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La
1. LOCAL ANAESTHETICS BY :DR SUHAIMI TAJUDIN MODERATOR : PROF MADYA DR SHAMSUL HUSM ANESTESIOLOGI DEPARTMENT
2. OUTLINE Introduction Ideal properties Comercial preparation Structure activity relationship Mechanism of action Pharmacokinetic Side effects Individual LA
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5. 3. Commercial Preparations Poorly soluble in water – marketed most often as water-soluble hydrochloride salt These HCl salts are acidic(pH 6) – contributing to the stability of LA An acidic pH also important if epinephrine is present in LA solution, becoz this cathecolamine is unstable at alkaline pH sodium metabisulphite (strongly acidic), may be added to LA-ephephrine solutions to prevent oxidative decomposition of epinephrine
6. COMERCIAL PREPARATION Alkalinization of LA solution By adding sodium bicarbonate Will shortens the onset (more nonionized form) Enhance dept of sensory and motor blockade (increase potency) Increase spread of epidural blockade
15. Connecting hydrocarbon chains .ester( -CO.O-) .amide(-NH.CO) the nature of this bond is the basis of classification of LA, relate to site of metabolism and potential to produce allergic reaction
33. About 70mV with the inside membrane being negative compare to outside
34. The Na-K-ATPase is electrogenic , pumps 3Na out of cell in exchange for 2K pumped intracellularly. This pump sets up concentration different of Na & K across the cell membrane
36. The membrane is permeable to Na and K, so these ion tend to leak across membrane down their concentration gradient
37. membrane is 100x more permeable to K, > K lost from cell than Na enters the cellNet result is larger amt of +ve charge left the cell than has entered it so inside of membrane left with net negative charge result in RMP are being negative compare to outside
40. Electrical or chemical trigger initially cause slow rise in membrane potential until threshold potential (50mV) is reached
41. Voltage sensitive Na channels then open, increasing Na pemeability dramatically and membrane potential briefly reaches +30mV, at which Na channels close
44. Mechanism of action LA selectively binds to Na channel in inactivated-closed state. It stabilizes it in this configuration and prevent their change to rested-closed and activated-open states in response to nerve impulse Na channel impermeable to Na Slows the rate of depolarisation, threshold potential not reached & action potential not propagated
45. Frequency- dependent blockade Defines a situation where the more frequent the channel are activated, the greater the degree of block produced After AP, Na channel develop a low affinity state where some drugs dissociate/ unbind and Na channel recover If another AP arrived before all LA dissociates it regain access into the Na channel at open activated state -> additional increment of block ↑ Frequency of AP - ↑ degree of blockade
46. Membrane volume expansion theory Lipophilic LA incorporated into lipid bilayer causing a volume expansion & distortion to the conformation of axonal membrane and hence the Na channel resulting in its inactivation Mode of action of Benzocaine, and other LA when given in high dosage
48. PHYSICOHEMICAL PROPERTIES The chemical structure and physicochemical characteristics of LA affect their clinical properties Modification of the chemical structure (lengthening of the hydrocarbon chain within critical length or increasing the number of carbon atoms in the aromatic ring or tertiary amine ) may alter lipid solubility, potency, rate of metabolism & duration of action In particular, these are modified by Lipid solubility Protein binding Dissociation constant (pKa value)
49. a) Lipid solubility Lipid solubility of different anaesthetics governs their ability to penetrate perineuronal tissues and neural membrane, and reaches their site of action in neuroplasm More lipid soluble – penetrates membrane more easily, less molecule requires for nerve conduction blockade i.e. more potent E.g. Bupivacaine, levobupivacaine and ropivacaine are app 3-4x as potent as lidocaine or prilocaine, dt differences in their lipid-solubility
52. Plasma protein binding acts as depotE.g Procaine is not extensively bound to tissue protein, has short duration of action Bupivacaine, levobupivacaine & ropivacaine are extensively bound to plasma and tissue protein --- prolonged effect
54. c) Dissociation constant (pKa value) pKa is equal to pH at which the concentration of ionized base and non-ionized base are equal Is the most important factor affecting rapidity of onset of axn pKa value governs the proportions of LA that is present in non-ionized form at physiological pH values and therefore available to diffuse across tissue barrier to its site of axn LA with a pKa near physiological pH will have a greater degree of unionized molecules -> More LA diffused across membrane -> rapid onset of action
56. ABSORPTION Absorption of LA from its site of injection into systemic circulation is influenced by; Site of injection Dosage Addition of vasoconstrictor Physicochemical properties of LA Vasoactive properties of the LA Pathophysiological process – acidity of tissue reduces absorption ( e.g. abscess, metabolic acidosis)
