Local Anesthetics: Mechanism and Nerve Anatomy
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Questions and Answers

How does increased lipophilicity of a local anesthetic typically affect its duration of action?

  • Decreases duration due to rapid systemic absorption.
  • Has no significant effect on duration of action.
  • Increases duration due to greater partitioning into nerve membranes and slower removal. (correct)
  • Increases duration due to enhanced binding to sodium channels.

Why do vasoconstrictors like epinephrine prolong the action of local anesthetics?

  • They stabilize the nerve membrane, making it less permeable to the local anesthetic.
  • They decrease the local blood flow, reducing the removal of the local anesthetic from the injection site. (correct)
  • They directly enhance the binding affinity of the local anesthetic to sodium channels.
  • They increase the systemic absorption of the local anesthetic.

During which stage of general anesthesia does loss of consciousness typically occur?

  • Stage II: Excitement or Delirium
  • Stage IV: Medullary Depression
  • Stage I: Analgesia (correct)
  • Stage III: Surgical Anesthesia

Which of the following properties of an intravenous anesthetic would make it more suitable for maintaining anesthesia rather than inducing it?

<p>A long context-sensitive half-time. (D)</p> Signup and view all the answers

What is the relationship between the minimum alveolar concentration (MAC) of an inhaled anesthetic and its potency?

<p>MAC is inversely proportional to potency; a lower MAC indicates higher potency. (B)</p> Signup and view all the answers

How does a high blood/gas partition coefficient of an inhalational anesthetic affect its onset of action?

<p>Results in a slower onset of action because more of the anesthetic is absorbed into the blood before reaching the brain. (D)</p> Signup and view all the answers

Which of the following best describes the mechanism of action of dantrolene in treating malignant hyperthermia?

<p>Inhibits the ryanodine receptor, reducing calcium release from the sarcoplasmic reticulum. (D)</p> Signup and view all the answers

Which of the following muscle relaxants primarily acts through a central mechanism rather than directly at the neuromuscular junction?

<p>Baclofen (C)</p> Signup and view all the answers

How does increased hydrophobicity, resulting from substitutions on the aromatic group of a local anesthetic, typically affect its properties?

<p>Increases potency and prolongs the duration of action. (D)</p> Signup and view all the answers

What is the primary mechanism by which local anesthetics exert their effect?

<p>Inhibiting voltage-gated sodium channels. (D)</p> Signup and view all the answers

What is the role of tertiary amine protonation in the function of local anesthetics?

<p>It is necessary for the drug to bind effectively to the sodium channel within the cell. (A)</p> Signup and view all the answers

How does the oil/gas partition coefficient of an inhaled anesthetic relate to its potency and induction time?

<p>A higher coefficient indicates higher potency and slower induction. (D)</p> Signup and view all the answers

What is the significance of 'context-sensitive half-time' in the pharmacokinetics of intravenous anesthetics?

<p>It describes the elimination half-life of the drug after prolonged infusion, which may be longer due to tissue accumulation. (D)</p> Signup and view all the answers

Which statement best describes the concept of 'use-dependent inhibition' in the context of local anesthetics?

<p>Local anesthetics are more effective when the nerve is frequently stimulated (phasic block). (C)</p> Signup and view all the answers

What does MAC (Minimum Alveolar Concentration) define in the context of inhaled anesthetics?

<p>The alveolar concentration required to prevent movement in 50% of subjects exposed to a noxious stimulus. (D)</p> Signup and view all the answers

How do ester-linked local anesthetics differ from amide-linked local anesthetics in terms of metabolism and potential for allergic reactions?

<p>Ester-linked anesthetics are metabolized more rapidly and have a higher potential for allergic reactions. (D)</p> Signup and view all the answers

Flashcards

Peripheral Nerve Anatomy: Epineurium

The outer layer of the peripheral nerve, providing a protective barrier.

Peripheral Nerve Anatomy: Fascicle

A bundle of nerve fibers within a peripheral nerve.

Peripheral Nerve Fiber Types

Nerve fibers include A-alpha, A-beta, A-delta, and C fibers, each with varying diameters and myelination, influencing conduction velocity and sensitivity.

Local Anesthetic Structure

Local anesthetics have an aromatic group, an ester or amide linkage, and a tertiary amine.

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Ester vs. Amide Linkage

Ester-linked local anesthetics are metabolized faster than amide-linked.

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Effect of Amine Protonation

Protonation of the tertiary amine increases water solubility and enhances binding.

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State-Dependent Inhibition

Local anesthetics bind preferentially to the open and inactivated states of sodium channels, blocking sodium influx and nerve signal propagation.

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Minimum Alveolar Concentration (MAC)

MAC is the concentration that prevents movement in 50% of subjects exposed to a noxious stimulus.

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Lipophilicity/Ionization of Anesthetics

Increase lipophilicity enhances drug penetration across nerve membranes, speeding up onset and increasing potency. Ionization affects the drug's ability to cross membranes; the un-ionized form penetrates more easily.

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Frequency- and Voltage-Dependence

Frequency-dependence: local anesthetics block more rapidly at higher nerve firing frequencies. Voltage-dependence: block is greater at more depolarized membrane potentials.

