Local Anesthesia and Nerve Fiber Classification
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Questions and Answers

What are some common systemic toxic effects of local anesthetics?

Systemic toxic effects may include lightheadedness, tinnitus, seizures, and hypotension.

What is the primary cause of tissue toxicity in local anesthetics?

Tissue toxicity occurs if local anesthetics are administered at high concentrations, often due to preservatives in the solution.

What characterizes true allergies to local anesthetics?

True allergies to local anesthetics are very rare, with most reported reactions stemming from the ester class or intravascular injections.

Define regional anesthesia.

<p>Regional anesthesia is the process of rendering a specific area of the body insensate to surgical stimuli.</p> Signup and view all the answers

List one use of regional anesthesia in medical procedures.

<p>Regional anesthesia is used to provide anesthesia for surgical procedures.</p> Signup and view all the answers

What is an intravenous block, commonly referred to as?

<p>An intravenous block is commonly referred to as a 'Bier' block.</p> Signup and view all the answers

Name two types of peripheral nerve blocks used in regional anesthesia.

<p>Two types of peripheral nerve blocks include brachial plexus block and lumbar plexus block.</p> Signup and view all the answers

What rare effect can occur due to systemic absorption of local anesthetics?

<p>Systemic absorption can lead to CNS or cardiovascular collapse.</p> Signup and view all the answers

What might cause a patient to mistakenly report an allergy to local anesthetics?

<p>Patients might confuse reactions from ester hydrolysis or intravascular injections as an allergy.</p> Signup and view all the answers

What are some symptoms of local anesthetic toxicity?

<p>Symptoms may include shivering, twitching, seizures, and numbness.</p> Signup and view all the answers

What is the primary function of local anesthetics?

<p>They produce loss of sensation to pain in a specific area of the body without causing loss of consciousness.</p> Signup and view all the answers

How do local anesthetics block pain signal transmission?

<p>They block sodium ion channels in peripheral neurons, preventing neuronal depolarization and action potential formation.</p> Signup and view all the answers

What role do hydrogen ions play in the function of local anesthetics?

<p>Local anesthetics must lose their hydrogen ions to diffuse across the cellular membrane into the neuron.</p> Signup and view all the answers

Which nerve fibers are generally more sensitive to local anesthetics?

<p>Smaller diameter nerve fibers, such as Aδ and unmyelinated C fibers, are typically more sensitive than larger, myelinated Aα fibers.</p> Signup and view all the answers

In terms of nerve fiber classification, which types are less sensitive to local anesthetics?

<p>Larger, myelinated fibers (Aα) are less sensitive compared to smaller unmyelinated fibers (C).</p> Signup and view all the answers

What sequence of sensitivity to local anesthetic inhibition is seen in spinal nerves?

<p>The sensitivity sequence is autonomic &gt; sensory &gt; motor.</p> Signup and view all the answers

How are ester local anesthetics metabolized?

<p>They are mostly metabolized by pseudocholinesterase, also known as plasma cholinesterase.</p> Signup and view all the answers

What happens to local anesthetics as they diffuse throughout the body?

<p>Their effect wears off gradually as they disperse and lose concentration.</p> Signup and view all the answers

Why are small diameter nerve fibers more sensitive to local anesthetics?

<p>Smaller diameter fibers have a higher susceptibility to the inhibitory effects of local anesthetics compared to larger ones.</p> Signup and view all the answers

What dictates the effectiveness of local anesthetics on different nerve fibers?

<p>Effectiveness is dictated by factors including axonal diameter, myelination, and individual anatomical variations.</p> Signup and view all the answers

What role does pseudocholinesterase play in the metabolism of acetylcholine?

<p>Pseudocholinesterase hydrolyzes acetylcholine more slowly than acetylcholinesterase, regulating its activity in the bloodstream.</p> Signup and view all the answers

How does hepatic function affect the metabolism of amide local anesthetics?

<p>Decreases in hepatic function reduce the metabolic rate of amide local anesthetics, potentially leading to increased blood concentrations and systemic toxicity.</p> Signup and view all the answers

What is the significance of pKa in determining the onset of action for local anesthetics?

