Local Anesthetic

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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.

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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.

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Chemistry
All local anesthetics are weak bases, classified
as tertiary amines. 
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• Esters
• These include cocaine, procaine, tetracaine, and
  chloroprocaine.
• They are hydrolyzed in plasma by
  pseudo-cholinesterase. One of the by-products of
  metabolism is paraaminobenzoic acid, the common
  cause of allergic reactions seen with these
  agents

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• 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)

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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 channellocal anesthetics
  bind directly to the intracellular
  voltage-dependent sodium channels
• Block primarily open and inactive sodium
  channels, at specific sites within the channel

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• 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.

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Order of sensory function block

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

Recovery in reverse order
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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

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Factors affecting local anesthetic action

• 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.
• more lipophilic agents are more potent as local
  anesthetics

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Factors affecting local anesthetic action

• Effect of protein binding - increased binding
  increases duration of action
• Effect of vasodilator activity - greater
  vasodilator activity decreased potency and
  decreased duration of action

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The adverse effects of local anesthetics fall
broadly into four groups

• (a) Effects attributable to the technique itself
  rather than to the agent used, for example needle
  damage to a vessel or nerve.
• (b) Local and regional effects of the drug, which
  may be related to its anesthetic activity or a
  consequence of irritation or allergy.
• (c) Systemic effects, most usually seen if the
  agent is inadvertently injected into a blood
  vessel in sufficient quantities.

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

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TOXICITIES OF LOCAL ANESTHETICS

• The result of over-dosage leading to high blood
  levels of the agent given.
• Therefore, to avoid a systemic toxic reaction to
  a local anesthetic, the smallest amount of the
  most dilute solution that effectively blocks pain
  should be administered.

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• Hypersensitivity. Some patients are
  hypersensitive (very rare cases)
• There are two basic types of local anesthetics
  (the amide type and the ester type). A patient
  who is allergic to one type may or may not be
  allergic to the other type.

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• Central Nervous System Toxicities.
• Local anesthetics, if absorbed systematically in
  excessive amounts, can cause central nervous
  system (CNS) excitement or, if absorbed in even
  higher amounts, can cause CNS depression.

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• Cardiovascular Toxicities. Local anesthetics if
  absorbed systematically in excessive amounts can
  cause depression of the cardiovascular system.
• Peripheral vascular action arteriolar dilation
  (except cocaine which is vasoconstrictive
• Hypotension.
• Hematological SE
• Methemoglobinemia

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Administraion of Local Anesthetcs

• 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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• 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.

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• 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.
• Peridural Anesthesia. This type of anesthesia is
  accomplished by injecting a local anesthetic into
  the peridural space.

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• 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.

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