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Local Anesthetics

Local Anesthetics. A Case-Based Review. The Na+ Channel- Site of LA Action. www.septodont.ca/.../ english/other/cea_dh01.html. Summary. Block Na+ channels with loss of nerve conduction Therefore we call the result of LA action… Conduction Block. Case 1: Mom with Liver Disease.

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Local Anesthetics

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  1. Local Anesthetics A Case-Based Review

  2. The Na+ Channel- Site of LA Action www.septodont.ca/.../ english/other/cea_dh01.html

  3. Summary Block Na+ channels with loss of nerve conduction Therefore we call the result of LA action… Conduction Block

  4. Case 1: Mom with Liver Disease • Plan is epidural for labor • What are key concerns related to use of 0.125% bupivacaine? • Structure and metabolism

  5. Structure: Local Anesthetics • Structure of the LA molecule • Head • Tail • Intermediate chain

  6. Intermediate Chain (Hydrocarbon) Ester (-CO-) Amide (-NHC-) R N H+ R Lipophilic Head (Benzene Ring) Hydrophilic Tail (Quaternary Amide) Adopted from:Barash PG, Cullen BF, & Stoelting RK. (eds) Clinical Anesthesia 1997. J.B. Lippincott

  7. Metabolism

  8. LA Structures: Esters vs. Amides • Main issue: metabolic pathway • Esters • Ester hydrolysis by plasma cholinesterase • Amides • N-dealkylation and hydroxylation in the liver • This patient has pre-existing liver disease- we would worry about accumulation of the drug

  9. Entiomers • Substances of opposite shape • Molecules existing in mirror image forms • Left and right handed • When dissolved in solution rotate polarized light • Optical isomers • Important concept in LA toxicity • Right handed molecules such as bupivacaine are more toxic

  10. Case 2: • You are working in a dental practice • The next patient has an abscessed tooth • You suggest deep sedation, the dentist insists on trying to get a decent block with local • You end up with deep sedation due to patient discomfort • Why? Pus

  11. How LA’s Work ie - How they get in there and do their job

  12. Ionized/Non-ionized Pairs

  13. Variables Affecting Ionization Local Anesthetic Molecule Non- H+ H+ Intrinsic structure pKa Adjuncts pH of site pH of solution

  14. Local Anesthetics and Ionization: Both ionized and non-ionized forms are needed for function In infected tissue, the LA is completely ionized and won’t work- hence the need for sedation or GA

  15. Onset Speed: 1/Ionization Adopted from: Datta, S (1993) Pharmacology of Local Anesthetic Agents in Barash, PG (ed) ASA Refresher Courses in Anesthesiology, 21:241-254., Young and MacKenzie, 1994; Carpenter and Mackey, 1990; Strichartz and Covino, 1992, Stoelting Pharmacology and Physiology 1999. Hemmings 2000, pg. 295

  16. Case 3: Differential Block • Your obstetric patient is a difficult epidural placement. • On the third attempt you get the epidural and administer a test dose with no change in BP or HR observed. • You then give a bolus dose of 8 cc of 0.125% bupivacaine. The patient begins complaining of blurred vision and a metallic taste • Her LOC then becomes depressed

  17. Systemic Toxicity • CNS toxicity: due to peak serum local anesthetic blood levels • Accidental intravascular injection is the most common etiologic factor in severe systemic reactions • Toxicity can be gradual • Same variables: • Site, vascularity, total dose, use of a vasoconstrictor….

  18. Systemic Toxicity • Symptoms: Mild neurological symptoms to coma or death • Excitatory symptoms • Selective depression of inhibitory neurons • Depression of CNS • Na channel block centrally • Depression of heart • Takes a much higher blood level • Lipid soluble agents are more potent and more toxic

  19. Clinical Potency: Clinical Toxicity Adopted from: Mulroy, MF (1996) Pharmacology and Toxicity of Local Anesthetics in Barash, PG (ed) ASA Refresher Courses in Anesthesiology, 24:193-204.

  20. Treatment of Toxicity • Get assistance • Airway • Oxygen, ventilation, intubation or airway equipment • Drugs to stop seizure • Benzodiazepines • Barbiturates • Other drugs/interventions • Post seizure management

  21. Toxic Dose Ranges- Amides Doses are in mg/kg, dose ranges change depending on site of injection

  22. Toxic Dose Ranges- Esters Doses are in mg/kg, dose ranges change depending on site of injection

  23. Site of Injection: Likelihood of Toxicity IV Tracheal Intercostal Caudal Paracervical Epidural Brachial plexus Sciatic/Femoral Subcutaneous Most Most Schematic is illustrative in nature and not intended to reflect absolute correlation Toxicity Liklihood Vascularity Least Least

  24. Case 4: Will the Spinal Last? • Your patient presents for a BKA • You administer a Lidocaine 2% (80mg) SAB with epi • The surgeon runs into trouble and the case approaches 90 minutes

  25. Duration = Protein Binding = Lipid Solubility • Protein binding. •  protein binding =  duration • Lipid solubility •  lipid solubility=  duration

  26. Lipid Solubility: Duration Adopted from :Datta, S (1993) Pharmacology of Local Anesthetic Agents in Barash, PG (ed) ASA Refresher Courses in Anesthesiology, 21:241-254.

  27. Protein Binding: Duration Datta, S (1993) Pharmacology of Local Anesthetic Agents in Barash, PG (ed) ASA Refresher Courses in Anesthesiology, 21:241-254. Stoelting RK, Pharmacology and Physiology in Anesthetic Practice, 1999

  28. Adjuncts also Prolong Duration • Vasoconstrictors • Epinephrine/Phenylephrine/ Levonordefrin • Prolong duration • Minimize effect of LA vasodilatation •  toxicity •  intensity of block •  bleeding

  29. Morgan 2006

  30. Morgan 2006

  31. And Don’t Give Me Any of Those Local Anesthetics! Get Me the Imported Stuff!

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