61. c. Addition of vasoconstrictor By addition of adrenaline 5g/ml (1:200000) Higher Dosage offers no additional benefits but increases symphatomimetic activites limit systemic absorption and maintain the drug concentration in nerve fibre and prolong the time the drug in contact with nerve fibre Ropivacaine & Cocaine has intrinsic vasoconstrictor activities Lignocaine, mepivacaine, bupivacaine, etidocaine exhibit vasodilator effects
62. Vasoactive properties of LA Influence potency and duration of action All LA has vasodilator effect except ropivacaine and coccaine More vasoactive like lidocaine more greater systemic absorption result in shorter duration of action Pathophysiological process Acidosis environment Will increase ionized fraction of the drug Result in poor quality of LA Physiocochemical properties Lipid solubility Protein binding Dissociation constant
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66. Lung extraction The lung capable of extracting local anesthetic such as lidocaine and bupivacaine from circulation Limit the concentration of drug that reaches systemic circulation to be distributed to coronary & cerebral circulation Placental transfer Highly plasma protein binding LA limits diffusion across placenta Esters undergo rapid hydrolysis hence not available for transfer across placenta Acidosis in fetus, which may occur during prolonged labour, can result in accumulation of LA molecules in the fetus (ion trapping)
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68. Rate of hydrolysis varies, and resulting metabolites are pharmacologically inactive
70. CSF – lacks estrase enzyme; so the termination of action of intrathecal injection of LA depends on absorption into bld. Stream
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73. EXCRETION Poor water solubility of LA limit renal excretion of unchanged drugs to < 5% of injected dose (except cocaine 10-12%) Water soluble para-aminobenzoic acid readily excreted
107. Following repeated doses of 5% lidocaine & 0.5% tetracaine used in continuous spinal anesthesia
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109. CVS Overdose of LA may cause profound hypotension, bradycardia, bradyarhythmias and even cardiac arrest, and usually follows sign of CNS toxicity E.g. – high systemic conc of bupivacaine are particularly ass with significant toxicity Produce prolonged blockade of Na channel also affects myocardial Ca and K channels, and is preferentially bound by cardiac muscle Myocardial contractility and conduction in junctional tissue is depressed, with widening of QRS complex and distortion of St segment
110. Predispose to re-entrant phenomena and ventricular arrthyhmias, which are potentiated by hypoxia, acidosis and hyperK Arrthyhmias and bradycardia may respond to iv atropine (1.2-1.8mg) and colloid/crystalloid infusions may be required to expand pl vol Current evidence suggest that use of LA enantiomers with (S)-configuration reduce risk of cardiac depression and cardiotoxicity, and ropivacaine (an S-isomer) and levoupivacaine (S-bupivacaine) may have significant advantages compare to racemic bupivacaine
111. Hematology --Methemoglobinemia Rare but potentially life treatening complication (decreased O2-carrying capacity) Cause by oxidation of Hb to methemoglobin more rapidly than methemoglobin is reduced by Hb Prilocaine ( > 600mg @ >10mg/kg) Amide LA that is metabolized to othotoluidine Othotoluidine is oxidizing compound, capable of converting Hb to methemoglobin ---- methemoglobinaemia Cause pt to appear cyanotic Also caused by Benzocaine ( topical application > 200-300mg) Is readily reversed by administration of iv methylene blue, 1-2mg/kg over 5 min (total dose 7-8mg/kg)
112. Respiratory Lidocaine at high plasma concentration depress ventilatory response to arterial hypoxaemia So patient with CO2 retention which resting ventilation depend on hypoxic drive may be at risk of ventilatory failure when lidocaine is administered for treatment of cardiac dysrythmia
113. Hepatotoxicity Continous or intermittent epidural administration of bupivacaine to treat postherpetic nuralgia has been associated with increase plasma concentration of liver transaminase enzyme that normalized when bupivacaine infusion was discontinued It could be due to a direct toxic injury or an allergic reaction Dysphoria Vivid fear of imminent death and a delusional belief of having died
133. Recommended dose –3.5mg/kg, 250mg (150mg for C-section under epidural), not currently intended for intrathecal admin and in children < 12 years
134. Clinical – sensory blockade similar in time course to that produced by bupivacaine; motor blockade is slower in onset & shorter in duration than after an equivalent dose of bupivacaine; less cardiotoxic than bupivacaine; Intrinsic vasoconstrictor, mild CNS Sx
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137. Excretion – 86% (mostly conjugated) excreted in the urine, 1% unchanged
146. Recommended max dose 3mg/kg (7mg/kg with adrenaline), toxic plasma level >10ug/ml
147. Acute ventricular dysrhythmia ( class 1)– 1mg/kg over 2 min followed by infusion 4mg/kg/min (30min), 2mg/kg/min ( 2 hr) & subsequently 1mg/kg/min.
