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Local Anesthetics

Topic updated on 06/12/17 11:16pm

Snapshot
  • A 36-year-old man presents to the emergency department with a wound laceration of the hand. He is a construction worker that accidently cut himself with his tools approximately 2 hours prior to presentation. His most recent tetanus vaccination was given 2 years ago. There does not appear to be any possible contamination or foreign body at the site. The wound extends beyond the dermis. Wound irrigation is commenced and a local anesthetic is administered prior to suturing.
Introduction
  • Local anesthetics
    • prevent sensory nerve impulses from reaching the central nervous system (CNS)
      • this is accomplished by blocking the inner portion of the sodium channel
        • which in turn prevents the propagation of an action potential
        • most effective in rapidly firing neurons
    • structure
      • a lipophilic group is joined to a hydropilic group via
        • an amide or ester linkage
          • biotransformation of amides mainly occur in the liver
            • tertiary amine local anesthetics cross membrane in uncharged form and
              • undergo ionic change in order to bind to sodium channel in charged form
          • biotransformation of esters are accomplished by plasma cholinesterases (pseudocholinesterase)
    • onset and duration of action
      • influenced by
        • tissue pH
          • infection can decrease pH (more acidic) in the affected tissue
            • alkaline anesthetics will therefore become charged, which
              • impairs its ability to penetrate the membrane to block sodium channels
                • more anesthetic would be needed
        • lipid solubility of the drug
        • drug concentration
        • nerve morphology
    • effects
      • local anesthetics can result in vasodilation, which
        • causes the drug to diffuse away from the site of action
          • vasoconstrictors (e.g., epinephrine) can correct this
            • thus enhancing the local action of the drug
    • order of nerve blockade
      • small-diameter fibers > large diameter fibers and myelinated fibers > unmyelinated fibers 
        • size predominates over myelination
    • order of loss
      • (first) pain → temperature → touch → pressure (last)
Medications
  • Medication
    • esters
      • benzocaine, cocaine, propcaine, and tetracaine
    • amides
      • bupivacaine, lidocaine, and mepivacaine
  • Clinical use
    • minor surgical procedures
    • spinal anesthesia
  • Adverse effects
    • CNS symptoms
      • excitation or depression
    • cardiovascular toxicity (bupivacaine)
    • arrhythmia
    • methemoglobinemia (benzocaine)


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