Local Anesthetic

1
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.

2
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.

3
Types of local anesthetics

• Esters
• These include cocaine, procaine, tetracaine, and
  chloroprocaine.
• They are hydrolyzed in plasma by
  pseudo-cholinesterase.
• One of the by-products of metabolism is
  para-aminobenzoic acid, the common cause of
  allergic reactions seen with these agents

4
Types of local anesthetics

• 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)

5
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

6
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.

7
Order of sensory function block

• 1. pain
• 2. cold
• 3. warmth
• 4. touch
• 5. deep pressure
• 6. motor

Recovery in reverse order
8
Factors affecting local anesthetic action

• Effect of pH charged (cationic) form binds to
  receptor site uncharged form penetrates membrane
  ,efficacy of drug can be changed by altering
  extracellular or intracellular pH
• Effect of protein binding - increased binding
  increases duration of action
• Effect of vasodilator activity - greater
  vasodilator activity decreased potency and
  decreased duration of action

9
Factors affecting local anesthetic action

1. 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.

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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.

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Adverse effects of local anesthetics

• Effects of additives, notably vaso- constrictors
  to prolong the local effect, hyaluronidase to
  promote penetration, and preservatives to prevent
  bacterial contamination or degradation

12
Adverse effects of local anesthetics

• 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.
• Peripheral vascular action arteriolar dilation
  (except cocaine which is vaso-constrictive)
• Hypotension
• Methaemoglobinaemia.

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Adverse effects of local anesthetics

• 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.

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Adverse effects of local anesthetics

• 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.

15
Administration of Local Anesthetics

• 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).

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Administration of Local Anesthetics

• 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.
• 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.

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Administration of Local Anesthetics

• Peridural Anesthesia. This type of anesthesia is
  accomplished by injecting a local anesthetic into
  the Peridural space.
• Spinal Anesthesia. In spinal anesthesia, the
  local anesthetic is injected into the
  subarachnoid space of the spinal cord

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Vasoconstrictors

• Vasoconstrictors decrease the rate of vascular
  absorption which allows more anesthetic to reach
  the nerve membrane and improves the depth of
  anesthesia.
• 1200,000 epinephrine (adrenaline) appears to be
  the best vasoconstrictor.