Local anaesthesia is defined as a reversible loss of sensation in a circumscribed area caused by depressing nerve excitation or inhibiting peripheral nerve conduction. It can be induced by low temperature, trauma, anoxia, neurolytic agents like alcohol and phenol, or chemical agents like local anaesthetics. Local anaesthetics work by altering the nerve membrane's resting potential, threshold potential, rate of depolarization, or rate of repolarization. They are available as salts and can exist as uncharged molecules or positively charged ions depending on pH. Local anaesthetics contain the active agent, a vasoconstrictor to decrease absorption, a reducing agent for stability, a preservative, and a vehicle for isotonicity. They are classified based

1. Local
anaesthesia

2. Definition :
Local anaesthesia is defined
as a reversible loss of
sensation in a circumscribed
area of a body caused by
depression of excitation in
nerve endings or an inhibition
of conduction process in
peripheral nerves.

3. Methods of inducing local anaesthesia:
◈ Low temperature
◈ Mechanical trauma
◈ Anoxia
◈ Neurolytic agents such as alcohol & phenol
◈ Chemical agents such as local anaesthetics

4. Properties of local anaesthesia :
◈ Should be non irritating to tissues
◈ Should not cause permanent alteration of nerve structure
◈ Systemic toxicity should be low
◈ Time of onset should be short
◈ Should be effective regardless of site of injection
◈ Duration of cation should be long

5. Electrophysiology of nerve conduction:
Resting potential of neuron= -70mV

6. Mechanism of action of local
anaesthesia
Local anaesthesia agent achieves interference with excitation process
in nerve membrane in one of following ways:
• Altering basic resting potential of nerve membrane
• Altering threshold potential
• Decreasing rate of depolarisation
• Prolonging rate of repolarization

7. Theories of mechanism of mechanism of
action of local anaesthetic agent
◈ Acetylcholine theory: there is no evidence that acetylcholine is involved in
neural conduction

8. Calcium displacement theory:

9. Surface charge (repulsion) theory:

10. Membrane expansion theory:
◈ Lipid soluble LA can easily penetrate the lipid portion of cell membrane
decreasing the diameter of sodium channel.

11. Specific receptor theory:

12. Dissociation of local anaesthetics:
• Local anaesthetics available as salts
for clinical use.
• Solution exists simultaneously as
uncharged molecule (RN) also called
as base and positively charged
(RNH+)
• In low pH equilibrium shifts and
solution exists in cationic form
• In high pH equilibrium shifts towards
base form
• Dissociation also depends on pKa of
local anaesthetic – pKa = affinity for
H+
• When pH= pKa concentration of
RNH+ = concentration of RN

13. Contents of local anaesthetic agent:
◈ Local anaesthetic agent (xylocaine, lignocaine 2%)
◈ vasoconstrictor ( adrenaline 1:80000) : to decrease the blood flow to injection site
absorption od LA into systemic circulation
decrease systemic toxicity
Maximum dose - 0.2mg & for patient with cardiovascular history - 0.4mg
◈ Reducing agent ( sodium metabisulphite) : vasoconstrictors are unstable in solution may get
oxidised on long standing and change the colour of solution. Hence, Na- bisulphite added it
competes for oxygen.
◈ Preservative ( methyl paraben, capryl hydrocuprienotoxin) : to maintain the sterility and allow
long standing pf LA . Some subjects may be allergic towards methylparaben.
◈ Fungicide ( thymol)
◈ Vehicle ( distilled water, NaCl) : to maintain the isotonicity which minimizes the discomfort during
injection

14. Classification of LA - based on duration of
action

15. Classification of LA - based on chemical
nature

17. MDR

18. Armanterium required
◈ Syringe
◈ Needle
◈ Cartrige
◈ Other- topical anaesthesia : ointment, gels, pastes, sprays
applicator tips
cotton gauze