Pharm - Local Anesthetics

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Features of Local Anesthetics

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basic (pH7.5-9); sparingly soluble
combine with acids to form water soluble salts
3 components - lipophilic; intermediate, hyddrophilia/amine terminus
before intermed - either AMIDE or ESTER bond
ester more subjected to lysis

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Solubility of Local Anesthetics

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AMIDES
mepivaine - methyl group
bupivacaine - butyl group; higher potency and duration b.c inc fat solubility

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pH of Local Anesthetics

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uncharged delievers cation to receptor
cation acts at receptor site

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pH of Local Anesthetics
reduced pH

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inflammation
inc amt of cation
reduces effect b.c less gets to receptor site

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pH of Local Anesthetics
Tachyphylaxis

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rapid dec in response to drug after it has been used over time
occurs in this case in body fluid with low buffering capacity (CSF)

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Local Anesthetics
Pharmaco
Effect of EPI

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alpha-1 vasoconstriction
reduces absorption; enhances neuronal uptake
prolongs duration;

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Local Anesthetics
Pharmaco
Distribution

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all cross barriers; BBB; placenta
highly distribued b.c lipophilic; goes to perfused tissues

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Local Anesthetics
Pharmaco
Liver Metab

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Liver metab
metab slowed in 1. liver disease; 2. reduced hepatic BF (halothane); 3. inhibit cytochromes

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Local Anesthetics
Pharmaco
Blood Metab

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butyrylcholinesterase - ester type hydrolyzed rapidly in blood.
Prilocaine metab by diff. mech; can result in methemoglobinemia

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Metab
Order from fastest to slowester

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prilocaine
lidocaine
mepivacaine
ropivacaine=bupivacaine
levobupivacaine

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Local Anesthetics
Site

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acts inside the pore near cytoplasmic opening OR at interface of channel and lipid core of neuronal membrane

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Local Anesthetics
Mech
frequency dependence

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block voltage gated sodium channels
enters channel when depolarizes
binds to inactivation gate (H) and holds gate in off position

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Local Anesthetics
Mech
voltage dependence

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if cell hyper pol. drug has less effect
depol. cell enhances local anes block b.c more channels are opened. occurs in anoxia; can lead to toxicity

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Local Anesthetics
Effect of block on diff. nerves

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smaller fiber, more block
inc firing frequence, more block
inc myeline inc block b.c acts as reservoir (except in large fibers)
Type B and C most sensitive

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Local Anesthetics
Use

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epidural/subarachnoid
block sympathetic fibers
neuropathic pain
cardiac dysarrhythmias
local pain control

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Local Anesthetics
Determinant of Toxicity

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can be absorbed by vascular system and taken to CNS and heart
interacts with drugs taht interfere with metab
hypoxia

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Local Anesthetics
CNSToxicity

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depression; nystagmus, m. twitching
local neurotoxicity in high conc.; prolonged sensory and motor deficit
Lidocaine - anticonvulstant; excitation, seizures, general CNS depression as dose inc.

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Local Anesthetics
CV Toxicity

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direct effect on vascular system;
indirect on ANS
Lidocaine - antiarrhythmic and low conc; myocardial depression; to inc vasodilation and CV collapse at high conc

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Local Anesthetics
Manage toxicity

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oxygen
diazepam
thipental - seizures

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Local Anesthetics
Bupivacaine Cardiotoxicity

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long duration of action
difficult resuscitation
inc. in preggers

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Local Anesthetics
Allergy

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in esters (PABA derivatives)

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Local Anesthetics
Cocaine

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sympathomimetic
CNS stimulant
topical effects - vasoconstriction

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Local Anesthetics
Esters

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Cocaine
Procaine

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Local Anesthetics
Amides

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Prilocaine
Lidocaine

Mepivacaine
Bupivacaine
Levobupivacaine

Etidocaine
Articaine