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Local Anaesthetics

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					LOCAL ANAESTHETICS

ANAESTHESIA
Anaesthesia means the absence of sensation.

Ref: http://www.oyston.com/anaes/local.htm http://www.facialplasticsurgery.net/anaesthesia.htm

Definition
► Local

anaesthetics are drugs which upon topical application or local injection cause reversible loss of sensory perception specially of pain ,in a restricted area of the body

GA Site of action CNS

LA Peripheral nerves

Area involved

Whole body

Restricted area

Consciousness Major surgery Minor surgery

lost preferred Not preferred

unaltered Cannot be used preferred

Classification
► Injectable ► Surface

anaesthetics

anaesthetics

Injectable anaesthetics
potency,short duration Procaine, Chlorprocaine ► Intermediate potency, duration Lignocaine, prilocaine ► High potency, long duration Tetracaine, bupivacaine, ropivacaine, dibucaine
► Low

Surface anaesthetics
Cocaine Lignocaine Tetracaine Benzocaine Oxethazaine

Chemical classification
► Ester

linked LA’s : Cocaine, procaine, chlorprocaine, tetracaine, benzocaine linked LA’s : Lignocaine, bupivacaine, dibucaine, prilocaine, ropivacaine

► Amide

Esters:
► They

are hydrolyzed in plasma by pseudocholinesterase. One of the by-products of metabolism is paraaminobenzoic acid (PABA), the common cause of allergic reactions seen with these agents

AMIDES
 Longer lasting anaesthesia  Rarely cause hypersensitivity reactions  No cross sensitivity with esters

Mechanism of Action
► Local

anesthetics block nerve conduction by reducing the influx of sodium ions into the nerve cytoplasm.

► Sodium

ions cannot flow into the neuron, thus the potassium ions cannot flow out, thereby inhibiting the depolarization of the nerve.

► Na+

channels exist in activated-open,

inactivated-closed and rested-closed states
► LAs

selectively bind to Na+ channels in

inactivated-closed states so are membrane stabilising

► Reducing

Na+ ion permeability slows

depolarization so threshold potential is not reached

► Prevention

of voltage dependent increase in Na+ ion conductance

Effect on action potential

is slowed ► Threshold and resting potential unchanged

► Depolarisation

► Resting

nerve is rather resistant to blockade and blockade develops rapidly when the nerve is stimulated repeatedly. of blockade is frequency dependent blockade at higher frequency of stimulation

► Degree

► Greater

local anesthetic action is affected
by Effect of pH
► charged

(cationic) form binds to receptor site ,uncharged form penetrates membrane ► efficacy of drug can be changed by altering extracellular or intracellular pH

Effect of lipophilicity
ANESTHETIC POTENCY
► Lipid

solubility appears to be the primary determinant of intrinsic anesthetic potency. Chemical compounds which are highly lipophilic tend to penetrate the nerve membrane more easily, such that less molecules are required for conduction blockade resulting in enhanced potency. lipophilic agents are more potent as local anesthetics

► more

Susceptibility to block by local anesthetics of types of nerve fibers
► In

general, small – non myelinated nerve fibers are more susceptible than large fibers; however,
    the type of fiber degree of myelination fiber length and frequency- dependence are also important in determining susceptibility

► Sensory

and motor fibres are affected

► Sensitivity

is determined based on diameter of fibre fibres are more sensitive than larger fibres and nonmyelinated fibres are bolcked easily than myelinated fibres

► Smaller

Order of sensory function block
pain ► 2. cold ► 3. warmth ► 4. touch ► 5. deep pressure ► 6. motor
► 1.

Recovery in reverse order

CNS
inhibition ► Apparent stimulation is due to inhibition of inhibitory neurons ► cocaine-powerful stimulant ► Euphoria, excitement, confusion, restlessness, twitching of muscles, tremor, unconsciousness, respiratory depression, death.
► Neuronal

CVS
depressants ► Decrease automaticity, excitability, contractility, increase refractory period ► Lignocaine is used as an antiarrythmic Large doses of LA s can induce arrhythmias
► Cardiac

Blood vessels
► Fall

in BP

► Cause

direct relaxation of arteriolar smooth muscle, due to sympathetic blockade dose- cardiovascular collapse

