relates to

intrinsic potency | – Nerve membrane is 90% lipid

Increased protein binding will increase

duration | – Nerve membrane is 10% protein

Recovering from nerve block | 2

– “reverse” of anesthetic induction pattern | – Intraneural concentration exceeds extraneural concentration

Recurrence of Immediate profound anesthesia | 2

– Reduced concentration at mantle fibers – Residual local + newly deposited supply = immediate profound anesthesia

Difficulty reestablish profound anesthesia | 2

– Surgical procedure outlasts effectiveness of med | – Tachyphylaxis

Tachyphylaxis

– ↑ Tolerance to drug after repeated administration

Factors | 6

``` – Edema – Localized hemorrhage – Clot formation – Transudation – Hypernatremia – ↓pH of the tissue ```