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Pharmacology > Local Anesthetics > Flashcards

Local Anesthetics Flashcards

1
Q

Local Anesthetic effect on neural electrophysiology

A

Increases excitation threshold
Slows impulse conduction
Decreases action potential rate of rise
Decreases action potential amplitude

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2
Q

LA Site of Action

A

Voltage-gated Na+ channels

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3
Q

*Characteristics of Nerve Fiber sensitivity to LA blockade

A
  • Nerve fibers differ greatly in their sensitivity to LA blockade based on the fibers: D.E.P.F
    1. Diameter-small fibers are first to be blocked
    2. Effects on other excitable membrane- weak NMB, antiarrythmic properties (lidocaine), potential for lethal arrhythmias (Bupivacaine)
    3. Position in nerve bundle- peripheral nerves of bundle are blocked first
    4. Firing frequency- blockade more pronounced at higher frequencies
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4
Q

*Classification of Nerve Fibers based on sensitivity to LA blockade

A

Autonomic Sympathetic B-fiber nerves-Most senstitive to blockade
Autonomic Sensory C fibers-Highly sensitive to blockade
Somatic sensory/motor nerve fibers Aa Ab-least sensitive
Clinically, LA produce sequential loss of:

Autonomic fx»>Sensory fx»>Motor fx

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5
Q

Structurally, All LA have a similar structure that consists of:

A

Lipophilic, Hydrocarbon, Hydrophilic portions

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6
Q

The hydrocarbon portion of LAs vary and can be classified as either:

A

Esters or Amides

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7
Q

Amide LA include:

A

Mepivacaine, Etidocaine, Prilocaine, Lidocaine, Bupivacaine, and Ropivacaine

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8
Q

Rapid acting Amide LAs include

A

Mepivacaine
Etidocaine
Lidocaine
Prilocaine

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9
Q

Ester LA include:

A

Chloroprocaine-Rapid
Procaine-Slow
Tetracaine-Slow

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10
Q

LA acid/base properties

A

Local anesthetics, all of which have a pKa > 7.4, become more charged at neutral or acidic pH (i.e. they accept protons when the pH is ~ 7.4), and are less charged at more basic pH (able to cross cell membrane)

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11
Q

Why is the acid/base properties of LA important?

A

This is important because the uncharged base forms are more soluble in lipid environments, and lipid solubility is correlated with potency

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12
Q

Because LAs are prepared in acidic HCL salts (pH 4-7), they tend to be

A

highly ionized & water soluble
this means a vial contains equal mixtures fo nonionized, lipid-soluble (free base) & ionized water-soluble (cationic) molecules

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13
Q

the uncharged base in LA is the most important molecule because

A

It readily crosses the lipid bilayer, and is the reactive species.

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14
Q

What does the uncharge base in LA react to once inside the cell

A

Intracellular Hydrogen Ions to form an ammonium ion complex

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15
Q

What does the newly formed LA ammonium complex react

A

Sodium channels

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16
Q

LA’s exert it’s effect on neurons by binding to Na channels and prolonging entry of sodium ions which ——-

A

repolorization, and therefore subquent depolorization of the cell

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17
Q

Increased Lipid Solubility (Pka >7.4) of LA correlates to

A

Increased potency

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18
Q

the solubility/potency relationship of LA is similar to what anesthetic

A

Inhalation agents

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19
Q

Cm is defined as:

A

the lowest drug concentration required for conduction blockade

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20
Q

Cm is influenced by

A 
nerve fiber:
Size 
Type
pH
Frequency of Action Potential
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21
Q

Larger nerves have lower or higher Cm

A

Higher

22
Q

Increased pH ——Cm

A

lowers

23
Q

Increased Action Potential frequency—Cm

A

lowers

24
Q

pKa of LA determines

A

Onset of action

25
Q

Protein binding determines

A

Duration of action

the greater the protein binding, the longer the duration of action

26
Q 
Predict the following LAs DOA based on their protein binding %
Chlorprocaine      0%
Lidocaine            65%
Mepivacaine       78%
Bupivacaine        96%
Ropivacaine        93%
A 
Clor.. (< 30 min-Short)
Lido.. (1hr-Medium)         
Mepiv...(1 hr-Medium)
Bupiv...(>2 hr-Long)
Ropiv...(>2 hr-Long)
27
Q

**LA systemic absorption varies by site of injection, name the order from greatest to least absorptive sites

A

ITIC.PEB.SFS
Intravenous>Tracheal>Intercostal>Caudal>
Paracervical>Epidural>Brachial Plexus>Sciatic/Femoral>Subcutaneous

28
Q

Common additives used to Increase LA activity include:

A

Sodium Bicarbinate
Opioids
Alpha-adrenergic agonist (Clonidi/Dexmed)
Epinephrine

29
Q

Epinephrine added to LA

A

useful for local and epidural analgesia where epinephrine can reduce the peak local anesthetic concentrations in blood and also serve as part of a test dose to detect acccidental intravenous injection of local anesthetic.
It can also prolong the duration of regional blocks

30
Q

Sodium Bicarbonate added to LA

A

Raises the pH of the mileau, thus less of the LA is charged, and more of it is lipid soluble. DO NOT ADD TO BUPIVACAINE (as precipitation may occur).

