Local Anesthetics(final pre)


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Local Anesthetics(final pre)

  2. 2.  t1/2 Elimination (h) Vdss (L) CL (L/min) Bupivacaine 28 3. 5 72 0.47 Lidocaine 10 1.6 91 0.95 Mepivacaine 7 1.9 84 0.78 Prilocaine 5 1.5 261 2.84 Ropivacaine 23 4.2 47 0.44 2
  3. 3. Local Anesthesia: Definition: Local anesthesia is any technique to render part of the body insensitive to pain without affecting consciousness. LOCAL ANESTHETICSThese are are the agents which produce transient and reversible loss 3
  4. 4. CONTINUED The following terms are often used interchangeably: Local anesthesia, is anesthesia of a small part of the body such as a tooth or an area of skin. Regional anesthesia is aimed at anesthetizing a larger part of the body such as a leg or arm. Conduction anesthesia is a comprehensive term which encompasses a great variety of local and regional anesthetic techniques. 4
  5. 5. Local anesthetics:amides vs. esters Common structure  Aromatic ring  Tertiary amine  Alkyl chain Linking bond  Amide bond (see lidocaine)  Ester bond (see procaine) 5
  6. 6. Types of Local Anesthetics ESTERS: Procaine chloroprocaine tetracaine Cocaine Benzocaine AMIDES: Etidocaine (Duranest) Lidocaine (Xylocaine) Mepivacaine (Carbocaine) Prilocaine (Citanest) Ropivacaine Bupivacaine (Marcaine) 6
  7. 7. Pharmacokinetics: PKa And Onset of Action: Local anesthetics with a pKa closest to physiological pH will have a higher concentration of nonionized base that can pass through the nerve cell membrane, and generally a more rapid onset. pKa > 7.4 more cations, pKa < 7.4 more anions 7
  8. 8. Duration and Protein binding: Amides: Esters: Bupivacaine, Etidocaine Chloroprocaine and and Ropivacaine- very Procaine- have low potency high potency and lipid and lipid solubility and also low solubility, very long duration and protein binding. duration and protein binding also. Cocaine- has intermediate Lidocaine, Prilocaine potency and solubility and and Mepivacaine- have intermediate duration and intermediate potency protein binding and lipid solubility and Tetracaine- has high potency intermediate duration of action and protein and lipid solubility along with a binding. long duration of action and high protein binding 8
  9. 9. Systemic absorptionRate of systemic absorption: Intravenous > tracheal > intercostal > caudal > paracervical > epidural> brachial plexus > sciatic > subcutaneous High tissue binding also decreases the rate of absorptionMetabolism: Amides… N-dealkylation and hydroxylation  P-450 enzymes, liver, slower process than esterase activity  Prilocaine>lidocaine>mepivacaine>ropivacaine>bupivacaine Esters…  Pseudocholinesterase 9
  10. 10. Pharmacokinetics: AGENT Pot. Onset pKa %PB P. coefProcaine 0.5-1% (Novocain) 1 Rap 8.9 5.8 0.02Chloroprocaine 2-3% (Nesacain) 4 Rap 8.7 ? 0.14Tetracaine 0.1-0.5% (Pontocain) 16 Slow 8.5 75.6 4.1Lidocaine 1-5% (Xylocaine) 1 Rap 7.9 64.3 2.9Mepivacaine 1.5% (Carbocaine) 1 Mod 7.6 77.5 0.8Bupivacaine 0.25-0.75% (Marcainesensorcaine) 4 Slow 8.1 95.6 27.5Etidocaine 0.5-1.5% (Duranest) 4 Rap 7.7 94 141Prilocaine 1 7.9 55 0.9 10Ropivacaine 0.75% (Naropin) 4 Mod 8.1 94 2.9
  11. 11. Mechanism of Action Voltage & time dependent blockade of resting membrane sodium channels binding to sodium channel receptors inside the cell Inc threshold for excitation Slowing of impulse conduction Decreased rate of rise of action potential inhibiting action potentials in a given axon. 11
  12. 12. 12
  13. 13. CONTINUED If the resting potential encounters the proper chemical, mechanical or electrical stimuli to reduce the membrane potential to less than -55 mV then an action potential is produced that allows the influx of sodium ions. LA act here to block the Na influx. The influx allows the membrane potential to further increase to +35mV temporarily. Sodium and potassium channels along with the sodium/potassium pump eventually returning a given axon back to it’s resting membrane potential after an action potential. 14
  14. 14. Regional anesthesia Definition: Rendering a specific area of the body, e.g. foot, arm, lower extremities, insensate to stimulus of surgery or other instrumentationUses: Provide anesthesia for a surgical procedure Provide analgesia post-operatively or during labor and delivery Diagnosis or therapy for patients with chronic pain syndromes 15