<p>The pKa indicates the pH at which the fractions of ionized and nonionized forms are equal, affecting the lipid solubility and onset of action of local anesthetics.</p> Signup and view all the answers

What factors can affect the potency and duration of action of local anesthetics?

<p>Potency correlates with lipid solubility, while duration of action is influenced by both potency and lipid solubility.</p> Signup and view all the answers

How do changes in pH affect the effectiveness of local anesthetics?

<p>Acidic pH antagonizes the block of nerve impulse conduction, making local anesthetics less effective.</p> Signup and view all the answers

What is the minimum concentration of local anesthetic necessary to block nerve impulse conduction influenced by?

<p>It is influenced by fiber size, type, myelination, pH, frequency of stimulation, and electrolyte concentrations.</p> Signup and view all the answers

What is the maximum dose range for local anesthetics in patients?

<p>The maximum dose typically ranges between 70mg to 500mg, depending on individual factors.</p> Signup and view all the answers

Why do less potent local anesthetics have a faster onset compared to more potent ones?

<p>Less potent, less lipid-soluble agents generally have a faster onset because they are more readily available in their nonionized form.</p> Signup and view all the answers

What effect does lipid solubility have on the toxicity of local anesthetics?

<p>Higher lipid solubility can prolong the duration of action, potentially leading to higher circulating levels and increased toxicity.</p> Signup and view all the answers

What is hydrolysis in relation to ester local anesthetics, and how is it executed?

<p>Ester local anesthetics undergo rapid hydrolysis in the presence of water, leading to the formation of metabolites that are excreted in urine.</p> Signup and view all the answers

What can be a rare systemic effect resulting from local anesthetic overdose?

<p>CNS/cardiovascular collapse.</p> Signup and view all the answers

What is one major use of regional anesthesia during surgical procedures?

<p>To provide anesthesia for a specific area of the body.</p> Signup and view all the answers

How does local anesthetic toxicity manifest in a patient?

<p>Symptoms can include lightheadedness, shivering, and seizures.</p> Signup and view all the answers

What factors can lead to a patient thinking they have an allergy to local anesthetics?

<p>Previous intravascular injections causing adverse reactions.</p> Signup and view all the answers

What is the role of preservatives in local anesthetic solutions?

<p>They can contribute to tissue toxicity if used at high concentrations.</p> Signup and view all the answers

What are two types of peripheral nerve blocks in regional anesthesia?

<p>Brachial plexus block and lumbar plexus block.</p> Signup and view all the answers

What do local anesthetics primarily produce in a specific area of the body?

<p>A loss of sensation to pain without loss of consciousness.</p> Signup and view all the answers

Can true allergic reactions to local anesthetics occur?

<p>No, true allergy is very rare.</p> Signup and view all the answers

What is the significance of the term 'Bier block' in regional anesthesia?

<p>It refers to an intravenous block technique.</p> Signup and view all the answers

Which ion channels do local anesthetics block to inhibit pain signal transmission?

<p>Sodium ion channels.</p> Signup and view all the answers

What physiological condition can cause hypotension during local anesthetic administration?

<p>Systemic toxicity from local anesthetic overdose.</p> Signup and view all the answers

How does the sensitivity of nerve fibers to local anesthetics vary among different types of fibers?

<p>Smaller diameter and more myelinated fibers are generally more sensitive to local anesthetics.</p> Signup and view all the answers

Describe one chronic pain application of regional anesthesia.

<p>It can be used for diagnosis or therapy in chronic pain syndromes.</p> Signup and view all the answers

What is the general sequence of sensitivity to local anesthetic inhibition in spinal nerves?

<p>Autonomic &gt; sensory &gt; motor.</p> Signup and view all the answers

In what form do local anesthetics need to be in order to block sodium channels effectively?

<p>In their protonated ionized form.</p> Signup and view all the answers

Why do larger Aα fibers exhibit less sensitivity to local anesthetics compared to smaller Aδ fibers?

<p>Their larger diameter and faster conduction speed make them less sensitive.</p> Signup and view all the answers

What characteristic of local anesthetics helps them to traverse cellular membranes?

