Local Anesthetic Agents

Local anesthetic agents block nerve conduction at the site, causing temporary loss of sensation without unconsciousness.

Definition of Local Anesthetic Agents:

• Local anesthetics (LAs) are drugs that block nerve conduction when applied locally to nerve tissues. They reversibly inhibit the perception of pain without causing loss of consciousness.

Mechanism of Action:

• Local anesthetics work by:
• Blocking voltage-gated sodium (Na⁺) channels in the neuronal membrane.
• This prevents depolarization and transmission of action potentials along nerve fibers.
• Sensory fibers (especially small, unmyelinated C fibers) are more sensitive than motor fibers.
• Sequence of loss of sensations:
• Pain → Temperature → Touch → Deep pressure

Classification:

1. Bases on  Chemical Structure

   • Esters
     • Examples: Procaine, Tetracaine, Cocaine
     • Metabolism: In the plasma by cholinesterase
   • Amides
     • Examples: Lidocaine, Bupivacaine, Ropivacaine
     • Metabolism: In the liver by CYP enzymes

2. Bases on Duration of Action

   • Short-acting
     • Drug: Procaine
   • Medium duration
     • Drugs: Lidocaine, Mepivacaine
   • Long-acting
     • Drugs: Bupivacaine, Ropivacaine

Pharmacokinetics

• Absorption: Depends on vascularity and whether a vasoconstrictor (like epinephrine) is used.
• Distribution: Highly protein-bound (especially amide types).
• Metabolism:
  • Esters: Rapid hydrolysis in plasma
  • Amides: Hepatic metabolism
• Excretion: Mostly via kidneys

Clinical Uses

• Infiltration anesthesia (minor surgeries)
• Nerve block anesthesia
• Spinal and epidural anesthesia
• Topical anesthesia (e.g., in eye, skin)
• Dental procedures

Routes of Administration:

• Topical: applied to skin or mucous membranes
• Infiltration: injected into tissue
• Nerve block: injected near a specific nerve
• Epidural and spinal anesthesia: administered near the spinal cord

Adverse Effects

• Systemic Toxicity (CNS & Cardiovascular):

  • CNS: Dizziness, restlessness, seizures, respiratory depression
  • Cardiovascular: Hypotension, arrhythmias, cardiac arrest (especially with Bupivacaine)
  • Allergic reactions: More common with ester LAs

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