Download Lecture notes of International Edcation Local Anesthetics Definition

Lecture notes of International Edcation
Local Anesthetics
1. Definition
Drugs that cause reversible loss of sensory perception, especially of pain in a
restricted area of the body, when applied topically or local injection.
LA if applied to a mixed nerve—sensory and motor impulses are
interrupted—resulting in muscular paralysis and loss of autonomic control.
2. history
Cocaine – First local anaesthetic – Niemann in 1860, Koller in 1884 as an
ophthalmic anesthetic. Procaine – Einhorn in 1905. Lidocaine – (Prototype)
Lofgren in 1943.
3. Chemistry and Classification
Local Anesthetics can be classified according to source , duration of action ,
chemistry and therapeutic , but the classic classification is according to
chemistry.
LAs usually include three basic moiety：①lipophilic aromatic ring；②middle
functional linker；③hydrophilic amino group. And the Las can be classified as
esters or amides based on the structure of this intermediate chain.
4. Mechanism of action
LAs work in general by decreasing the entry of sodium ions
and thereby
inhibiting action potentials in a given axon.
LA if applied to a mixed nerve—sensory and motor impulses are
interrupted—resulting in muscular paralysis and loss of autonomic control .
5. Pharmacological Effects : local actions and systemic actions
The pharmacological action of local anesthetics on the CNS is depression . At
high levels, local anesthetics will produce tonic-clonic convulsions . Procaine,
Lidocaine,
Mepivacaine,
Prilocaine
and
Cocaine
generally
produce
anti-convulsant properties; this occurs at a blood level considerably below that
at which the same drugs cause seizure
6.
Pharmacokinetics
Absorption : all local anesthetics possess some degree of vasoactivity; most producing
some level of vasodilation . Ester local anesthetics are potent vasodilating drugs
Procaine (Novocaine) possesses tremendous vasodilating abilities which are
employed to halt arteriospasm (accidental IA injection).*Cocaine is the only
local anesthetic that consistently produces vasoconstriction
vasodilation


initial
intense vasoconstriction. Vasodilation leads to an increased
rate of absorption of the local anesthetic into the blood, thus decreasing the
duration and depth of pain control while increasing the anesthetic blood
concentration and potential for overdose (toxic reaction).
Distribution: Once in the blood, local anesthetics are distributed to all tissues. Brain,
head, liver, lungs, kidneys and spleen have high levels of local anesthetics due
to their high level of perfusion . Skeletal muscle has the highest level because it
has the largest mass of tissue in the body
Metabolism: the metabolic degradation of LAs depends on whether the
compound has an ester or an amide linage.
Excretion: kidneys are the major excretory organs for LAs.Esters appear in
very small concentrations in the urine because they are almost completely
hydrolyzed in plasma
7. Therapeutic Uses：
(1)Surface anaesthesia:
Application to mucous membranes and abraded skin. Only
the superficial layer is anaesthetised.
(2)Infiltration anaesthesia:
Dilute solution of LA is infiltrated under the skin
in the area of operation and blocks sensory nerve ending.Procaine, Lidocaine
2% are used either with or without Adrenaline 1:200000.Used for minor
operations (e.g. incisions, excisions, hydrocele, etc.)
(3)Conduction block （a.Field block
b.Nerve block）: Drug is injected close to
the nerve or big nerve trunks eg. Brachial Block, Sciatic, Femoral, Obturator Nerve.
Choice of drug: Depends on the type of surgery whether short duration or long
duration.Most common drug: 0.5% Bupivacaine
(4)Spinal anaesthesia: LA is injected into the subarachnoid space.
(5)Epidural anaesthesia: When the anaesthetic injected outside the dura, the
technique is known as Epidural anaesthsia. Most common drug: Lidocaine,
Bupivacaine
8. Averse reactions
(1)CNS effects: light-headedness, dizziness, auditory and visual disturbances,
mental confusion, disorientation, et.
(2)Cardiovascular toxicity: bradycardia, hypotension, cardiac arrhythmias and
vascular collapse.
(3)Local tissue toxicity(low)
(4)Hypersensitivity
reactions:
rashes,
angioedema,
sensitization, asthma and anaphylaxis(rarely).
dermatitis,
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