► Toxic

► Bupivacaine

causes more fall in BP than lignocaine

► Cocaine

has sympathomimetic property – reuptake blocker: causes local vasoconstriction ,marked rise in BP and tachycardia ► Special = causes vasoconstriction by being a reuptake inhibitor – no need to give an alpha agonist

Adverse effects
► CNS-

Light headedness, dizziness, auditory and visual disturbances , mental confusion, disorientation, shivering, twitching, involuntary movements, convulsions, respiratory arrest
bradycardia, hypotension, vascular collapse

► CVS:

► Injection

is painful, ► Addition of vasoconstrictors like adrenaline will cause local tissue damage, mucosal sloughing, necrosis, delayed wound healing = less blood. ► Therefore vasoconstrictors should not be added for ring block of hands , feet , fingers, toes ,pinna and penis = organs with end arterial supply, w/o any collaterals ► Hypersensitivity reactions – more common with ester type LA’s

Advantages of adding vasoconstrictor
► Slow

absorption from local site,hence prolonged duration of action bleeding in surgical field

► Decreased ► Slow

absorption reduces systemic toxicity

Disadvantages of adding vasoconstrictor
► Intense

vasospasm and ischemia in tissues with end arteries can cause gangrene especially in fingers, toes , tip of nose pinna and penis

► Adrenaline

absorption can sause systemic toxicity – tachycardia, palpitation, rise in BP, precipitation if angina, arrhythmias wound healing by decreasing blood flow.

► Delays

Precautions and interactions
► Aspirate

slightly before injecting LA to avoid intravascular injection LA slowly

► Inject

► Propranolol

(ß blockers ) reduce metabloism of lignocaine by reducing hepatic blood flow

► Vasoconstrictor

(adrenaline) containing LA should be avoided for patients with ischemic heart disease , cardiac arrhythmias, thyrotoxicosis, hypertension, and those receiving beta blockers ( rise in BP)

TECHNIQUES OF LOCAL ANAESTHETICS

Surface anaesthesia(Topical anaesthesia)
by topical application of surface anaesthetics to mucous membrane and abraded skin ► Only superficial layer is anaesthetized ► Eg- lignocaine sprayed in mouth or throat Acts in 2-5 min, lasts for 30-45 min Used for intubation, endoscopies, tonsillectomy, urethra for catheterization Available as ointment, gel, cream, lozenges
► Produced

Infiltration anaesthesia
►

►

Infiltration anesthesia is the injection of local anesthetic directly into tissue without taking into consideration the course of cutaneous nerves. Infiltration anesthesia can be so superficial as to include only the skin Block sensory nerve endings

► ► ►

Used for minor operations Eg: incisions, excisions, hydrocele, herniorraphy Lignocaine , bupivacaine can be used

► Is

suitable for small areas

► Requires

large amount of drug to anaesthetize small area in local infection and inflammatory disorders – b/c change in pH = LA don’t work there = 1st – ab, 2nd - LA

► C/I

Conduction block
► A)

Field block anesthesia is produced by subcutaneous injection of a solution of local anesthetic in order to anesthetize the region distal to the injection for dental procedures , appendicectomy, scalp stitching

► Done

► B)

Nerve block: injection of LA around the nerve plexus or nerve trunks

► Brachial

plexus blocks are particularly useful for procedures on the upper extremity and shoulder. nerve blocks are effective for anesthesia and relaxation of the anterior abdominal wall.

► Intercostal

Spinal anesthesia
: injection into subarachnoid space in space between L3-4 ► Commonly used are lignocaine, bupivacaine, tetracaine Uses: lower limb surgery, obstetric procedures, prostatectomy, appendicectomy
► Site

Advantages
► No

loss of consciousness analgesia relaxation

► Good

► Complete

► Co

morbid patients tolerate better

Complications
due to seepage of CSF, reduced by using fine needles ► Hypotension due to sympathetic blockade Treated by raising the foot end and if necessary with sympathomimetics ► Respiratory paralysis ► Septic meningitis ► Post operative urinary retention ► Cauda equina syndrome
► Headache

Epidural anesthesia
► Injection

into epidural space used are lignocaine, bupivacaine

► Commonly
► Safer

but difficult than spinal anaesthesia ► a/v = any level


				
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posted:11/22/2008
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