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Local Anesthetics

Local Anesthetics. Joseph Haake, MS4 February 21, 2007. Esters Procaine Cocaine Tetracaine Chloroprocaine Queen Others Diphenhydramine Benzyl alcohol. Amides ( i in prefix) Lidocaine Bupivacaine Mepivacaine Etidocaine Prilocaine Ropivacaine. Types of Local Anesthetics.

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Local Anesthetics

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  1. Local Anesthetics Joseph Haake, MS4 February 21, 2007

  2. Esters Procaine Cocaine Tetracaine Chloroprocaine Queen Others Diphenhydramine Benzyl alcohol Amides (i in prefix) Lidocaine Bupivacaine Mepivacaine Etidocaine Prilocaine Ropivacaine Types of Local Anesthetics

  3. Chemical Structure • Most esters and amides are synthetic derivatives of cocaine • Consist of: • Aromatic head (lipophilic) • Terminal amine tail (hydrophobic) • Hydrocarbon chain attached to the aromatic acid (via amide or ester bond) • Metabolism • Esters: hydrolysis in plasma • Amides: biotransformation via liver

  4. Mechanism of Action • Block transmission of action potential by binding to voltage-gated sodium channels • Anesthesia occurs when enough drug molecules occupy sodium channels to interrupt and temporarily stop conduction • Activity is based on pKa, lipid solubility, and protein binding

  5. Clinical Characteristics • Onset • pKa: lower pKa results in faster travel thru lipid layers → faster onset • Potency • Lipid solubility: higher lipid solubility results in increased concentrations inside nerve → higher potency • Duration • Protein binding (affinity LA has for Na channel): higher affinity → longer duration

  6. Clinical Characteristics of Common Local Anesthetics

  7. Friends of Locals • Epinephrine • Added to provide longer duration of anesthesia, promote hemostasis, & slow systemic absorption • May increase pain of injection by lowering pH • Avoid in “end-arterial fields” (digits, nose, ears, penis); if trouble arises, apply nitro paste or inject intravascular phentolamine • Sodium Bicarbonate • Mix with lidocaine (9 mL lido 1% to 1 mL bicarb 8.4%) • Increases pH, thus faster diffusion into nerve & faster onset of action

  8. Differential Nerve Blockade • Large, myelinated nerve fibers more sensitive to blockade than smaller, unmyelinated fibers • Anesthetics diffuse from the outer surface toward the center after deposition near a peripheral nerve • Proximal anesthesia occurs before distal (e.g. axillary block produces anesthesia of shoulder before hand) • Skeletal muscle paralysis may precede sensory blockade

  9. Toxicity • Systemic toxicity often results from high plasma concentrations • Related to potency (lipid solubility) & duration of action (protein binding) • Often from accidental intravascular injection • Less likely from local absorption • Dependent on vascularity (high vascularity = high absorption) • Vascularity of common sites: intercostal > epidural/caudal > brachial plexus > mucosal > distal peripheral nerve > subcutaneous

  10. Systemic Toxic Effects • CNS • Visual change, numb tongue, lightheadedness, restlessness • Perioral paresthesia, muscle twitch, slurred speech, excitability, drowsiness • Seizures, cardiorespiratory depression, coma • Cardiovascular • Palpitations, vasodilation, HTN, ventricular arrhythmias, myocardial depression, hypotension, bradycardia, cv collapse • Respiratory • Hypoventilation, resp arrest • Allergy • More common in esters

  11. Questions?

  12. Sources • Tintinalli JE, Kelen GD and Stapczynski JS. Emergency Medicine: A Comprehensive Study Guide. American College of Emergency Physicians. 6th ed. 2004. • Stoelting RK and Miller RD. Basics of Anesthesia. Churchill Livingstone / Elsevier. 5th ed. 2007.

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