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Local anaesthetics

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Lecture slides for MBBS students for pharmacology...

Lecture slides for MBBS students for pharmacology...

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Local anaesthetics Local anaesthetics Presentation Transcript

  • Local Anaesthetics Dr Mayur Chaudhari Assistant Professor Department of Pharmacology Government Medical College, Surat Available at www.slideshare.net 1
  • At The End Of Class….. • Local Anaesthesia • Mechanism of Action • Pharmacodynamic • Pharmacokinetic • Individual Agents • Techniques and uses 2
  • Local Anaesthesia 3
  • Local Anaesthesia • Reversible loss of sensation (Sensory) • In a local area • Without loss of consciousness • Without loss of control of vital functions • Topical/Injection/Infiltration 4
  • Ideal Local Anaesthetic Non irritant / Negligible Local irritation  Negligible local tissue damage  minimal systemic toxicity Rapid onset of action Prolonged action  water soluble  Sterilizable by heat Without after effects 5
  • Local Vs General Advantages Disadvantages • • • • • • • Uncooperative patient – No • Minor Surgery only • Some major Surgery • Also Have Side effects Consciousness Localized No Altered Physiology Monitoring of Vitals Safe in poor GC Response can be modified 6
  • Historical Aspects • South American natives chewed coca leaves for stimulant and euphoric action • Albert Niemann – isolated cocaine in 1860 • Niemann noted that it causes numbing of tounge • Sigmund Freud – tried it for psychic energizing activity unsuccessfully • Carl Koller – used cocaine for Ocular surgery in 1884 • Halstead – infiltration anaesthesia 7
  • Classification  Injectable  Low Potency, Short Duration Procaine, Chloroprocaine  Intermediate Potency and Duration Lignocaine, Prilocaine  High Potency, long Duration Tetracaine, Bupivacaine, Ropivacaine, Dibucaine  Surface Anaesthetic  Soluble: Cocaine, Lignocaine, Tetracaine, Benoxinate  Insoluble: Benzocaine, Butamben, Oxethazaine 8
  • Classification - II Ester Linked Amide linked • Cocaine, Procaine, Chloroprocaine, Tetracaine • Lignocaine, Bupivacaine, Prilocaine, Ropivacaine – Short acting – Metabolized by plasma esterase – Can be used in poor liver function – Hypersensitivity - ↑ – Longer acting – Metabolized by liver enzymes – Avoided in poor liver function – Hypersensitivity - ↓ 9
  • Mechanism Of Action 10
  • Mechanism Of Action  Prevent generation and conduction of Nerve impulses by acting at the cell membrane:  Decrease the entry of Na+ ions during action potential  Increase in LA conc. decreases the maximum depolarization causing slowing of conduction  Finally depolarization fails to reach threshold potential 11
  • Mechanism Of Action 12
  • Mechanism Of Action Degree of blockade is frequency dependent: Greater blockade at higher frequency of stimulation Higher concentration of Ca++ reduces inactivation of Na+ channel Blockade is not due to hyperpolarization RMP is unaltered as K+ channels are not blocked 13
  • Factors Influencing Action of LA  Lipid Solubility  Lipid solubility helps in nerve penetration, faster action  Non ionized form can easily cross nerve membrane pH  Lower pKa (7.6 – 7.8) – faster acting (lidocaine, mepivacaine)  Higher pKa (8.1 – 8.9) – slower acting (procaine, tetracaine, bupivacaine) 14
  • Factors Influencing Action of LA  Vasoconstrictors (Adrenaline, Phenylephrine)  Tissue Necrosis, Systemic Side effects  CI in areas with terminal arteries (Fingers, Toe, Nose, Penis) - Hypoxic injury - Tissue Necrosis and May Produce gangrene  Felypressin (Vasopressin Analogue) - Used as vasoconstrictor in CV Dz Patients 15
  • Factors Influencing Action of LA Inflammation  Acidic environment  ionized LA, Penetration decreased  Alkalization  Hasten onset of nerve block  Limited increase in unionized form  precipitation of LA 16
  • Pharmacodynamics Functions lost by LA (Local)  Pain perception  Temperature  Touch sensation  Proprioception  Skeletal muscle tone 17
  • Pharmacodynamics Sensory > Motor Nonmyelinated > Myelinated Small fibres > Large fibres Autonomic fibres > Somatic Fibres 18
  • Pharmacodynamics (Systemic) CNS • Inhibition of inhibitory neurons • Euphoria, Dysphoria, Muscle twitches • Stimulation – Restlessness, tremors, Convulsions • Respiratory depression in high doses • Respiratory failure - death 19
  • Pharmacodynamics (Systemic) CVS • ↓ Automaticity, Conductivity, Excitability, Contractility, Conductivity • ↑ Effective refractory period • Prolonged QTc interval • Ventricular Tachycardia, Ventricular Fibrillation • ↓ in Blood Pressure by Sympathetic blockade • Cocaine ↑ Blood pressure 20
  • Pharmacodynamics (Systemic) Smooth Muscle • ↓ contraction of bowel • Relaxation of vascular and bronchial smooth muscle  Sympathetic System • Blockade – Spinal, Epidural anaesthesia, local infiltration in peritoneal cavity  Neuromuscular Junction • Block NMJ, Inhibit ganglionic transmission 21
  • Pharmacokinetics • Surface anesthetics from mucus membrane and abraded areas • Depends on Blood flow to the area, total dose and specific drug characteristics • Widely distributed in the body: (lipophilic) • Enters brain, heart, liver and kidney • Followed by muscle and other viscera 22
