1. Local anesthesia
Local anesthesia is defined as transient reversible loss of sensation in a
circumscribed area of the body caused by either a depression of
excitation in nerve endings, or an inhibition of conduction process in the
peripheral nerves, without lss of consciousness.
2. Theories of pain
1. Specificity theory – by Descartes
it is based on specific set of peripheral nerve fibres that are nociceptive in
function, this theory cannot explain any pathological pain, referred pain and
also do nt explain paroxysmal episodes of pain.
2. Pattern theory- by Goldscheider, 1894
This theory explains that pain is produced by the summation of sensory input
within dorsal horn of the spinal column.
3. Gate control theory- by Melzack and Wall, 1965
This theory contains elements of both specificity and pattern theory.
It proposes spinal gating mechanism in substantia gelatinosa that modulates
sensory input by the balance of activity of smaller diameter (A-delta C) an
larger diameter (A-delta).
3. Theories of local anesthesia
Acetylcholine theory- acetylcholine is a neurotransmitter at the nerve
synapses , supposed to be involved in nerve conduction.
Membrane expansion theory- LA penetrates the membrane , expands it and
narrows the sodium channels, thus decreases the permeability to sodium ions.
Calcium displacement theory – La molecules displace calcium inside the nerve
which in turn controls sodium channels.
Electrical potential theory/ surface charge theory- the cations RNH+ of LA
align themselves on the nerve membrane and make outside potential more
positive , thus the threshold potential increases.
Specific receptor theory – the specific receptors are sodium channels which re
bound by LA and controls the sodium channels. It is the most acceptable
theory which explains the actions of LA.
7. LA substance according to biological site and mode of action
classification Definition Chemical substance
Class A Agents acting on the tetradoxin
External surface of sacridoxin
Nerve membrane
Class B Agents acting on the Quarternary ammonium
Internal surface of nerve Analogues of lidnocaine
membrane
Class C Receptor-independent Benzocaine, N-butanol
Physio chemical Benzyl alcohol
mechanism
Class D Both receptor mediated Most clinically
And receptor-
independent
Useful LA agents-
physiochemical Lidnocaine,
mechanism prilocaine
8. LA blocks
Nerve block – LA deposited near main trunk
Field block – LA is deposited near large branch of peripheral nerves
Infiltration- LA is deposited near small nerve endings.
9.
10. Composition of Local Amesthesia
Lignocaine HCL IP 21.3 mg Anesthetic agent
Adrenaline (as Adr. Bitartrate IP
0.09 mg) 0.005
Vasoconstrictor
NaCL IP 6.0 mg isotonic
Sodium metabisulphite IP 0.5 mg (germicidal) reducing agent,
antioxidant for adrenaline
Methyl Paraben IP 1.0 mg preservative
Thymol Fungicide
Distilled water for injection IP to make
0.1mL
vehicle