  15. 15. Types: Application of local anesthetic to mucous membrane - cornea, nasal/oral mucosa: Uses :  awake oral, nasal intubation, superficial surgical procedure Advantages :  technically easy  minimal equipment Disadvantages :  potential for large doses leading to toxicity 17
  16. 16. Subcutaneously Application of local subcutaneously to anesthetize distal nerve endings Uses:  Suturing, minor superficial surgery, more extensive surgery with sedation Advantages:  minimal equipment, technically easy, rapid onset Disadvantages:  potential for toxicity if large field 18
  17. 17. IV Block Injection of local anesthetic intravenously for anesthesia of an extremity Uses  any surgical procedure on an extremity Advantages:  technically simple, minimal equipment, rapid onset Disadvantages:  duration limited by tolerance of pain, toxicity 19
  18. 18. Peripheral nerve block Injecting local anesthetic near the course of a named nerve Uses:  Surgical procedures in the distribution of the blocked nerve Advantages:  relatively small dose of local anesthetic to cover large area; rapid onset Disadvantages:  technical complexity 20
  20. 20. Plexus Blockade Injection of local anesthetic adjacent to a plexus, e.g cervical, brachial or lumbar plexus Uses :  surgical anesthesia or post- operative analgesia in the distribution of the plexus Advantages:  large area of anesthesia with relatively large dose of agent Disadvantages:  technically complex, potential for toxicity and neuropathy. 22
  21. 21. PLEXUS BLOCK 23
  22. 22. Central neuraxial blockade - “Spinal” Injection of local anesthetic into CSF Uses:  profound anesthesia of lower abdomen and extremities Advantages:  technically easy, high success rate, rapid onset Disadvantages:  “high spinal”, hypotension due to sympathetic block, post dural puncture headache. 24
  23. 23. 25
  25. 25. Central Neuraxial Blockade -“epidural Injection of local anesthetic in to the epidural space at any level of the spinal column Uses:  Anesthesia/analgesia of the thorax, abdomen, lower extremities Advantages:  Controlled onset of blockade, long duration when catheter is placed, post-operative analgesia. Disadvantages:  Technically complex, toxicity, “spinal headache” 28
  26. 26. 29
  27. 27. Systemic Toxicity of LocalAnesthetics Drugs-not a great difference in toxicity between equally potent local anesthetics-one of low toxicity when a large dose is required Site of injection-vascular sites lead to rapid absorption  accidental I.V. injection is the most common cause of toxicity 30
  28. 28. Signs and Symptoms ofLocal/Regional Anesthesia Toxicity CNS Toxicity: Unconsciousness Generalized convulsions Coma Apnea Numbness of the mouth and tongue, metal taste in the mouth 31
  29. 29.  Light-headednes Tinnitus Visual disturbance Muscle twitching Irrational behavior and speech 32
  30. 30. Cardiovascular toxicity slowing of the conduction in the myocardium myocardial depression peripheral vasodilatation usually seen after 2 to 4 times the convulsant dose has been injected 33
  31. 31. Hypersensitivity/Allergy: true allergy is very rare. esters ---- sensitivity to their metabolite, para- aminobenzoic acid (PABA), and does not result in cross-allergy to amides.. allergy to paraben derivatives, which are often added as preservatives to local anesthetic solutions. 34
  32. 32. Methemoglobinemia: prilocaine metabolite, o-toluidine, is known to cause methemoglobinemia. . Seen with larger not recommended for use in infants. 35
  33. 33. Prevention and Treatment ofLocal/Regional Anesthesia ToxicityPrevention: use recommended dose Aspirate through the needle or catheter before injecting the local anesthetic. large quantity required, divide the dose into small increments, increasing the total injection time inject slowly (<10 ml/min) 36
  34. 34. Treatment: Manage airway and give oxygen Stop convulsions if they continue for more than 15 to 20 seconds  Thiopental 100 mg to 150 mg IV  or Diazepam 5 mg to 20 mg IV 37
  35. 35. Drug Interactions Chloroprocaine may interfere with the analgesic effects of intrathecal morphine Opioids and α2 agonists potentiate LA’s Propranolol and cimetidine decrease hepatic blood flow and decrease lidocaine clearance Pseudocholinesterase inhibitors decrease Ester LA metabolism Dibucaine (amide LA) inhibits pseudocholinesterase used to detect enzyme potentiate nondepolarizing muscle relaxant blockade 38
  36. 36. THE END 39