<p>Their ability to lose hydrogen ions.</p> Signup and view all the answers

How are ester local anesthetics primarily metabolized in the body?

<p>By pseudocholinesterase.</p> Signup and view all the answers

What happens to local anesthetics as they diffuse throughout the body?

<p>Their effect wears off gradually.</p> Signup and view all the answers

What anatomical factors influence the sensitivity of nerve fibers to local anesthetics?

<p>Axonal diameter and myelination.</p> Signup and view all the answers

How does liver function impact the metabolism of amide local anesthetics?

<p>Reduced hepatic function decreases the metabolic rate of amide local anesthetics, potentially raising blood concentrations and the risk of systemic toxicity.</p> Signup and view all the answers

What is the relationship between lipid solubility and the potency of local anesthetics?

<p>Increased lipid solubility generally correlates with higher potency of local anesthetics because it enhances their ability to permeate lipid membranes.</p> Signup and view all the answers

Explain why highly lipid-soluble local anesthetics tend to have a longer duration of action.

<p>Highly lipid-soluble local anesthetics diffuse more slowly from lipid-rich environments into the aqueous bloodstream, extending their duration of action.</p> Signup and view all the answers

What effect does pH have on the effectiveness of local anesthetics?

<p>An acidic pH can antagonize the blockade of local anesthetics, as it affects the ionization of the drug.</p> Signup and view all the answers

What determines the fraction of ionized and nonionized drug in solution for local anesthetics?

<p>The pKa of the local anesthetic indicates the pH at which the fraction of ionized and nonionized forms is equal.</p> Signup and view all the answers

List two factors that can influence the minimum concentration of local anesthetic required to block nerve impulse conduction.

<p>Fiber size and type, as well as myelination, are key factors influencing the minimum concentration needed for nerve blockade.</p> Signup and view all the answers

How do changes in electrolyte concentrations affect the action of local anesthetics?

<p>Electrolyte concentrations, such as hypokalemia and hypercalcemia, can antagonize the blockade produced by local anesthetics.</p> Signup and view all the answers

Describe the process of N-dealkylation in the metabolism of amide local anesthetics.

<p>N-dealkylation is a metabolic process performed by microsomal P-450 enzymes in the liver that modifies amide local anesthetics for elimination.</p> Signup and view all the answers

What is the importance of understanding patient variables like age and health when administering local anesthetics?

<p>Age, weight, and overall health can influence the maximum safe dose and response to local anesthetics, impacting efficacy and safety.</p> Signup and view all the answers

Why might the onset of less potent local anesthetics be faster than that of more potent agents?

<p>Less potent, less lipid-soluble local anesthetics generally have a faster onset because they require a lower concentration of the nonionized form.</p> Signup and view all the answers

Common toxic effects of local anesthetics include light headedness, shivering, and ______.

<p>seizures</p> Signup and view all the answers

True allergy to local anesthetics is very ______.

<p>rare</p> Signup and view all the answers

Regional anesthesia is used to render specific areas of the body ______ to surgical stimuli.

<p>insensate</p> Signup and view all the answers

A common method used to provide local anesthesia is ___ nerve block.

<p>peripheral</p> Signup and view all the answers

Hypotension, or low ______, can occur as a toxic effect of local anesthetics.

<p>blood pressure</p> Signup and view all the answers

One of the local anesthetic toxicity symptoms is ______.

<p>tinnitus</p> Signup and view all the answers

In regional anesthesia, a ______ block is a technique where anesthesia is administered intravenously.

<p>Bier</p> Signup and view all the answers

Local anesthetics can lead to ______ toxicity, which is related to the blood level of the drug.

<p>systemic</p> Signup and view all the answers

Most reactions attributed to allergy in local anesthetic use are due to ______ injections.

<p>intravascular</p> Signup and view all the answers

Local anesthetics can cause ______ of the body if administered in high enough concentrations.

<p>tissue toxicity</p> Signup and view all the answers

Local anesthetics produce loss of sensation to pain in a specific area of the body without the loss of __________.

<p>consciousness</p> Signup and view all the answers

Local anesthetics block pain signal transmission by blocking __________ ion channels in the peripheral neurons.

