Download 13. Local Anesthetics III

Local Anesthetic
DR. ISRAA
Local Anesthetic
• A local anesthetic is an agent that interrupts pain
impulses in a specific region of the body without
a loss of patient consciousness.
• Normally, the process is completely reversible.
History
• The first local anesthetic was Cocaine which was
isolated from coca leaves.
• It was first introduced to clinical ophthalmology as
a topical ocular anesthetic.
• In 1884, Dr. William Stewart Halsted was the first
to describe the injection of cocaine into a sensory
nerve trunk to create surgical anesthesia.
Types of local anesthetics
1. Esters:
– These include cocaine, procaine, tetracaine,
and chloroprocaine.
– They are hydrolyzed in plasma by pseudocholinesterase.
– One of the by-products of metabolism is
para-aminobenzoic acid, the common cause
of allergic reactions seen with these agents.
Types of local anesthetics
2. Amides:
– These include lidocaine, mepivicaine,
prilocaine, bupivacaine, and etidocaine.
– They are metabolized in the liver to inactive
agents.
– True allergic reactions are rare (especially
with lidocaine)
Mechanism of Action
• Local anesthetics work to block nerve
conduction by reducing the influx of sodium
ions into the nerve cytoplasm.
• They block the sodium channel
• Anesthetics bind directly to the intracellular
voltage-dependent sodium channels
• Block primarily open and inactive sodium
channels, at specific sites within the channel.
Mechanism of Action
• Local anesthetics abolish sensation (and in
higher concentrations, motor activity) in a
limited area of the body without producing
unconsciousness.
• The small, un-myelinated nerve fibers, that
conduct impulses for pain, temperature, and
autonomic activity, are most sensitive to
actions of local anesthetics.
Order of sensory function block
1. Pain
2. Cold
3. Warmth
4. Touch
5. Deep pressure
6. Motor
Recovery in reverse order
Factors affecting
local anesthetic action
1. Effect of pH : local anesthetics are basic drugs. The
charged (cationic) form binds to receptor site,
uncharged form penetrates membrane, efficacy of
drug can be changed by altering extracellular or
intracellular pH
2. Effect of protein binding: increased binding
increases duration of action
3. Effect of vasodilator activity: greater vasodilator
activity = decreased potency and decreased
duration of action
Factors affecting
local anesthetic action
4. Lipid solubility: appears to be the primary
determinant of intrinsic anesthetic potency.
Chemical compounds which are highly
lipophilic tend to penetrate the nerve
membrane more easily, such that less
molecules are required for conduction
blockade resulting in enhanced potency.
Adverse effects of local
anesthetics
• Effects attributable to the technique itself
rather than to the agent used, for example
needle damage to a vessel or nerve.
• Local and regional effects of the drug, which
may be related to its anesthetic activity or a
consequence of irritation or allergy.
• Systemic effects, most usually seen if the
agent is inadvertently injected into a blood
vessel in sufficient quantities.
Adverse effects of local
anesthetics
• Effects of additives:
– Vasoconstrictors to prolong the local effect
– Hyaluronidase to promote penetration
– Preservatives to prevent bacterial contamination
or degradation
Adverse effects of local
anesthetics
1. Hypersensitivity.
• Ester type local anesthetics are metabolized to
product which can stimulate immune system
• This is very rare and can be prevented by using
agents from amide subclass.
2. Peripheral vascular action: arteriolar dilation
(except cocaine which is vaso-constrictive)
3. Hypotension
4. Methaemoglobinaemia.
Adverse effects of local
anesthetics
5. Central Nervous System Toxicities:
• Local anesthetics, if absorbed systematically in
excessive amounts, can cause light headedness
or sedation, restlessness, nystagmus, and
convulsions
which may be followed by
respiratory and cardiovascular depression.
Adverse effects of local
anesthetics
6. Cardiovascular Toxicities:
• Local anesthetics with exception of cocaine are
vasodilators.
• Local anesthetics with exception of cocaine can
cause brady-arrhythmia and heart block
• Cocaine in abuser can cause severe
hypertension, MI, cerebral hemorrhage and
arrhythmias.
Administration of Local
Anesthetics
1. Infiltration Anesthesia:
– Local infiltration occurs when the nerve
endings in the skin and subcutaneous tissues
are blocked by direct contact with a local
anesthetic, which is injected into the tissue.
– Infiltration Anesthesia is used primarily for
surgical procedures involving a small area of
tissue (for example, suturing a cut).
Administration of Local
Anesthetics
2. Topical Block:
– This technique is often used during examination
procedures involving the respiratory tract, the
eye etc.
– For topical application, the local anesthetic is
always used without epinephrine.
3. Nerve Block:
– In this type of anesthesia, a local anesthetic is
injected around a nerve that leads to the
operative site.
– Usually more concentrated forms of local
anesthetic solutions are used for this type of
anesthesia.
Administration of Local
Anesthetics
4. Peridural Anesthesia.
– This type of anesthesia is accomplished by
injecting a local anesthetic into the Peridural
space.
5. Spinal Anesthesia.
– In spinal anesthesia, the local anesthetic is
injected into the subarachnoid space of the
spinal cord
Vasoconstrictors
• Vasoconstrictors decrease the rate of vascular
absorption which allows more anesthetic to
reach the nerve membrane and improves the
depth of anesthesia.
• 1:200,000 epinephrine (adrenaline) appears to
be the best vasoconstrictor.
GOOD LUCK