Local anaesthetics

1. LOCAL ANAESTHETICS
OPTOM FASLU MUHAMMED

2. • LA are agents which produce reversible block
of nerve conduction without any structural
damage to the neuron concerned and without
any loss of consciousness.
• local anesthetics are used to block all
sensation in the part supplied by the nerve.

3. • Most local anesthetic agents consist of a
lipophilic group (eg, an aromatic ring)
connected by an intermediate chain via an
ester or amide to an ionizable group (eg, a
tertiary amine)

4. CLASSIFICATION
• ESTER TYPE
-COCAINE, TETRACAINE, PROCAINE,
BENZOCAINE etc .
 AMIDE TYPE
-LIDOCAINE, BUPIVACAINE, PRILOCAINE,
DIBUCAINE etc.

5. MECHANISM OF ACTION
 Sensory information passes along nerve fibers via
electrical impulse, or action potential.
 When nerve is at rest , the interior has a negative
charge.
 An action potential is generated by the influx of Na
ions in to the interior of the nerve, giving it a positive
charge-Depolarization
 The nerve fiber is returned to its resting potential by
efflux of k ions- repolarization.
 The action potential is then generated along the axon
by successive depolarization & repolarization of
adjacent regions.

6. • The primary mechanism of action of local
anesthetics is blockade of voltage-gated
sodium channels .
• Bind to receptors near the intracellular
end of the voltage gated Na channels.
• This reduce the permeability of cell
membrane to Na ions , so action
potential is not generated.

7. USES
• FOREIGN BODY REMOVAL
• A drop of LA will make location & removal of
the offending object very much easier.
• TONOMETRY
• Topical anesthesia is essential for the
assessment of IOP
• CONTACT LENS FITTING- eye impressions for
the constriction of scleral lenses
• Certain diagnostic procedures: LA are used
before a schirmer test of lacrimal function.

8. Commonly used local anesthetics in
ophthalmic practice.
• Lidocaine- -1-4%
• Bupivacaine-0.25-0.75%
• Etidocaine-1-1.5%
• Prilocaine-1-2%
• Mepivacaine-1-2%
• Hexylcaine-1-2%
• Proparacaine-0.75%
• Tetracaine-0.5%

9. Common topical anesthetics.
Lignocaine-2-4%
Tetracaine-0.5%
Proparacaine-0.75%
Common regional anesthetics
Lignocaine-1-2%
Bupivacaine-0.5%
Hexylcaine-1-2%

10. Pharmacokinetics
ABSORPTION
Systemic absorption of injected local anesthetic
from the site of administration is determined
by several factors, including dosage, site of
injection, drug-tissue binding, local blood flow
• Application of a local anesthetic to a highly
vascular area such as the tracheal mucosa or
the tissue surrounding intercostal nerves
results in more rapid absorption

11. DISTRIBUTION
• The amide local anesthetics are widely
distributed after intravenous bolus
administration.
• After an initial rapid distribution phase, which
consists of uptake into highly perfused organs
such as the brain, liver, kidney, and heart, a
slower distribution phase occurs with uptake
into moderately well-perfused tissues, such as
muscle and the gastrointestinal tract.

12. METABOLISM AND EXCRETION
• The local anesthetics are converted in the liver
(amide type) or in plasma (ester type) to more
water-soluble metabolites and then excreted
in the urine.
• Ester type LA is metabolized by
pseudocholinesterase and amide type by
hepatic microsomal enzymes and enzyme
amidase.

13. SIDE EFFECTS
• Cardiovascular
-depression of heart, bradycardia, hypotension,
cardiac arrhythmias etc.
 CNS
-rapid absorption produce restlessness, tremor,
convulsions.
 Anaphylactic reaction
-common with ester type.
-causes asthma, dermatitis, skin rash etc.

14. • Corneal change
-very rarely reversible corneal change may occur.

15. ROUTES OF APPLICATION
• SURFACE ANAESTHESIA
-Most popular method in ophthalmology.
-lignocaine 4% is used .
INFILTRATION ANESTHESIA
 Nerve endings are anaesthetized by direct
exposure to drug.
 Drug is infiltrated subcutaneously
Procaine [2%] and lignocaine [2%] used.

16. NERVE BLOCK ANAESTHESIA
Drug is injected very close to nerve eg; brachial
plexus.
SPINAL ANAESTHESIA
Drug is injected to the subarachnoid spaces.
Eg; procaine and lignocaine.
EPIDURAL ANAESTHESIA
Drug is injected outside the dura.

17. Commonly used drug with LA
 Adrenaline –to prolong the effect of LA and to
reduce the toxicity by reducing absorption
from local area.
 Hyaluronidase-enzyme which cause
depolymerization of hyaluronic acid and
increase the permeability of injected fluid.

18. TETRACAINE [Amethocaine]
 Topical anaesthetic [0.5%] in ophthalmic
practice.
 Onset of action is 30 minutes.
 Side effects-stinging sensation, punctate ,drug
allergy.

19. PROPARACAINE
Topical anaesthetic. [0.75%]
Onset of action-30 sec
COCAINE
First local anaesthetic obtained from leaves of
plant Erythroxylon Coca.
It is no more in use nowadays because of its
corneal toxicity, addicting nature etc.

20. PROCAINE
• First synthetic local anaesthetic
• Used as a small area infiltration and spinal
anaesthetic.
• It is poorly absorbed from mucous membrane-
so no topical use.
• 2% injection is the usual preparation.

21. LIGNOCAINE
 Most commonly used LA
 In ophthalmology 4% [topical] and 2%
[infiltration] solutions are commonly used.
 It has quick onset of action and high degree of
penetration.
 The drug is recommended for topical, nerve
block, infiltration and epidural injection and
for dental analgesia.
 It may cause drowsiness.

22. BUPIVACAINE
• Onset of action is slow
• A 0.75% solution produce anaesthesia and
akinesia for 8-12 hours.
• In ophthalmic practice, usually a combination
of 2% lignocaine,0.75% bupivacaine and 7.5
TRU of hyaluronidase is used for periocular
injection, and surgery of any kind may be
performed on such type of anaesthetized eye.