Transcript Local Anaesthetics

Local Anaesthetics
Local anaesthetics are drugs,
when applied directly to peripheral nervous
tissue,
block nerve conduction and
abolish all sensations in the part supplied by
the nerve.
Classification
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Local anaesthetics are Natural : Cocaine
Procaine, Amethocaine
Ethyl aminobenzoate (Benzocaine)
Lignocaine (lidocaine,Xylocaine))
Clove oil
Ethyl chloride spray
Classification-clinical uses
Topical anaestheticsBenzocaine,Lidocaine,Ethyl chloride
Infiltration & block anaestheticsProcaine,Lidocaine
Spinal anaestheticsTetracaine,Lidocaine
Epidural & caudal anaestheticsLidocaine,Mepivacaine,Procaine
General properties of L A
Varying degrees of water and lipoid solubility
Lipoid solubility helps the drug to move in
to neuronal fibre
Water solubility helps to get the drug to site
of action from site of injection or
application
These drugs are bases and form soluble salt
with acids
In the tissues, as the pH is alkaline, the free
base is liberated and produces its
pharmacological action on the tissues
Mechanism of Action
Local anaesthetics block both the generation
and the conduction of the nerve impulse
Sodium channel block
Reduce the membrane permeability changes
of the cell membrane to Na+ ion, which is
the first event in depolarization. An action
potential is not generated
Affecting the process of depolarization
Membrane stabilizing effect
L A compete with Ca+ for a site controlling
passage of Na across the membrane
By preventing binding of Ca to this site(rec),Na
influx is checked
Decrease in Na conductance
Depression of rate of electrical depolarization
Failure to achieve threshhold potential level
Lack of development of propagated action
potential
-conduction blockade
Smaller fibers are blocked first.
Autonomic fibers
Sensory fibers conducting temperature and
pain
Sensory fibers carrying touch, pressure and
vibration sensations
Motor fibers
Recovery of function appears to occur in the
reverse order
Local anaesthetics are less effective when
injected into an inflammed area
Pharmacological actions
L A cause blocked of nerve conduction at
the site of application--exert effect on
restricted area
Producing local effect
-Loss of pain,temperature ,touch
Vasodilatation
Loss of motor power
Important actions on other systems
C N S :By blocking inhibitory controlCNS stimulation- produces restlessness, tremors
in toxic doses- convulsions.
Stimulation is followed by depression and death is usually
due to respiratory depression
CV S :
Direct Myocardiac depressants. HR and force of
contraction- decreased
Refractory period- increased ,conduction is slowed
(membrane stabilizing effect).
Procainamide and lignocaine are used for cardiac
depressant action( antiarrhythmic action).
When used by infiltration the absorption can
be retarded by using adrenaline with the
drug.
Anticholinesterases increases the duration of
action of procaine by inhibiting its
destruction by plasma pseudocholinesterase
Adverse reactions
Allergic reactions
CVS: Fall of BP and cardiac arrest can
sometimes occur
CNS: - convulsions,
IV diazepam or IV thiopentone is used
Oxygen is given to prevent hypoxia, artificial
ventilation is necessary in the presence of
respiratory arrest
Therapeutic uses
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Surface anesthesia:
LA –solution,ointment,cream or powder directly
to site
Amethocaine as a surface anaesthetic for cornial
surface, throat, urethra, rectum for piles,fissures and
skin.
Benzocaine and Lidocaine are used as all purpose
surface anaesthetics except for eye
Dibucaine is used for ear, rectum and skin,
Proparacaine is used exclusively for the eye
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Infiltration anaesthetic:
By injecting drug throughout the area to be used- Nerve
endings are anaesthetized by their direct exposure to the
drug.
The drug is infiltrated S C
Procaine 2 % and lignocaine 2 % are most commonly
used, mixed with adrenaline 1:200,000 to 250,000 to
prolong action.
Adrenaline should be avoided
when LA
are used to produce ring block to anaesthetize digits or
penis, to avoid local ischaemia
in patients with known myocardial disease.
Nerve Block or conduction
block
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Field blockwhere the drug is injected into surrounding
area,
e.g. scalp,ant. abdominal wall.
Nerve blockLA is injected very close to the nerve
e.g. brachial plexus,radial nerve block
Choice of the anaesthetic is determined by
the duration of anesthesia needed.
Cervical Block
Spinal anesthesia :
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Subarachnoid or intrathecal anaesthetiathe drug is injected into subarachnoid space so it
reaches to roots of spinal nerves
Its level in the space is adjusted by using solutions
as vehicles with higher (hyperbaric) or lower
(hypobaric) specific gravity than that of CSF.
the injection is made ‘heavy” by adding dextrose
or ‘light’ by adding saline.
The position of the patient is also important in
limiting the block to the desired level.
e.g. Procaine and lignocaine
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This can best be achieved
by sitting the patient on
the operating table and
placing their feet on a
stool. If they then rest their
forearms on their thighs,
they can maintain a stable
and comfortable position.
The sitting position is
preferable in the obese.