LAST: PREVENTION AND TREATMENT

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LAST PREVENTION AND TREATMENT

• PART A

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PART A OVERVIEW

• What are local anesthetics
• Classification
• MOA
• Anesthetic potency
• Clearance
• Uses
• Prolongation of action

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PART B ADVERSE EFFECTSSYSTEMIC TOXICITY

• Allergic reaction
• Local toxicity
• Systemic toxicity

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What are LA

• Weak bases which produce a transient and
  reversible loss of sensation (analgesia) in a
  circumscribed region of the body without loss of
  consciousness.
• Normally, the process is completely reversible.

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Classification

• Local anesthetics - esters or amides.
• Major difference is their potential for producing
  adverse effects and the mechanisms of their
  metabolism.

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Esters

• Eg Procaine, Cocaine, tetracaine
• The ester linkage is cleaved by plasma
  cholinesterase. Short half life( abt 1min).
• Amides
• Eg lidocaine, Marcaine(Bupivacaine),
  ropivacaine. Amide linkage is cleaved in the
  liver. Half life is about 2-3 hrs

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MECHANISM OF ACTION

• LA block nerve conduction by impairing
  propagation of action potential in axons.
  Interact directly with Na channels and stop Na
  ion influx. May also act on K Ca channels.
• LA need to diffuse passively in uncharged state
  (lipophilic) to reach target-axoplasmic side of
  Na channel.

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HOW MUCH IS TOO MUCH?
ESTERS MAX DOSE(mg/kg) DURATION (HOURS)
Chloroprocaine 12 0.5 1
Procaine 12 0.5 1
Cocaine 3 0.5 1
Tetracaine 3 1.5 6
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Amides Max Dose (mg/kg) Duration (h)
Lidocaine 4.5/(7 with epi) 0.75 1.5
Mepivacaine 4.5/(7 with epi) 1 2
Prilocaine 8 0.5 1
Bupivacaine 3 1.5 8
Ropivacaine 3 1.5 8
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ANESTHETIC POTENCY

• The more LA is lipophilic the more potent it is (
  increased rate of diffusion).
• More protein binding prolongs the effect.
  Bupivacaine is approximately 95 protein-bound.
• Intermediate-duration LAs (lidocaine and
  mepivacaine) have a smaller protein-bound
  fraction (60-70).
• pH higher pH speeds action (keeps LA uncharged).
• pKa lower pKa faster onset.
• Concentration higher conc. rapid onset (mass
  effect).

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CLEARANCE

• ESTERShydrolysis via tissue cholinesterase
• AMIDESmetabolism via hepatic enzymes

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USES

• Surface anesthesia
• Infiltration anesthesia
• Regional anesthesia

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SURFACE ANESTHESIA

• On intact skin
• eutectic mixture of
• Local anesthetics
• (EMLA)
• Slow absorption,
• Action up to ½ Hr

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INFILTRATION ANESTHESIA

• LA infiltrates locally into tissues to numb the
  area.
• Purpose of pain free procedures-before injections
  with large needles, lumbar puncture

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REGIONAL ANESTHESIA

• Nerve block
• Intravenous
• Epidural/Extradural
• Intrathecal block/ spinal anaesthesia

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NERVE BLOCKS

• Inject a drug around the nerve
• Anaesthetise a region

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INTRAVENOUS ANESTHESIA

• 0.5-1 lidocaine without adrenaline

• Biers Block

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EPIDURAL

• Thoracic, lumbar, sacral
• Act on nerve roots
• Less hypotention

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SPINAL BLOCK

• Sympathetic nerve block
• hypotension