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15 Cards in this Set
- Front
- Back
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Local anesthetics
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Drugs used to block the conduction/transmission of pain.
*Blocking the generation of action potentials w/out affecting resting potentials (preventing the ↑ in Na+ permeability w/o affecting K+ channels). |
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The 3 components of a local anesthetic
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1. Lipophilic end
2. Hydrophillic end 3. Ester or Amid linkage bwt the first two |
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Lipophilic component
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Important for the duration and potency of a local anesthetic
Also accounts for its toxicity |
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Ester or Amide linkage component
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Important for the metabolism, hypersensitivity, and chemical stability of a local anesthetic
*Esteratic: metabolized by plasma and hepatic cholinesterases (more hypersensitivity) vs. Amide: metabolized by liver microsomal enzymes |
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Hydrophilic component
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Important for the mechanism of action and the onset of action in a a local anesthetic.
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Sites where local anesthetics work
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1. Receptor at external surface
2. Expansion of axonal membrane 3. Receptor's internal surface 4. Combination of (3) and (4) |
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The pKa of a local anesthetic is 7.6. What proportion of the drug is in the unportonated (non-ionized form at physiological pH?
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Physiological pH = 7.4
log [acidic form: LA]/[basic form: LA] = pKa - pH log [acidic form: LA]/[basic form: LA] = 7.6 - 7.4 log [acidic form: LA]/[basic form: LA] = .2 [acidic form: LA]/[basic form: LA] = 1.6 1/1.6 = ***.631*** |
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The pKa of a local anesthetic is 7.9. What proportion of the drug is in the unportonated (non-ionized form at physiological pH?
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Physiological pH = 7.4
log [acidic form: LA]/[basic form: LA] = pKa - pH log [acidic form: LA]/[basic form: LA] = 7.9 - 7.4 log [acidic form: LA]/[basic form: LA] = .5 [acidic form: LA]/[basic form: LA] = 3.16 1/3.16 = ***.316*** |
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Explain the frequency dependent of local anesthetics
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Local anesthetics bind with greater affinity of the inactivated state of the Na+ channel.
Neurons that fire more rapidly are blocked more readily, because they have a higher proportion of inactive state channels |
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Effect of fiber diameter on local anesthetics
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- small diameter fibers are blocked first
- myelinated nerves are blocked before unmyelinated of the same diameter* *This is b/c the Na+ channels of myelinated neurons are concentrated at nodes of ravier and so not as many channels need to be blocked in order to stop conduction. |
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Cocaine
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Blocks the reuptake of catecholamines, hence it is the only local anesthetic which provies its own vasoconstriction
*Available for topical use only |
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Short-acting local anesthetics
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- Procaine
- Chloroprocaine *Short acting b/c of their esteratic linkage |
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Intermediate-acting local anesthetics
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- Lidocaine: rapid onset, high intrinsic activity to vasodialate (used frequently with EPI)
- Prilocaine - Amide linkages |
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Long-acting local anesthetics
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- Tetracaine: esteratic linkage, but its high pKa give is a slow onset of action
- Bupivacaine: amide linkage *Note: though they are longer lasting, they are also more toxic |
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EMLA
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Eutectic Mixture of Local Anesthetics
Mixture of equal portions of lidocaine & prilocaine - oil at room temperature, applied topically: 45-60min onset |