<p>sodium</p> Signup and view all the answers

The effect of local anesthetics lasts as long as the molecules remain in sufficient __________.

<p>concentration</p> Signup and view all the answers

Sensitivity to local anesthetics is determined by axonal diameter, myelination, and other anatomical and __________ factors.

<p>physiological</p> Signup and view all the answers

In spinal nerves, local anesthetic inhibition generally follows the sequence: __________ > sensory > motor.

<p>autonomic</p> Signup and view all the answers

Larger, faster Aα fibers are less sensitive to local anesthetics than smaller, slower-conducting __________ fibers.

<p>Aδ</p> Signup and view all the answers

Esters are a class of local anesthetics that are primarily metabolized by __________.

<p>pseudocholinesterase</p> Signup and view all the answers

Small unmyelinated C fibers are relatively __________ to inhibition by local anesthetics compared to larger myelinated fibers.

<p>resistant</p> Signup and view all the answers

Local anesthetics often need to lose their __________ ions in order to diffuse through cellular membranes.

<p>hydrogen</p> Signup and view all the answers

Once inside the neuron, local anesthetics regain their ionized __________ form and block sodium ion entrance.

<p>active</p> Signup and view all the answers

Ester hydrolysis is very rapid, and the water-soluble metabolites are excreted in the ______.

<p>urine</p> Signup and view all the answers

Pseudocholinesterase is an enzyme that hydrolyzes ______ more slowly than acetylcholinesterase.

<p>acetylcholine</p> Signup and view all the answers

Amide local anesthetics are metabolized by microsomal ______ enzymes in the liver.

<p>P-450</p> Signup and view all the answers

Decreases in hepatic function or liver blood flow will reduce the metabolic rate and potentially predispose patients to having greater blood ______ concentrations.

<p>anesthetic</p> Signup and view all the answers

The minimum concentration of local anesthetic that will block nerve impulse conduction is affected by fiber size, type, and ______.

<p>myelination</p> Signup and view all the answers

The onset of action of local anesthetics depends on factors including lipid solubility and the relative concentration of the nonionized and ______ forms.

<p>ionized</p> Signup and view all the answers

Less potent, less lipid-soluble agents generally have a ______ onset than more potent, more lipid-soluble agents.

<p>faster</p> Signup and view all the answers

Duration of action correlates with potency and lipid ______.

<p>solubility</p> Signup and view all the answers

Maximum dose for an individual is usually between ______ mg to 500 mg.

<p>70</p> Signup and view all the answers

Toxicity is defined as the peak circulation levels of local ______.

<p>anesthetics</p> Signup and view all the answers

Local anesthetics produce loss of sensation to pain in a specific area of the body without the loss of ______.

<p>consciousness</p> Signup and view all the answers

Local anesthesia blocks pain signal transmission by blocking _____ ion channels in the peripheral neurons.

<p>sodium</p> Signup and view all the answers

Once inside the neuron, local anesthetics regain their ionized active form and become _____ inside.

<p>trapped</p> Signup and view all the answers

In spinal nerves, local anesthetic inhibition generally follows the sequence: autonomic > sensory > _____.

<p>motor</p> Signup and view all the answers

Small unmyelinated C fibers are relatively _____ to inhibition by local anesthetics compared to larger myelinated fibers.

<p>resistant</p> Signup and view all the answers

Ester local anesthetics are mostly metabolized by _____ in the plasma.

<p>pseudocholinesterase</p> Signup and view all the answers

Larger, faster Aα fibers are less sensitive to local anesthetics than smaller, slower-conducting _____ fibers.

<p>Aδ</p> Signup and view all the answers

Local anesthetics are weak _____ and must lose their hydrogen ions to diffuse across cellular membranes.

<p>bases</p> Signup and view all the answers

The sensitivity of nerve fibers to local anesthetics is determined by axonal diameter, myelination, and other _____ and physiological factors.

<p>anatomic</p> Signup and view all the answers

The local anesthetic effect wears off gradually as the molecules _____ throughout the body.

<p>diffuse</p> Signup and view all the answers

Some common toxic effects include light headedness, shivering, and ______.

