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Local anaesthetics

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Local anaesthetics

  1. 1. LOCAL ANAESTHETICS
  2. 2. INTRODUCTION ANAESTHESIA : GENERAL ANAESTHESIA LOCAL ANAESTHESIA
  3. 3. DIFFERENCE BETWEEN GENERAL ANAESTHESIA & LOCAL ANAESTHESIA FEATURES Gen.Anaesthsia Local Anaesthsia Site of action CNS Peripheral nerves Area of body involved Whole body Restricted area Consciousness Lost Unaltered Care of vital functions Essential Usually not needed Poor health patients Risky Safer Use in non cooperative patients Possible Not possible Major surgery Preferred Cannot be preferred Minor surgery Not preferred preferred
  4. 4. LOCAL ANAESTHETICS  DEFINITION: are drugs which, when applied directly to peripheral nervous tissue, block the nerve conduction and abolish all sensations in the part supplied by the nerve without loss of consciousness.
  5. 5. HISTORY Erythroxylon coca 500’s B.C Coca leaves first used by Peruvians for psychotropic properties Cocaine –first discovered in 19 th century- local anasthetic action Andean natives –chewed extract of leaves- stimulatory &euphoric action 1850’s Cocaine isolated, hypodermic needle developed Albert niemann-isolated -1860 1884 Sigmund Freud studies the effects of cocaine it was used as ocular anasthetic 1884 Carl Koller introduces cocaine into medical practice 1884 Local anesthesia- used in dentistry by Halsted and Hall 1905 Procaine synthesized by Einhorn
  6. 6. FEATURES OF LOCAL ANAESTHETICS  Should have quick onset of action  Should not be irritating to skin & mucous membranes  Duration of action must be long enough to allow desired surgery to be completed  Should be effective on both injection & local application  Should have low Systemic toxicity
  7. 7.  Should not cause any permanent damage on any tissue.  Should be relatively free from producing allergic reaction.  Should be stable in solution and readily undergo biotransformation. No LA in use today satisfy all of these criteria , however all anesthetics do meet a majority of them. Contd…
  8. 8. Chemistry All local anesthetics are weak bases,  they have amphiphilic property Consist of hydrophilic secondary or tertiary amine on one side Lipophilic aromatic residue on other side Two are joined by an alkyl chain through an ester or amide linkage
  9. 9. Based on linkage they can be classified as
  10. 10. Esters:  cocaine, procaine, tetracaine, and chloroprocaine.  hydrolyzed in plasma by pseudo-cholinesterase.
  11. 11. Amides:  lidocaine, mepivicaine, prilocaine, bupivacaine, and etidocaine.  metabolized in the liver to inactive agents.  True allergic reactions are rare (especially with lidocaine)
  12. 12. ADVANTAGE OF AMIDE LAs OVER ESTER LAs  Produce more intense and longer lasting anaesthesia .  Bind to α1 acid glycoprotein in plasma.  Not hydrolysed by plasma esterases.  Rarely causes hypersensitivity reaction.
  13. 13. CLASSIFICATION 1. INJECTABLE ANAESTHETIC:  LOW POTENCY, SHORT DURATION procaine chloroprocaine  INTERMEDIATE POTENCY AND DURATION Lidocaine prilocaine  HIGH POTENCY, LONG DURATION tetracaine bupivacaine ropivacaine dibucaine
  14. 14. 2. SURFACE ANAESTHETIC: SOLUBLE INSOLUBLE cocaine benzocaine lidocaine butylaminobenzoate tetracaine oxethazaine benoxinate
  15. 15. MECHANISM OF ACTION OF LAs 
  16. 16. MECHANISM OF ACTION OF LAs  LA blocks the nerve conduction by reducing entry of Na+ through the voltage gated channels  Due to this, they block the initiation & propagation of nerve impulse.  At higher doses it also blocks 1. Voltage gated Ca2+ channels 2. K+ channels
  17. 17. VASOCONSTRICTOR  Vasoconstrictor used – adrenaline (1:50,000 to 1:200,000).  prolongs duration of action of LAs by decreasing rate of removal from local site into circulation  Enhances intensity of nerve block  Reduces systemic toxicity of LAs  Provides more bloodless field for surgery
  18. 18. Order of sensory function block  1. pain  2. cold  3. warmth  4. touch  5. deep pressure  6. motor  If applied to the tongue, bitter taste is lost first followed by sweet & sour and salty taste last of all Recovery in reverse order
  19. 19. SYSTEMIC ACTIONS  CNS: All LAs are capable of producing a sequence of stimulation followed by depression  CVS: LAs are cardiac depressants, but no significant effects are observed at conventional doses. But at high doses they decrease automaticity, excitability, contractility, conductivity.  BLOOD VESSELS: LAs tend to produce fall in BP. This is primarily due to sympathetic blockade, but higher doses cause direct relaxation of arteriolar smooth muscle
  20. 20. SYSTEMIC TOXICITIES OF LOCAL ANESTHETICS Central Nervous System Toxicities: - Excitement: Tremors, shivering, and convulsions characterize the CNS excitement. - Depression: respiratory depression at higher doses CVS derangement— High plasma titers may depress the cardiovascular system directly.  Blood pressure may fall because of arteriolar dilation, myocardial depression, and/or cardiac conduction disruption.  Treatment includes patient positioning, IV fluids, and vasopressors.
