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Updated: Jan 26 2021

Local Anesthetics

  • 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, procaine, 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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