<p>seizures</p> Signup and view all the answers

True allergies to local anesthetics are considered ______.

<p>rare</p> Signup and view all the answers

Local anesthetics can cause ______ if administered in high enough concentrations.

<p>tissue toxicity</p> Signup and view all the answers

Regional anesthesia aims to render a specific area of the body ______ to stimulus.

<p>insensate</p> Signup and view all the answers

One common type of regional anesthesia is ______ block.

<p>peripheral nerve</p> Signup and view all the answers

Symptoms related to systemic toxicity of local anesthetics can range from ______ to seizures.

<p>lightheadedness</p> Signup and view all the answers

The ______ block is a type of intravenous regional anesthesia.

<p>Bier</p> Signup and view all the answers

Common reactions to the ester class of local anesthetics are often due to ______ injections.

<p>intravascular</p> Signup and view all the answers

Local anesthetic toxicity may lead to hypotension, also known as ______ blood pressure.

<p>low</p> Signup and view all the answers

Regional anesthesia can be used for diagnosis or therapy in patients with chronic ______ syndromes.

<p>pain</p> Signup and view all the answers

Ester hydrolysis is very rapid, and the water-soluble metabolites are excreted in the ______.

<p>urine</p> Signup and view all the answers

Pseudocholinesterase is an enzyme present in the blood that hydrolyzes ______ more slowly than acetylcholinesterase.

<p>acetylcholine</p> Signup and view all the answers

Amide local anesthetics are metabolized by microsomal P-450 enzymes in the ______.

<p>liver</p> Signup and view all the answers

The onset of local anesthetic action depends on the relative concentration of the nonionized lipid-soluble form (B) and the ionized water-soluble form (BH+), expressed by the ______.

<p>pKa</p> Signup and view all the answers

Toxicity refers to the peak circulation levels of local ______.

<p>anesthetics</p> Signup and view all the answers

Factors affecting the minimum concentration of local anesthetic that will block nerve impulse conduction include fiber size, type, myelination, and ______.

<p>pH</p> Signup and view all the answers

Less potent, less lipid-soluble agents generally have a faster ______ than more potent ones.

<p>onset</p> Signup and view all the answers

Highly lipid-soluble local anesthetics have a longer duration of action because they diffuse more slowly from a lipid-rich environment to the ______.

<p>aqueous bloodstream</p> Signup and view all the answers

Maximum dose for an individual is usually between ______ mg to 500 mg.

<p>70</p> Signup and view all the answers

Potency correlates with octanol ______, which reflects the ability of the local anesthetic molecule to permeate lipid membranes.

<p>solubility</p> Signup and view all the answers

Study Notes

Local Anesthesia

  • Local anesthetics are drugs that induce temporary loss of sensation in a specific body region, without affecting consciousness.
  • Local anesthetics diminish pain signals by blocking sodium channels in peripheral neurons where they are applied.
  • This blocking action prevents neuronal depolarization and action potential formation, interrupting the transmission of pain signals to the central nervous system.
  • Local anesthetics are weak bases, which require a change in their structure to cross cellular membranes.
  • Once inside neurons, they revert to their ionized form and block sodium ion entry in the ion channel.
  • The duration of anesthetic effect depends on the concentration of molecules at the site of action.
  • Nerve fiber sensitivity to local anesthetics is influenced by factors like axonal diameter, myelination, and other physiological factors.

Nerve Fiber Classification

  • Within the same type of nerve fiber, smaller diameter fibers are more sensitive to local anesthetic effects compared to larger diameter fibers.
  • Larger, faster conducting Aα fibers are less sensitive to local anesthetics than smaller, slower conducting Aδ fibers.
  • C fibers, which are small and unmyelinated, are relatively resistant to anesthetic inhibition compared to larger myelinated fibers.

Local Anesthetics - Classes

  • Ester local anesthetics are metabolized primarily by pseudocholinesterase in the plasma.
  • Amide local anesthetics are metabolized by liver enzymes, primarily in the microsomal P-450 system.
  • Pseudocholinesterase is an enzyme found in blood and certain organs, responsible for breaking down acetylcholine at a slower rate than acetylcholinesterase.
  • Acetylcholinea is a neurotransmitter found throughout the nervous system.