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Differential Sensitivity

Smaller, myelinated fibers (like pain fibers) are more sensitive because they have less distance between nodes, increasing effectiveness of the block .

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Effect of pH

In acidic environments (e.g., infected tissue), local anesthetics become more ionized, reducing their ability to penetrate nerve membranes and decreasing effectiveness.

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Vasoconstrictors

Vasoconstrictors (e.g., epinephrine) reduce blood flow, decreasing systemic absorption of the anesthetic, thus prolonging its local action and minimizing systemic toxicity.

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Blood/Gas Partition Coefficient

Blood/gas partition coefficient determines how quickly an inhalational anesthetic enters and exits the bloodstream; a lower coefficient results in faster onset and recovery.

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Malignant Hyperthermia

A rare, life-threatening reaction to certain general anesthetics (e.g., halothane, succinylcholine) causing hyperthermia, muscle rigidity, and metabolic acidosis. Treated with Dantrolene.

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Study Notes

  • The session reviews peripheral nerve anatomy/function, neuronal excitability, voltage-gated channels in action potential generation/propagation, and pharmacology of voltage-gated sodium channel blockers for local anesthesia.

Peripheral Nerve

  • General anatomy is outlined in Figure 12-6.
  • Types of peripheral nerve fibers are in Table 12-1.

Local Anesthetics: Chemistry and Structure-Activity Relationship

  • Figure 12-4 describes the chemistry and structure-activity relationship.
  • Local anesthetics can be ester-linked or amide-linked.
  • Tertiary amine protonation impacts activity.
  • Substitutions on the aromatic group influence hydrophobicity.

Local Anesthetics: Mechanism of Action

  • Figures 12-7 describes the mechanism of action.
  • State-dependent inhibition occurs at voltage-gated sodium channels.
  • Use-dependent inhibition can be tonic or phasic.

General Anesthetics

  • Parenteral (intravenous) anesthetics differ from inhaled (gases/volatile) anesthetics.
  • Stages of anesthesia are outlined in Golan Figure 17-1.
  • Potency of inhaled anesthetics are outlined in Golan Figure 17-2.
  • MAC is defined and related to potency.
  • The oil/gas partition coefficient affects potency and induction time.
  • Molecular actions of general anesthetics are outlined in Golan Figure 17-15.
  • Pharmacokinetics of intravenous anesthetics include:
    • Tissue distribution (Katzung Figure 25-7)
    • Context-sensitive half-time (Katzung Figure 25-8)

Post-Class Learning Objectives

  • Describe local anesthetics' mechanism of action and effects on voltage-gated ion channel kinetics.
  • Describe basic mechanisms and clinical implications of Tonic vs. Phasic inhibition of nerve signals.
  • Know the differential sensitivity of nerve fibers to local anesthetics.
  • Know local anesthetics' general chemical structure and its relation to onset, duration of action, potency, and toxicity.
  • Describe how lipophilicity/ionization of local anesthetics influence its pharmacokinetic and pharmacodynamic properties including:
    • Frequency- and voltage-dependence of local anesthetic action
    • Differential sensitivity of nerve fibers to local anesthetics
    • Effect of pH
    • Prolongation of action by vasoconstrictors
  • Know the process of general anesthesia including stages, depth, and adjunctive drug commonly used.
  • Differentiate between intravenous and inhalational anesthetics and their proposed mechanisms of action (unitary hypothesis, lipid-solubility hypothesis, and inhibition/stimulation of ion-channel).
  • Describe how pharmacokinetics properties (e.g. context-sensitive half-time) of an intravenous anesthetic affect it is use for induction vs. maintenance of anesthesia.
  • Define minimum alveolar concentration (MAC) of inhalational anesthetics and to describe the relationship between MAC and potency.
  • Describe how pharmacokinetics properties (e.g. blood/gas partition coefficient) of inhalational anesthetics influence onset and duration of anesthesia.
  • List and discuss serious side effects (e.g. malignant hyperthermia) associated with the use of general anesthetics.
  • Recognize the role of dantrolene in the treatment of malignant hyperthermia.
  • Know the mechanisms of action of the drugs and how drug binding at these targets is related to their desired and/or side effects

Clinically Important Drugs:

  • Local Anesthetics include:
    • Lidocaine
    • Tetracaine
    • Bupivacaine
  • Muscle relaxants include:
    • Benzodiazepines
    • Baclofen
    • Tizanidine
    • Dantrolene
    • Gabapentin
    • Methocarbamol
    • Cyclobenzaprine
    • Carisoprodol
  • Intravenous General Anesthetics include:
    • Barbiturates
    • Etomidate
    • Propofol
  • Inhaled General Anesthetics include:
    • Nitrous oxide
    • Halothane
    • Isoflurane
    • Desflurane
    • Sevoflurane

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Description

Review of peripheral nerve anatomy/function, neuronal excitability, and the pharmacology of voltage-gated sodium channel blockers for local anesthesia. Topics include: nerve fibers, chemistry, and mechanism of action.

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