Local Anesthetics

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Key Points

  • Local Anesthetics
    • Composed of lipophilic aromatic ring joined to a hydrophilic group via an amide or ester linkage
    • Ester Drug Names
      • Breakdown of ester anesthetics are accomplished by plasma cholinesterases (pseudocholinesterase)
      • Benzocaine
      • Procaine
      • Tetracaine
      • Chloroprocaine
    • Amide Drug Names
      • Breakdown of amide anesthetics mainly occurs in the liver
      • Lidocaine
      • Prilocaine
      • -vacaine drugs: mepivacaine, bupivacaine, ropivacaine
      • Mnemonic: “amides have two I’s in the name”
    • Mechanism
      • Block sodium (Na+) channels
        • Blocking sodium influx inhibits depolarization and action potentials in sensory neurons
      • Onset and duration of action influenced by tissue pH
        • Most local anesthetics are alkaline tertiary amines which cross membrane in uncharged form, and undergo ionic change in order to bind to sodium channel in charged form
        • Infection decreases pH (more acidic), which decreases amount of drug available in uncharged form → requires more drug
    • Clinical Use
      • Analgesia/Anesthesia
        • Used directly in minor surgical procedures or administered as spinal/epidural anesthetic
        • Order of sensation loss: (1) pain → (2) temperature → (3) touch → (4) pressure (last)
          • Related to relative action of drug on nerves with different properties
          • Drug activity depends on
            • small-diameter fibers > large diameter fibers
            • myelinated fibers > unmyelinated fibers
              • size predominates over myelination
            • Pain and Temperature are carried by small, myelinated fibers (Aδ fibers), the afferent fibers of nociceptors and free nerve endings
            • Touch is carried by large myelinated fibers
      • Vasodilation
        • All local anesthetics (except cocaine) are vasodilators, causing drug to diffuse away from the site of action
        • Often administered with epinephrine
          • To induce vasoconstriction and prevent diffusion into systemic circulation
    • Adverse Effects
      • Esters → allergic reactions
        • Give amides in people allergic to esters
      • Cardiovascular toxicity
        • Especially with bupivacaine
        • Due to Na+ channel block in cardiomyocytes, which may impair conduction
      • Methemoglobinemia (benzocaine)