Amide vs. Ester Local Anesthetics

  • Amide local anesthetics undergo N-dealkylation and hydroxylation by microsomal P-450 enzymes in the liver for metabolism.
  • Reduced liver function or blood flow can impair metabolism and increase drug concentrations, potentially leading to increased toxicity.

Properties of Local Anesthetics

  • Potency is correlated with the anesthetic's lipid solubility, which influences its ability to cross cell membranes.
  • Onset is influenced by lipid solubility and the ratio of ionized and nonionized forms of the drug, represented by the pKa.
  • Duration is closely related to potency and lipid solubility. Highly soluble anesthetics maintain their effect longer because they diffuse more slowly from the lipid-rich environment into the bloodstream.

Toxicity

  • Toxicity is linked to the peak blood levels of local anesthetics, which vary based on patient factors like age, weight, and health.
  • Tissue toxicity can occur at high concentrations but is rare in clinical practice.
  • Systemic toxicity arises from high blood levels of the drug and can range from mild symptoms like lightheadedness to severe complications like seizures and cardiovascular collapse.
  • Allergy to local anesthetics is extremely rare. Most reported reactions are associated with the ester class, linked to ester hydrolysis.

Regional Anesthesia

  • Regional anesthesia involves administering a local anesthetic to a specific area of the body, making that region insensitive to surgical stimulation or other instrumentation.
  • It can serve as anesthesia for surgical procedures, provide postoperative pain relief, or manage chronic pain syndromes.
  • Different types of regional anesthesia include topical applications, field blocks, intravenous blocks (Bier block), peripheral nerve blocks, plexus blocks (brachial, lumbar), and central neuraxial blocks (epidural, spinal).

### Local Anesthetics

  • Local anesthetics transiently inhibit sensory, motor, or autonomic nerve function
  • Local anesthetics produce loss of sensation to pain in a specific area of the body without loss of consciousness
  • Local anesthetics block pain signal transmission by blocking sodium ion channels in the peripheral neurons
  • This prevents neuronal depolarization and action potential formation
  • Local anesthetics are weak bases, and due to their positive charge, they cannot readily traverse the cellular membrane
  • Local anesthetics must lose their hydrogen ions in order to diffuse
  • After entering the neuron, local anesthetics regain their ionized active form and are trapped inside
  • In the protonated form, the molecules bind to the local anesthesia binding site on the inside of the ion channel near the cytoplasmic end and block sodium ion entrance
  • Local anesthesia effects persist as long as the molecules remain in a sufficient concentration and wear off gradually as they diffuse throughout the body
  • Sensitivity of nerve fibers to inhibition by local anesthetics is determined by axonal diameter, myelination, and other anatomical and physiological factors
  • Small diameter nerve fibers of the same type are more sensitive to local anesthetics
  • Larger, faster Aα fibers are less sensitive to local anesthetics than smaller, slower-conducting Aδ fibers
  • Larger unmyelinated fibers are less sensitive than smaller unmyelinated fibers
  • Small unmyelinated C fibers are relatively resistant to inhibition by local anesthetics as compared with larger myelinated fibers
  • In spinal nerves, local anesthetic inhibition (and conduction failure) generally follows the sequence autonomic > sensory > motor

### Local Anesthetic Classes

  • Esters: mostly metabolized by pseudocholinesterase
    • Ester hydrolysis is very rapid, and the water-soluble metabolites are excreted in the urine
  • Amides: metabolized by microsomal P-450 enzymes in the liver
    • Decreased hepatic function (eg, cirrhosis of the liver) or liver blood flow (eg, congestive heart failure, β blockers, or H2-receptor blockers) will reduce the metabolic rate and potentially predispose patients to having greater blood concentrations and a greater risk of systemic toxicity

