Slay

• Local anesthetics (LA)


  Reversibly block the fast voltage-gated Na+ channels in the neuronal membrane

  
• Local anesthetics


  • They are a specific receptor for Na+ channel
  • This leads to the impairment of action potential, thus inhibiting impulse conduction in peripheral nerves & CNS without causing CNS depression
  • Sequence of blockade: autonomic impulses > somatic sensory impulses > somatic motor impulses

  
• Local anesthetics


  Most are weak bases (pKa 8-9)
• Local anesthesia is difficult to achieve if the tissue is inflamed
• Infected and inflamed tissue has an acidic pH; this results in an increased proportion of cationic local anesthetic component that has a low permeability
• Laryngospasm


  • Characterized by closure of the glottis during intubation
  • A common complication encountered in cats being intubated
  • May lead to hypoxia and cyanosis
• Prevention of laryngospasm


  Injection (1 ml) or spray of 2% lidocaine to the glottis before placing the endotracheal tube
• Representative larynx of cats

  • Laryngeal spasm in cats, not medicated, in which intubation at the first attempt was not possible
  • Laryngeal opening after desensitization with lidocaine hydrochloride (0.1 mL of 10% topically)
  • Laryngeal relaxation after paralysis with rocuronium bromide (0.6 mg kg−1 IV)
• Pharmacodynamics of local anesthetics

  • Thin nerve fibers are more easily blocked than thick ones (THIN > THICK)
  • Autonomic fibers (B & C fibers), small unmyelinated (C fibers), & small myelinated fibers (B & Aδ fibers) will be more readily blocked than thick, myelinated fibers (AUTONOMIC, SMALL unmyelinated > THICK myelinated)
• Epinephrine added to local anesthetics


  • Decreases the rate of drug absorption & thereby prolongs the effect of lidocaine by ~50%
  • Reduces the concentration of local anesthetic that enters the circulation at a given time, thereby reducing local anesthetic toxicity
• The dose of epinephrine in the combination should not be more than 1:100,000
• Dosage forms of local anesthetics

  • Injectable
  • Topical
• Chemical structure of local anesthetics

  An ester (-CO-) or an amide (-NHC-) bond links the hydrocarbon chain to the lipophilic aromatic ring
• Esters


  • Procaine
  • Cocaine
  • Chloroprocaine
  • Tetracaine
• Amides


  • Lidocaine/Lignocaine
  • Bupivacaine
  • Mepivacaine
  • Prilocaine
  • Etidocaine
• Synthetic local anesthetics are structurally related to cocaine (adrenergic agonist)
• Cocaine


  • Causes vasoconstriction
  • No longer used as a local anesthetic because of its potential for abuse
• Lidocaine/Lignocaine


  • Most-widely used
  • Possesses reasonably rapid onset of action (5-10 min)
  • Has good spreading properties
  • Duration of action (1-2 hr)
  • May cause a stinging sensation when injected

  
• Bupivacaine


  • Used when long action is required (post op analgesia; prolonged surgery etc.)
  • Has a prolonged duration of action (up to 8 hrs when combined with epinephrine)
  • Onset 20 min, lasts for 4-6 hr
• Prilocaine


  • Has slower onset of action & spreads less well than lidocaine
  • Mainly used in the horse as it causes less swelling
  • May cause dose-dependent methemoglobinemia which limits its clinical usefulness
• Mepivacaine


  • Most widely used drug in the horse as it causes very little swelling & edema in injection area
  • Has a faster onset of action & greater reliability of block than prilocaine
• Methods of producing local anesthesia

  1. Surface or topical anesthesia

  2. Infiltration anesthesia

  3. Peripheral nerve block

  4. Spinal (intrathecal) anesthesia

  5. Epidural anesthesia
• Surface or topical anesthesia


  • Applied on skin & mucous membrane of ear, eye, larynx
  • Eye drop, spray, cream, ointment
• Infiltration anesthesia


  • Drug is injected into skin or deeper structures in proximity to the target nerve endings
  • To facilitate minor surgery (e.g. skin biopsy, suturing wounds, removal of small animal digits)
• Peripheral nerve block


  • Drug is injected to the area surrounding the nerve or plexus
  • Anesthetic diffuses to nerve trunks & anesthetize the area innervated by the nerve
  • Commonly used in large animals
  • Caution: avoid injecting directly into the nerve since it could lead to temporary or permanent loss of nerve function
• Spinal (intrathecal) anesthesia


  • Drug is injected into the CSF of the subarachnoid space
  • Causes sensory & motor blockade with rapid onset
  • Seldom used in animals
• Epidural anesthesia


  • Drug is injected into the epidural space posterior to the end of the spinal cord
  • Used in obstetric, perineal & lower extremity procedures
• Biotransformation of local anesthetics

  • Esters - hydrolyzed by pseudocholinesterases in the plasma
  • Amides - mainly metabolized in the liver