  21. 21. Haemotological: large dose of prilocaine->10mg/Kg- Acumalation of it’s metabolite- o toludine-oxidises haemoglobin to methehaemoglobin Patient – cyanosed , blood –choclate colouerd –decompensate in patients with preexisting cardiac /res. Disease Treatment: I.V methylene blue /ascorbic acid-a reducing agent – convert methhemoglobin to Hb
  22. 22. Hypersensitivity: Some patients are hypersensitive (allergic) to some local anesthetics.  There are two basic types of local anesthetics (the amide type and the ester type). A patient who is allergic to one type may or may not be allergic to the other type.
  23. 23. PHARMACOKINETICS  Absorption  Local anesthetics are absorbed when ingested. Some local anesthetics may be absorbed in toxic amounts after topical use.  Absorption after an injection depends on drug solubility in lipid and in water, tissue vascularity and local anesthetic and vasoconstrictor effects on local circulation.
  24. 24. Distribution: amides-wide distribution –I.V-lipophilics taken up by highly perfused organs-then moderately perfused Ester type- short plasma half life Metabolism and excretion  Esters are hydrolyzed by plasma and liver esterases. Longer-acting esters are often metabolized more slowly.. Patients with altered pseudo-cholinesterase activity may be highly sensitive to these drugs.  Amides are metabolized in the liver by cyp450.-N- dealkylation then hydrolysis except prilocaine- hydrolysis first-o toludine-can cause methhamoglobinemia
  25. 25.  Patients with severe hepatic damage or advanced congestive heart failure may be unusually sensitive to these drugs.  Some amides are partially excreted unchanged in the urine.  Acidification can enhance excretion PK properties of amide LAs :
  26. 26. PRECAUTIONS AND INTERACTIONS  Aspirate lightly to avoid intravascular injection.  Inject the LA slowly &take care not to exceed the maximum safe dose, especially in children.  Propranolol may reduce metabolism of lidocaine and other amide LAs by reducing hepatic blood flow.  Vasoconstrictor (Adr) containing LA should be avoided for patients with ischemic heart disease, cardiac arrhythmia, uncontrolled hypertension those receiving β-blockers or tricyclic antidepressants
  27. 27. Techniques of Local Anaesthesia
  28. 28. Surface Anaesthesia
  29. 29.  Application of a local anesthetic to nose, mouth, throat, tracheobronchial tree, esophagus.  Onset & duration depends on the site, the drug, its concentration and form.  Absorption of soluble LAs from mucous membrane is rapid.
  30. 30.  Except for eutectic lidocaineprilocaine , no other LA is capable of anaesthetizing intact skin.  used to relieve itching, burning, and surface pain (for example, as seen in minor sunburns).
  31. 31. Infiltration Anaesthesia
  32. 32.  Injection of LA directly into tissue under the skin.  used primarily for surgical procedures.  LAs most frequently used are lidocaine (1%), bupivacaine (0.25%), etidocaine(0.5-1%), ropivacaine(0.5-1%), mepivacaine(1-3%) and prilocaine(1-4%).  mix with adrenaline (1:20000) to prolong the action.
  33. 33. Conduction block:
  34. 34.  Injected around nerve trunks so that area distal to injection is anaesthetised and paralyzed. - Choice of LA and concentration is mainly determined by the required duration of action. - Lidocaine for intermediate duration of action - longer lasting anesthesia bupivacaine may be selected.
  35. 35. Field block: - produced by injecting the LA subcutaneously in the surrounding area of nerve so that all nerves coming to particular field are blocked. - herniorrhaphy, appendicectomy, dental procedures, scalp stitching, operations on forearms and legs etc. - - Larger area can be anaesthetized with lesser drug compared to infiltration.
  36. 36. Nerve Block:  local anesthetic is injected around a nerve that leads to the operative site.  Usually more concentrated forms of local anesthetic solutions are used  eg: radial nerve block, ulner nerve block so on. Nerve block lasts than field block or infiltration anaesthesia.  Lidocaine (1.5%), mepivacaine(1.5%), bupivacaine (0.25- 0.35%) can be used.
  37. 37. Epidural Anaesthesia.
  38. 38.  spinal dural space is filled with semi liquid fat through which nerve root travel.  Injected in this space- acts primarily on nerve roots and small amounts permeates through intravertebral foramina to produce multiple paravertebral blocks.  used to produce analgesia or anaesthesia in surgical and obstretric.  Divided into 3 categories depending on site of action: 1. Thoracic: 2. Lumbar: 3. Caudal:
  39. 39. Spinal Anaesthesia.
  40. 40.  Injected into the subarachnoid space between L2-3 or L3-4 of the spinal cord .  Suitable LA like lidocaine (3-5%), bupivacaine (0.5- 0.8%), tetracaine(0.3-0.5%).  Primary site of action is cauda equina rather than spinal cord.  Used to anaesthetize lower abdomen and hind limbs.
  41. 41.  Use of hyperbaric(in7.5-10% glucose) or hypobaric (in distilled water) solution of LA .  proper positioning of the patient is also limiting the block to the desired level.  Advantages over general anaesthesia are:  Safer  Produces good analgesia and muscle relaxation without loss of consciousness.  Cardiac, pulmonary, renal disease and diabetic pose less problem.
  42. 42. complication of spinal anaesthesia:  Respiratory paralysis  Hypotension  Headache  Cauda equina syndrome  Septic meningitis Contraindications:  Hypotension & hypovolemia  Infant & childrens- control of level is difficult  Vertebral abnormalities - kyphosis
  43. 43. Intravenous regional anaesthesia
  44. 44.  Also referred as Bier’s block & used for upper limb and orthopedic procedures.  Regional analgesia produced within 2-5min and last till 5- 10min.  Only ¼ of the injected drug enters systemic circulation when tourniquet is removed.  Bradycardia can occur and bupivacaine should not be used because of higher cardio toxicity.
  45. 45. LAs IN A NUT SHELL
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