### Local Anesthetic Action

  • Potency: correlates with octanol solubility, which reflects the ability of the local anesthetic molecule to permeate lipid membranes
  • Minimum concentration: affected by fiber size, type, and myelination; pH (acidic pH antagonizes block); frequency of nerve stimulation; and electrolyte concentrations (hypokalemia and hypercalcemia antagonize blockade)
  • Onset of action: depends on lipid solubility and the relative concentration of the nonionized lipid-soluble form (B) and the ionized water-soluble form (BH+), expressed by the pKa
    • The pKa is the pH at which the fraction of ionized and nonionized drug is equal
    • Less potent, less lipid-soluble agents generally have a faster onset than more potent, more lipid-soluble agents
  • Duration of action: correlates with potency and lipid solubility
    • Highly lipid-soluble local anesthetics have a longer duration of action because they more slowly diffuse from a lipid-rich environment to the aqueous bloodstream

### Local Anesthetic Toxicity

  • Toxicity is related to peak circulation levels
    • Levels of local anesthetic concentration administered to patients are varied according to age, weight, and health
    • Maximum dose for an individual is usually between 70 mg to 500 mg
  • Common toxic effects: lightheadedness, shivering or twitching, seizures, hypotension (low blood pressure), numbness
  • Tissue toxicity: can occur if administered in high enough concentrations (greater than those used clinically)
    • Usually related to preservatives added to solution
  • Systemic toxicity: related to blood level of drug secondary to absorption from the site of injection
    • Range from lightheadedness, tinnitus to seizures and CNS/cardiovascular collapse

### Local Anesthetic Allergy

  • True allergy is very rare
  • Most reactions are from the ester class - ester hydrolysis
  • Patient reports of “allergy” are frequently due to previous intravascular injections

### Regional Anesthesia

  • Regional anesthesia is rendering a specific area of the body insensate to stimulus for surgery or other instrumentation
  • Regional anesthesia uses include anesthesia for a surgical procedure, analgesia post-operatively or during labor and delivery, diagnosis or therapy for patients with chronic pain syndromes
  • Types of regional anesthesia: topical, local/field, intravenous block (“Bier” block), peripheral nerve block, plexus (brachial, lumbar), central neuraxial (epidural, spinal)

Local Anesthetics

  • Local anesthetics are a group of drugs that temporarily block sensory, motor, or autonomic nerve function.
  • They produce loss of sensation to pain in a specific area of the body without loss of consciousness.

Mechanisms of Local Anesthetic Action

  • Local anesthetics block pain signal transmission by blocking sodium ion channels in the peripheral neurons.
  • This prevents neuronal depolarization and action potential formation.
  • Local anesthetics are weak bases and require a change in charge to enter the neuron.
  • Once in the neuron, they regain their ionized form and block sodium ion entrance.
  • Local anesthetic effects last as long as the molecules remain in a high enough concentration and wear off gradually as they diffuse throughout the body.

Nerve Fiber Classification

  • Sensitivity of nerve fibers to local anesthetics is influenced by axonal diameter, myelination, and other anatomical and physiological factors.
  • Larger, faster Aα fibers are less sensitive to local anesthetics than smaller, slower-conducting Aδ fibers.
  • Smaller unmyelinated fibers are more sensitive than larger unmyelinated fibers.
  • Smaller unmyelinated C fibers are relatively resistant to inhibition by local anesthetics compared to larger myelinated fibers.
  • In spinal nerves, local anesthetic inhibition follows the sequence of autonomic nerves, then sensory, and lastly motor nerves.

Local Anesthetics - Classes

  • Ester local anesthetics: Primarily metabolized by plasma cholinesterase (butyrylcholinesterase).
    • Ester hydrolysis is rapid, and water-soluble metabolites are excreted in the urine.
  • Amide local anesthetics: Metabolized by microsomal P-450 enzymes in the liver.
    • Decreased liver function or blood flow can reduce the metabolic rate, potentially leading to higher blood concentrations and increased risk of systemic toxicity.

Local Anesthetic Properties

  • Potency: Correlates with octanol solubility, reflecting the ability of the molecule to permeate lipid membranes.
  • Onset of Action: Depends on lipid solubility and the concentration of the nonionized lipid-soluble form (B) and the ionized water-soluble form (BH+).
    • Less potent, less lipid-soluble agents generally have a faster onset than more potent, more lipid-soluble agents.
  • Duration of Action: Correlates with potency and lipid solubility.
    • Highly lipid-soluble local anesthetics have a longer duration of action as they diffuse more slowly from a lipid-rich environment to the bloodstream.