  • Pharmacokinetics • Ester linked LA – inactivated by hydrolysis by plasma esterases, cholinesterase • Spinal anaesthesia – absorbed into systemic circulation • Amide linked LA – Degraded in liver by CYP450 • Use restricted in Liver disease 23
  • Pharmacokinetics • Amide linked LA – bind with α1 acid glycoprotein • α1 acid glycoprotein (↑) – MI, Trauma, Cancer, Surgery, Smoking • α1 acid glycoprotein (↓) – Oral Contraceptive Pill, Infants • Termination of action depends on rate of absorption and elimination 24
  • Break Time Available at www.slideshare.net 25
  • Toxicity  CNS  Numbness in circumoral area and tongue  Metallic taste  Drowsiness, Lightheadedness, Restlessness  Visual and auditory disturbances, Nystgmus  Respiratory depression, convulsions  Death due to respiratory failure 26
  • Toxicity CVS  Hypotension, Bradycardia, Cardiac Dysrhythmia  CV Collapse Methaemoglobinaemia  Prilocaine and Benzocaine  AE due to Vasoconstrictor 27
  • Toxicity Hypersensitivity  Esters> Amides (Methyl Paraben)  Asthmatic attack  Allergic dermatitis 28
  • Prevention of Toxicity  Proper History, Allergy Testing  4 hour fasting, Premedication  Avoid in Hepatic and cardiac disease  Administration at Proper site  Wait for development of effect  Look for signs of toxicity  Observation post operatively 29
  • Cocaine Natural alkaloid from Erythroxylon coca Medical use limited to surface or topical anesthesia Avoid with adrenaline A toxic action on heart may induce rapid and lethal cardiac failure Marked pyrexia is with cocaine overdose Not used presently 30
  • Procaine  Topically ineffective  Used for infiltration because of low potency and short duration  Most commonly used for spinal anesthesia  Produces significant vasodilation. Adrenaline used to prolong effect  Systemic toxicity negligible because rapidly destroyed in plasma  Procaine penicillin 31
  • Lignocaine  Effective by all routes.  Faster onset (3 Vs 15 min), more intense, longer lasting  Good alternative for those allergic to ester type  Quicker CNS effects than others  Overdose (muscle twitching, cardiac arrhythmia, fall in BP, coma and respiratory arrest)  Antiarrhythmic  Available as Injections, topical solution, jelly and ointment etc. 32
  • Bupivacaine  No topical effect  Slower onset and one of longer duration agents  Used for infiltration, spinal, nerve block and epidural  Unique analgesia without significant motor blockade (popular drug for analgesia during labor)  High lipid solubility, high distribution in tissues and less in blood (benefit to fetus)  More cardio toxic than other LA (prolong QT interval) 33
  • Eutectic Lignocaine/Prilocaine  Eutectic Mixture – Lowering of melting point of two solids when they are mixed  Lignocaine+Prilocaine at 25o C in equal proportion  Oil is emulsified in water to form a cream  Occlusive dressing prior to procedure  IV Canulation, Split Skin graft harvesting, Superficial Procedure  Up to 5mm  last for 1-2 hour 34
  • Benzocaine, Butamben  Low aqueous solubility – Not absorbed from mucosa or broken skin  Long lasting anaesthesia without systemic toxicity  Lozenges for stomatitis, Sore throat  Dusting powder on wounds/ Ulcerated surfaces  Suppositories for anorectal lesions 35
  • Techniques Surface Anaesthesia  Mucous membranes and abraded skin  Nose, mouth, bronchial tree, cornea and urinary tracts  Lignocaine, Tetracaine 36
  • Infiltration Anaesthesia  Injection of LA directly into tissues irrespective of the course of nerve  Superficial or deeper structure  Amides are preferred  Should not be injected into tissues supplied by end arteries  Adequate anaesthesia without affecting normal function  Dose required is more  Chances of Systemic Toxicity 37
  • Field Block  Injection of LA subcutaneously  Anaesthetize the region distal to the site of injection  Anaesthesia starts 2-3 cm distal to site of injection  All nerves coming to the field are blocked  Dose required is less, Prolonged duration  Forearm, anterior abdominal wall, scalp and lower extremity  Knowledge of neuroanatomy is required 38
  • Nerve Block  LA injected around individual Nerve/ Plexus..Not in the Nerve  Sensory and motor block distal to site of injection  Block depends on Proximity, Conc. And Volume of LA  Degree of ionization and Time  Trigeminal nerve blocks (face)  Cervical plexus block and cervical paravertebral block (shoulder and upper neck) 39
  • Spinal Anaesthesia  Subarachnoid space  between L2-3 or L3-4  Site of action – nerve root in the cauda equina  Level of anaesthesia –  vol. & speed of injection;  Baricity of drug soln. with CSF  Posture of patient  Order of anaesthesia – sympathetic > motor 40
  • Spinal Anaesthesia  Uses – lower limbs, pelvis, lower abdomen, prostatectomy fracture setting and obstetric procedures  Spinal headache, hypotension, bradycardia and respiratory depression, cauda equina syndrome and nausea-vomiting  Drugs - Lidocaine, Tetracaine 41
  • Epidural Anaesthesia  Site- sacral hiatus (caudal) or lumber, thoracic or cervical region  Catheters are used for continuous infusion  Used like spinal and also painless childbirth.  Side effect similar to Spinal, Less Chances  Lidocaine, bupivacaine, Ropivacaine 42
  • Regional anaesthesia (IV)  Injection of LA in a vein of a tourniquet occluded limb  Mostly limited to upper limb  Orthopedic procedures 43
  • Summary -Caine … Na+ Channel Blockers Esters and Amides Local and Systemic Actions (Toxicity)  Techniques Available at www.slideshare.net 44