170. Clinical – inhibit uptake of adrenaline and noradr by central and peripheral symphatetic nerve endings, and enhances effect of sympt nerve stimulation
171. CNS – increased neuronal activity in symphatetic pathways in hypothalamus and medulla
172. It may produce mental stimulation, euphoria, hallucinations, vasoC, pupillary dilatation, hypertension, tachycardia and arrhythmias
186. May produce blanching (addition of nitroglycerine ointment may promote venodilation) and increase in metHb (esp < 3mths- immature reductase pathway)
189. Uses described include analgesia for venopuncture, venous and arterial cannulation, lumbar puncture, epidural injection, superficial skin surgery and relief of tourniquet pain during IVRA
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191. mcq 1. Lidocaine can cause: a) sedationb) convulsionsc) slowed A-V conductiond) prolongation of the cardiac action potentiale) shortening of the refractory phase f) Is more potent than ropivacaine
192. mcq 1. Lidocaine can cause: a) sedation Tb) convulsions Tc) slowed A-V conduction Td) prolongation of the cardiac action potential Fe) shortening of the refractory phase T f) Is more potent than ropivacaine F
193. 2. Regarding local anaesthetic agents (LA):a) the potency of LAs is proportional to their lipid solubilityb) the duration of action is dependent on protein bindingc) agents with low pKa have a faster onset of actiond) all local anaesthetics are vasodilatorse) the depth of local anaesthetic block is increased by increasing the dose
194. 2. Regarding local anaesthetic agents (LA):a) the potency of LAs is proportional to their lipid solubility Tb) the duration of action is dependent on protein binding Tc) agents with low pKa have a faster onset of action Td) all local anaesthetics are vasodilators Fe) the depth of local anaesthetic block is increased by increasing the dose T
195. 3. Concerning local anaesthetics: a) they are absorbed more rapidly after intercostal block than after caudal administration b) in the foetus they are able to cross the placenta as readily as from the mother c) they are weak acids d) those which are esters are rapidly metabolised by liver enzymes e) pKa is the pH at which more than half of a local anaesthetic exists in non-ionised form
196. 3. Concerning local anaesthetics: a) they are absorbed more rapidly after intercostal block than after caudal administration Tb) in the foetus they are able to cross the placenta as readily as from the mother Fc) they are weak acids Fd) those which are esters are rapidly metabolised by liver enzymes Fe) pKa is the pH at which more than half of a local anaesthetic exists in non-ionised form F
197. 4.Cocaine: A. Blocks reuptake of dopamine and noradrenaline B. Central effects are due to noradrenaline C. Crosses lipid soluble membranes because its pKa is 2.8 D. Is not metabolised by plasma pseudocholinesterase E. Rapidly absorbed by nasal mucosa
198. Cocaine: A. Blocks reuptake of dopamine and noradrenaline T B. Central effects are due to noradrenaline F C. Crosses lipid soluble membranes because its pKa is 2.8 F D. Is not metabolised by plasma pseudocholinesterase F E. Rapidly absorbed by nasal mucosa ?
199. 5.Ropivacaine A. Is a pure R isomer B. Is an isomer of bupivacaine C. Provides more motor block than bupivacaine D. Has more toxicity than bupivacaine E. Has similar physico-chemical properties to bupivacaine
200. 5.Ropivacaine A. Is a pure R isomer F B. Is an isomer of bupivacaine F C. Provides more motor block than bupivacaine F D. Has more toxicity than bupivacaine F E. Has similar physico-chemical properties to bupivacaine T