Toxicity

  • Tissue Toxicity: Can occur with high concentrations of local anesthetics.
  • Systemic Toxicity: Related to blood levels of the drug due to absorption from the injection site.
    • Effects range from lightheadedness and tinnitus to seizures and cardiovascular collapse.

Allergy

  • True allergy to local anesthetics is rare.
  • Most reactions are to ester-class agents due to ester hydrolysis.
  • Patient reports of “allergy” are often due to previous intravascular injections.

Regional Anesthesia

  • Definition: Rendering a specific area of the body insensitive to stimuli for surgery or other procedures.
  • Uses: Provide anesthesia for surgical procedures, analgesia post-operatively or during labor and delivery, and diagnosis/therapy for chronic pain syndromes.

Regional Anesthesia Types

  • Topical
  • Local/Field
  • Intravenous block (“Bier” block)
  • Peripheral nerve block
  • Plexus block (brachial, lumbar)
  • Central neuraxial (epidural, spinal)

Local Anesthetics - Mechanisms of Action

  • Local anesthetics produce loss of sensation to pain in a specific area of the body without the loss of consciousness.
  • They block pain signal transmission by blocking sodium ion channels in the peripheral neurons.
  • This prevents neuronal depolarization and action potential formation.
  • Local anesthetics are weak bases and need to lose their hydrogen ions in order to diffuse through the cellular membrane.
  • Once inside the neuron, they regain their ionized active form and become trapped.
  • In the protonated form, the molecules bind to the local anesthetic binding site on the inside of the ion channel near the cytoplasmic end and block sodium ion entrance.
  • The local anesthetic effect persists as long as the molecules remain in sufficient concentration and wears off gradually as they diffuse throughout the body.

Nerve Fiber Classification

  • Sensory nerves are classified into four main groups: Aα, Aβ, Aδ, and C.
  • Larger diameter fibers (Aα) are less sensitive to local anesthetics than smaller diameter fibers (Aδ and C).
  • Myelinated fibers are more sensitive to local anesthetics than unmyelinated fibers.
  • In spinal nerves, local anesthetic inhibition (and conduction failure) generally follows the sequence autonomic > sensory > motor.

Local Anesthetics - Classes

  • Esters: metabolized by pseudocholinesterase (plasma cholinesterase or butyrylcholinesterase) in the blood.
  • Amides: metabolized by microsomal P-450 enzymes in the liver.

Factors Affecting Local Anesthetic Action

  • Potency: correlates with octanol solubility, which reflects the ability of the local anesthetic molecule to permeate lipid membranes.
  • Onset of Action: depends on lipid solubility and the relative concentration of the nonionized lipid soluble form (B) and the ionized water-soluble form (BH+).
  • Duration of Action: correlates with potency and lipid solubility. Highly lipid-soluble local anesthetics have a longer duration of action.

Local Anesthetics - Toxicity:

  • Systemic Toxicity: related to blood levels of the drug due to absorption from the site of injection.
  • Tissue Toxicity: can occur if administered in high enough concentrations greater than those used clinically.

Regional Anesthesia - Definition and Uses:

  • Regional anesthesia renders a specific area of the body insensitive to stimulus for surgical procedures, post-operative analgesia, labor and delivery, or chronic pain syndromes.

Regional Anesthesia - Types

  • Topical: applied to the skin or mucous membrane
  • Local/Field: injected into a specific area.
  • Intravenous Block ("Bier Block"): injected into a peripheral vein.
  • Peripheral Nerve Block: injected near a specific nerve.
  • Plexus Block: injected into a nerve plexus (e.g., brachial, lumbar).
  • Central Neuraxial Block: injected into the epidural or spinal space.

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Local Anesthetics PDF

Description

This quiz explores the concepts of local anesthesia and the classification of nerve fibers. It covers the mechanisms by which local anesthetics block pain signals and factors affecting nerve fiber sensitivity. Test your understanding of these essential topics in neuropharmacology.

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