31
Q

Ester LA metabolism

A

Rapid hydrolysis by plasma cholinesterase

Chloroprocaine>Procaine>Tetracaine

32
Q

Amides LA metabolism

A

Undergoe pulmonary extraction (Lidocaine, Bupivacaine, Prilocaine)
Hepatic metabolism
*Caution with sever liver dx

33
Q

Systemic toxicity and LA

A

Systemic toxicity (CNS and/or CVS)

Local Toxicity- nervous tissue injury

34
Q

CNS toxicity and LA

A

(tinnitus, paresthesias, agitation, lethargy, seizures, and coma)

  • Neurologic manifestations precede cardiovascular manifestations
  • Hypoxia, Acidosis, Hypercapnea potentiate LA toxicity
35
Q

CVS toxicity and LA

A

A-V block, arrhythmias, myocardial depression, cardiac arrest

  • bupivacaine being the most cardiotoxic
  • Hypoxia, Acidosis, Pregnacny, HyperKalemia can exacerbate both cardiac toxicities
36
Q

LA toxicity: Maximum Safe Doses (mg/kg)

A

Lidocaine (3-5) x2 w/ epi
Bupivacaine (1-2.5) 3 w/ epi
Ropivacaine (3) 3.5 w/ epi
Chloroprocaine (20-22)

37
Q

LA (local) toxic effects

A

Transient Neurologic Symptoms (M-S lower back pain, buttocks, & post. thighs) begin within 36 hours after recovery from spinal anesthesia, but self-limiting

Cauda Equina Syndrome

  • diffuse lumbar sacral injury with bowel/bladder sphincter dysf., & paraplegia.
  • Assoc. with hyperbaric 5% lidocaine via continuous spinal microcatheters
  • increased risk w/ lidocaine and the lithotomy position.
38
Q

LA and Allergic rxns

A

Occurs with esters>amides

*cross -sensitivities do not exist between classes of LA

39
Q

LA and Adverse Effects

A

Methemoglobinemia-seen w/ Prilocaine use

  • avoid in OB use
  • Tx w/ Methylene Blue IV
  • Amide- can cross the placental barrier and can cause Ion-trapping in fetus.
  • Esters don’t cross PB
40
Q

Lidocaine

A
  • Most widely used
  • Rapid onset
  • Immediated action and toxicity
  • Lidocaine toxicity (and all local anesthetic toxicity) can cause circumoral numbness, facial tingling, restlessness, vertigo, tinnitus, slurred speech, and tonic-clonic seizures. Local anesthetics are actually CNS depressants, thus tonic-clonic seizures are thought to be caused by depression of inhibitory pathways.
  • Used in liposuctions, and ETT to blunt sympathetic response
41
Q

*Lidocane’s dose-dependent (mcg/mL) systemic effects

A 
1-5 mcg/mL>>> Analgesia
5-10 mcg/mL>>>light headed,tinnitus, circum. numbness, muscle twithing, hypoten, myocardial depress.
10-15 mcg/mL>>> Seizures,unconscious.
15-25 mcg/mL>>> Coma, Resp. arrest
>25 mcg/mL>>> CV depression
42
Q

Mepivacaine

A
  • Rapid onset

- In contrast to lidocaine, no Vasodilator properties

43
Q

Bupivacaine

A
  • Slow onset (need to work)
  • Long DOA
  • Highly Toxic
44
Q

Ropivacaine

A
  • (S)- enatiomer of bupivacaine
  • Less Cardiotoxic than Bupivacaine
  • Less potent & DOA than Bup..
  • Less Moter blockade than Bup..
45
Q

LevoBupivacaine

A
  • (S)- enatiomer of Bupivacaine
  • Identical physio-chemical properties of Bu.
  • Less systemic/cardiotoxicity than Bu.
46
Q

Etidocaine

A
  • similar to bupivicane

- More potent motor block than sensory block

47
Q

Cocaine

A
  • ONSET slow
  • DOA short
  • Significant SNS stimulation-blocks presynaptic reuptake of NE & Dopamine
  • *Avoid in HTN, CAD, patients receiving catecholamines
  • TOPICAL use ONLY
48
Q

Cocaine Toxicity

A

Nitroglycerin
Beta-adrenergic blockers
Alpha-adrenergic blockers
Benzodiazeines

49
Q

Procaine

A
  • DOA short
  • ONSET slow
  • Hydrolyzed to PABA by cholinesterase
50
Q

Chloroprocaine

A
  • ONSET rapid
  • DOA short
  • Metabolized rapidly by plasma cholinesterase- reason for low toxicity profile
  • Preservatives»neurotoxicity
51
Q

Of the Esters Tetracaine

A

has highest potency, but onset slow

52
Q

Of the Amides, Lidocaine

A

has the lowest potency, but onset most Rapid

Pharmacology (27 decks)

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