Local Anesthetics Flashcards

1
Q

Local anesthesia general definition/mechanism

A
  • = loss of sensation in circumscribed area of the body
  • results from block of AP initiation or propogation @ nerves
    • block voltage-gated Na+ @ peripheral nerves
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2
Q

Chemical structure of local anesthetics

A
  1. lipophilic aromatic portion
  2. intermediate alkyl chain
    1. ester vs. amide group
  3. hyrdophilic amine portion
  4. naming
    1. “-caine”
    2. one i = intermediate ester (cocaine)
    3. two i’s = intermediate amine (lidocaine)
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3
Q

Effect of pH on local anesthetic action

A
  • local anesthetics = weak base
    • pKa ~7.7 - 9.0
  • thus, local anesthetics are partly ionized @ pH 7.4 ==> more drug molecules are charged vs. neutral
  • both forms are needed for proper action
  • changes in tissue pH ==> ratio of neutral/charged ==> alter effectiveness of local anesthetics
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4
Q

MOA of local anesthetics

A
  • binding site = @ wide region of water-filled pore of Na+
    • route of access = intracellular pore entrance
    • binding site accessed when channel is open
  • cross plasma membrane when in neutral form
    • lipophillic portion ==> membrane solubility
  • charged (cationic) form of binds with more affinity w/in Na+ channel pore
    *
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5
Q

Characteristics of use-dependent block

A
  • more the channel is open, thre greather degree of local anesthetic binding and block
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6
Q

Physiocochemical properties ==> potency, onset of drug action

A
  • lipid solubility ==> potency
  • pKa determines speed of drug to target ==> onset of action
    • lipid solubility correlates w/pKa and onset
  • protein binding ==> longer duration of action
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7
Q

Procaine: potency, onset, duration, pKa, lipid solubility, protein binding

A
  • low potency
  • slow onset
  • short duration
  • high pKa
  • low lipid solubility
  • low protein binding
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8
Q

Lidocaine: potency, onset, duration, pKa, lipid solubility, protein binding

A
  • medium potency
  • intermediate onset
  • medium duration
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9
Q

Etidocaine: potency, onset, duration, pKa, lipid solubility, protein binding

A
  • high potency
  • fast onset
  • long duration
  • low pKa (7.7)
  • high lipid solubility
  • high protein binding
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10
Q

Methods of local anesthetic application

A
  • topical anesthesia
  • infiltration anesthesia
  • nerve block anesthesia
  • IV regional anesthesia
  • Spinal anesthesia
  • epidural anesthesia
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11
Q

Topical anesthesia mechanism/disadvantage

A
  • directly onto skin, cornea, mucous membranes of nose, mouth, etc.
  • disadvantage: considerable amount can get into circulation ==> toxic effects
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12
Q

Infiltration Anesthesia

A
  • Injection of local anesthetic into tissue without consideration of the location of cutaneous nerves.
  • advantage = very superficial anesthesia but function of underlying organs is unaffected
  • Disadvantage is that large doses are needed and there may be significant absorption into the circulation.
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13
Q

Nerve block anesthesia

A
  • injection of a high concentration of local anesthetic near a peripheral nerve or nerve plexus
  • advantage of this technique = larger body regions can be anesthetized as compared to infiltration anesthesia.
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14
Q

IV regional anesthesia

A
  • blood is squeezed out of a limb or part of a limb using a very tight elastic bandage, and a tourniquet is placed proximally.
  • Local anesthetic is then injected via a catheter, and limb anesthesia develops within 5-10 minutes.
  • Tourniquet pain and ischemic injury limit the use of this method to a maximum duration of 2 hours.
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15
Q

Spinal anesthesia technique + advantages/disadvantages

A
  • injection of a local anesthetic into the cerebrospinal fluid bathing the lumbar section of the spinal cord.
  • advantage = large body regions can be anesthetized while maintaining a low plasma level of drug
  • disadvantage = useful for surgical procedures of the lower abdomen, perineum and lower extremities.
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16
Q

Epidural anesthesia technique + advantages/disadvantages

A
  • injection of a local anesthetic just outside the dura-enclosed spinal canal, and at the base of the canal.
  • advantage = can use catheters, allowing repeated bolus or continuous application of anesthetic.
  • disadvantage vs. spinal anesthesia = plasma levels of anesthetic are higher, which can lead to toxicity (e.g. cardiovascular).
17
Q

Vasoconstrictor use with local anestheti

A
  • vasoconstrictor prolongs the duration of conduction blockade (by a factor of perhaps 2) by reducing blood flow in the vicinity of the injection,
    • ==> slower systemic absorption of the anesthetic
  • vasoconstrictor also helps to prevent plasma anesthetic levels from rising to potentially toxic levels.
  • Epinephrine (5 μg/ml) is usually the chosen vasoconstrictor
18
Q

Side effects of local anesthetics

A
  • convulsion
    • action @ inhibitory interneurons @ CNS
  • interfere w/autonomic nervous system fxn
  • CV = actions @ heart
    • can be pro-arrhythmic
    • some (e.g. lidocaine) are used as antiarrhythmics
  • arteriolar dilatation
    • action @ vascular smooth muscle
  • impact fetus by crossing placenta
  • inhibit neuromuscular transmission
    • block nicotinic ACh receptors
  • possibility of hypersensitivity
19
Q

Local anesthetics vs. Tetrodotoxin and Saxitoxin

A
  • TTX blocks NaV channels from extracellular side, not selective at all or use dependent
  • LA are use dependent and attack from cytoplasm.
  • Bottom line TTX and SAX are NOT SELECTIVE; no descrimination at all