5. LOCAL ANAESTHESIA
Dr. Pritam Haval Dr. Smruti Haval
Local
anaesthetic (LA) drugs are most commonly used in primary care practice for
various procedures. They are classified on basis of their chemical structure
and duration of action. Chemically LA consists of a benzene ring separated from
tertiary amide either by ester or amide ring linkage. Hence based on that they
are classified as aminoesters and aminoamides.
Based on chemical structure
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Amino esters
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Amino amides
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Procaine
Chloropracaine
Benzocaine
Cocaine
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Lignocaine
Mepivacaine
Prilocaine
Bupivacaine
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Metabolized by psudocholinesterase (except cocaine which use liver
for metabolism)
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Metabolized primarily in liver
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High incidence allergic reaction
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Low incidence allergic reaction
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Unstable solution
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Stable solution
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Have shorter duration of action
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Have longer duration of action
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Mechanism of action
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Drug with non-ionized form penetrates axonal
membrane and inside it gets ionized.
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This ionized form binds to receptor situated in
sodium channel in inactivated state from inner side, blocking the channel and preventing
depolarization and action potential.
·
LA enters at node of Ranvier.
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Potency of LA depends upon lipid solubility.
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Onset of action depends on pH & pKa.
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Local anesthetic drugs are weak bases, agents with
pKa closer to physiological pH will have more drugs in non-ionized form which
can diffuse through axonal membrane and so onset will be rapid. That is why
soda bicarbonate is added to LA which increases pH and more drugs is available
to use in non-ionized form.
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Type of fiber decides susceptibility to block.
Small diameter axons are more susceptible to block than large diameter fibers.
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However myelinated fibers are more sensitive than
non myelinated fibers. Based on the diameter of fiber they are classified as
type A, B, C.
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HENCE SEQENCE OF BLOCAKGE IS TYPE B> type C
>type A.
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Functionally system wise
autonomic>sensory>motor and in recovery motor>sensory>autonomic.
· Among sensory fibers sequence of blockage
temperature (cold>hot) >pain>touch >deep pressure>proprioception
·
Duration of action depends upon dose, plasma
protein binding capacity and vasoconstrictor ability.
· Systematic absorption depends on site if injection
and additional vasoconstriction. Additional vasoconstriction increases margin
of safety by decreasing systematic absorption.
Systemic effects and toxicity
Cardiovascular system:
Negative
inotropic effect myocardium; depression of conduction system; Ventricular
arrthmias at high dose; hypotension, cardiac arrest, bradycardia.
Central nervous system:
This is
the first to get involved and affected system. Depression of cerebral tissues
(inhibitory more than excitatory);Symptoms and signs are dizziness, circumoral
numbness, tongue parasthesia, visual & auditory disturbances, muscle
twitching, tremors, convulsions and coma.
Respiratory system:
Lignocaine
depressed hypoxic drive; Direct depression of medullary respiratory center can
occur at high dose.
Immunological:
Allergic reactions are
common with esters than amides; Cross sensitivity does not exist between amides
and esters but do exist between drugs of same class.
Local toxicity:
When
injected directly into nerve, LA can damage nerve. When directly injected into
muscle they are myotoxic.
LA and other drugs
Adrenaline
In
concentration of 1:200000 duration of both sensory and motor blockade is
increased by addition of epinephrine to lignocaine. But only sensory block is
prolonged if epinephrine is added to bupivacaine with no effect on motor
blockade.
Xylocaine
with adrenaline should not be used for Ring block (for fingers, toe, penis,
pinna [absolute contraindications]); with inhalational agent (especially
halothane which sensitizes myocardium to adrenaline is used); myocardial
ischemia; hyperthyroid; severe hypertensive and intravenous regional
anesthesia.
Soda bicarbonate
1 ml of 8.4%
to 10 ml of lignocaine enhances onset of action,duration of action and improve quality
off block, decrease pain of injection.
Modes of administration of
LA
·
Topical application at skin: Prilocaine 5% and
lignocaine 5% equal amount (1:1 ratio)
·
At mucous membrane of mouth, pharynx, larynx in
form of xylocaine sprays 4% or benzocaine lozenges.
·
For catheterization and proctoscopy, lignocaine
jelly 2%.
·
Eye drops tetracaine ointments and cocaine drops.
·
For anal canal & rectum like piles, fissures
lignocaine 4%,dibucaine 1%,benzocaine 5% ointment.
·
For gastritis oxethazaine 0.2%.
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Infiltration at operation site & nerve block
by injecting around nerve.
·
Central neuraxial blocks like spinal &
epidural.
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Beir’s block by intravenous regional anaesthesia.
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Intravenous preservative free lignocaine (xylocard
2%) used dosage 2 mg/kg .Its use blunts cardiovascular response to laryngoscopy
and intubation.
Further reading
Butterworth JF, Mackey DC
Classification Based on duration of action and
potency
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Drug
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Duration
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Potency
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Duration of action
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Max safe dose (mg/kg)
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Comment
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Without adrenaline
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With adrenaline
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Chloroprocaine
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Shortest
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Low
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15-25 min
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30-90 min
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12 mg/kg
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Shortest acting
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Procaine
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Short
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Low
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15-30 min
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30-90 min
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12 mg/kg
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Lignocaine
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Intermediate
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Intermediate
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45-60 min
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2-3 hour
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3 mg/kg without adrenaline
7 mg/kg with adrenaline
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Not to be used in patients with h/o malignant hyerthermia,can cause
cauda eqina syndrome after continuous spinal. doc ventricular a tachycardia
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Mepivaccaine
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Intermediate
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Intermediate
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45-60 min
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2-3 hour
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4.5 mg/kg
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Cocaine
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Intermediate
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Intermediate
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3 mg/kg
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Potent vasoconstrictor
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Prilocaine
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Intermediate
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Intermediate
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45-60 min
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2-3 hour
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8 mg/kg
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Can cause methhaemoglobinemia
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Bupivacaine
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Long
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High
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2-3 hour
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3-5 hour
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2 mg/kg
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Very commonly used LA
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Dibucaine
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Longest
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High
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2.5-3.5hour
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3.5-5.5 hour
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1 mg/kg
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Longest acting, most potent, most toxic
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