Local anesthetic

1. LOCAL ANAESTHETICS:
 Drugs that block nerve conduction when applied locally
to nerve tissue in appropriate concentrations.
 It is the loss of sensation without the loss of
consciousness. Their action is completely reversible.
 They act on every type of nerve fibre and can cause both
sensory and motor paralysis.
 They act on axons, cell body, dendrites, synapses and
other excitable membranes that use sodium channels as
the primary means of action potential generation.
 Cocaine was the first agent to be used which had
addiction potential, then procaine was synthesized, but
was not that effective. The best product isolated was
lignocaine and is still dominating today.

2. CLASSIFICATION:
 Injectables:
 Short acting: procaine, chloroprocaine.
 Intermediate acing: lignocaine, prilocaine.
 Long acting: tetracaine, bupivacaine, dibucaine
 Surface anaesthetics:
 Cocaine, Lignocaine, Tetracaine, Dibucaine
 Used on eye:
 Proparacaine HCl
Depending on linking chain structures:
 Ester linked: cocaine, procaine, tetracaine,
benzocaine.
 Amide linked: lignocaine, bupivacaine, ropivacaine.

3. MOA:
 local anaesthetics prevent the generation and the
coduction of nerve impulses.
 Primary mechanism of action is blockade of voltage
gated sodium channels.
 Rise in threshold for excitation.
 Impulse conduction slows down.
 Rate of rise of action potential slows down.
 Ability to generate an action potential is abolished.
 Thus prevents the generation of an action potential
and its conduction.
 Smaller fibers are blocked first because of more
surface area

4.  Addition of vasoconstrictor like adrenaline or
phenylehrine:
 prolongs the duration of action (by slowing the rate of
absorption) and
 reduces systemic toxicity (since absorption rate is
reduced)
SYSTEMIC ACTIONS:
 Depending on the concentration attained in plasma
any LA can produce systemic effects.
 Las interfere with the function of all organs in which
conduction or transmission of impulses occur.

5. CNS EFFECTS:
 LA depresses the inhibition from cerebral cortex. This loss
of inhibition results in unopposed excitatory activity
which is manifested as restlessness, tremors and may
proceed to convulsions. This central stimulation is
followed by generalized CNS depression and death may
result from respiratory failure.
CVS EFFECTS:
 The primary site of action is myocardium.
 Lignocaine decreases excitability, conduction rate and
force of contraction. It also causes arteriolar dilatation.
SMOOTH MUSCLES:
 Decreases contraction in intact bowel.
 They also relax vascular and bronchial smooth muscles

6. ADVERSE EFFECTS:
 Hypersensitivity reactions: skin rashes, dermatitis,
asthma, allergy.
 CNS: dizziness, auditory and visual disturbances,
mental confusion, anxiety, muscle tremors,
convulsions and respiratory failure.
 CVS: hypotension, bradycardia, arrhythmias and
cardiac arrest.
 LOCAL IRRITATION: irritation may be seen with
bupivacaine, wound healing may be delayed

7. USES:
 Local anesthetics are found to be useful depending on
their site and technique of administration.
 Surface anaesthesia: used in anaesthesia of mucous
membrane of the eye, nose, mouth, tracheobroncheal
tree oesophagus, genitourinary tract. (tetracaine,
lignocaine)
 Infiltration anaesthesia: by using these, it is possible to
provide anaesthesia without disturbing normal body
activities. Its or 2 types: superficial and deeper
injections.(procaine, lignocaine) these may cause
systemic toxicities. Requires more dose

8.  Field block anaesthesia: subcutaneous injection of LA
solution proximal to the site to be anaesthetized,
interrupts nerve transmission in the region distal to the
injection. Here, lesser dose can be used.
 Nerve block: injection of a solution of a LA around
individual peripheral nerves produces larger areas of
anaesthesia with the smaller amount of drug required
when compared to above techniques. Anaesthesia starts
a few cm distal to the site of injection.
 Spinal analgesia: LA is injected into sub arachnoid space
between L2-3 or L3-4 below the lower end of spinal cord.
The drug acts on nerve roots. Lower abdomen and lower
limbs are anaesthetized and paralysed

9.  Epidural analgesia: LA is injected into the spinal
extradural space and it acts on the nerve roots. It is
technically more difficult and comparatively larger
volumes of the anesthetic are needed.
 Intravenous regional anaesthesia: useful for rapid
anaesthetization of an extremity. A rubber bandage is
used to force the blood out of the limb (veins) and a
tourniquet is applied to prevent the reentry of blood. A
dilute solution of anesthetic is injected intravenously.
It diffuses into extravascular tissue and produces
anaesthesia in 2 minutes. (has a disadvantage of
causing pain due to tourniquet )