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4. HDS Multisystem > Local Anesthetics > Flashcards

Local Anesthetics Flashcards

1
Q

Local Anesthetics

Overview

A

Used for relief of pain in specific regions or areas of the body

Often in order to perform a particular procedure

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2
Q

Structure & Metabolism

A
  • Divided into amides and esters
  • All local anesthetics have an aromatic group at one end (benzyl derivative)
  • With one exception (benzocaine), an amine at the other end
  • All weak bases
  • Strong lipophilic character
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3
Q

Amides

A
  • Amides contain two “i’s” as in lidocaine
  • Inactivated in the liver by cytochrome p450 system
  • Action will be prolonged in pts w/ ↓ liver function
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4
Q

Esters

A
  • Esters contain only one “i” as in tetracaine
  • Inactivated mainly by the plasma cholinesterease
  • More rapidly inactivated than the amides
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5
Q

Epinephrine Effects

A

For some local anesthetics:

  • Co-administration of epinephrine ⇒ vasoconstriction
  • Prevents local anesthetic from diffusing away from site of injection
  • ⊕ of α2 adrenergic receptors may also have an analgesic effect
  • Epinephrine should not be used in an area that lacks collateral blood flow
    • Digits, nose, ears, and penis
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6
Q

Lipophilic Anesthetics

A

Anesthetics that are more lipid soluble:

  • More potent
  • Longer duration of action
  • Take longer to achieve the clinical effect
  • Extensive protein binding also ↑ duration of action
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7
Q

General

Mechanism of Action

A
  • Un-ionized (neutral) form enters nerve
    • Onset of action strongly dependent upon local pH
      • Alkaline ⇒ fast
      • Acidic ⇒ slow
    • Bicarbonate could be used as an adjuvant to ↑ onset of action
    • Infections which lower the pH can slow the onset of action
  • Gains a proton ⇒ ionized (charged) form
  • Ionized form of weak base blocks Na+ channels by binding to an internal sequence
    • High affinity binding to the inactivated state
    • Depends on Na+ channel activity ⇒ use-dependent (frequency-dependent)
    • Very important for antiarrhythmic activity of some local anesthetics
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8
Q

Fiber Types

A
  • Small, high frequency fibers more susceptible to local anesthetic block than are larger fibers
    • Ex. type C fibers that carry nociceptive signals (pain)
  • In fibers of equal size, myelinated are more susceptible than unmyelinated fibers
  • Nerves of small diameter and very active (high frequency impulses) will be preferentially blocked
    • Ex. pain fibers carrying signals from damaged tissue
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9
Q

Fiber Location

A

Effect partially dependent on position of fibers in nerve bundle:

  • In the extremities:
    • Nerves innervating proximal sites are on outside
    • Nerves innervating distal sites are on the inside
    • Larger motor fibers are on the outside
    • Smaller sensory fibers are on the inside
  • Medial site admin (eg “spinal” block) ⇒ block may proceed from proximal to distal
  • Direct injection @ site of pain ⇒ knock out signals mediated by nerve endings right there, bringing immediate relief
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10
Q

Order of Anesthesia

A

The loss of sensation occurs in the following order:

Pain

Temperature

Touch

Movement

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11
Q

Local Anesthetics

Uses

A
  • Topical (Surface) ⇒ skin and mucosa
  • Infiltration (infl) ⇒ direct injection (eg knee)
  • Peripheral Nerve Block (PNB) ⇒ injected close to nerve trunks (eg Brachial)
  • Spinal ⇒ injection into subarachnoid space near spinal cord
  • Epidural ⇒ injection just above dura surrounding spinal cord, near spinal nerve roots
  • Systemic ⇒ may be used for postoperative pain and chronic pain
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12
Q

Adverse Effects

A
  • Major side effects result from escape of the drugs into the circulation
  • Distribution into the circulation occurs w/ most applications (except topical)
  • Risk of adverse effects is dose- and age-dependent
  • Effects are system specific
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13
Q

CNS Effects

A
  • Low doses:
    • Tremors
    • Oral numbness
    • Dizziness
    • Confusion
    • Agitation
  • High doses:
    • Muscle twitching
    • Convulsions
    • CNS depression & respiratory arrest
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14
Q

Cardiovascular Effects

A
  • Vasodilation (exception = cocaine)
  • Myocardial depression (less often)
    • Possibly leading to ↓ CO
  • Ventricular arrhythmias and cardiac arrest (Rare)
    • W/ unintentional high plasma levels of local anesthetics
    • Can occur w/ normal IV doses of bupivacaine
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15
Q

Hypersensitivity Reactions

A

Local dermatitis w/ some topicals

Rare systemic allergic response w/ injected esters

(due to p-aminobenzoic acid metabolite)

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16
Q

Drug Interactions

A
  • Local anesthetic blockade ⇒ loss of visceral and skeletal muscle tone
    • Potentiate effect of neuromuscular blocking drugs
    • Use w/ caution in myasthenia gravis pts
  • Drug that ↓ hepatic blood flow (such inhaled anesthetics) will ↓ the elimination of amide anesthetics
17
Q

Lidocaine

A
  • Potency:
    • Intermediate
  • Duration:
    • Short (0.75-2 hrs)
  • Uses:
    • All uses including topical
  • Adverse effects:
    • Cardiac and respiratory arrest, arrhythmias, decreased cardiac contractility, hypotension
    • Seizure, tinnitus, dizziness, paresthesia, tremor, somnolence
    • Skin irritation, constipation
  • Comments:
    • Used with epinephrine
    • Used for neuropathic pain
18
Q

Prilocaine

A
  • Potency:
    • Intermediate
  • Duration:
    • Short (0.5 -1 hrs)
  • Uses:
    • Infiltration
    • Topical
    • Combined with lidocaine (EMLA)
  • Adverse effects:
    • Same as lidocaine, but methemoglobinemia due to o-toluidine
  • Comments:
    • Does not require coadministration of epinephrine because it has vasoconstrictive properties
19
Q

Mepivicaine

A
  • Potency:
    • Intermediate (slightly greater than lidocaine)
  • Duration:
    • Short (1-2 hrs)
  • Uses:
    • All uses
  • Adverse effects:
    • Same as lidocaine
  • Comments:
    • Less vasodilation than lidocaine, so does not require epinephrine
    • Unsuitable for prolonged epidural infusion because of accumulation in fetus
20
Q

Bupivacaine

A
  • Potency:
    • High
  • Duration:
    • Long (1.5 – 8 hrs)
  • Uses:
    • All uses
  • Adverse effects:
    • Same as lidocaine but greater cardiotoxicity especially at high doses
  • Comments:
    • Epinephrine adds little to duration
    • Lower concentration used in obstetrics are less toxic and provide pain relief w/o motor block
    • Postoperative pain / Pain of surgical incision
21
Q

Ropivacaine

A
  • Potency:
    • High
  • Duration:
    • Long (1.5 -8 hrs)
  • Uses:
    • All uses
  • Adverse effects:
    • Less cardiotoxic than bupivicaine
  • Comments:
    • Alternative to bupivicaine
22
Q

Amide Drugs

A
23
Q

Cocaine

A
  • Potency:
    • Low
  • Duration:
    • 0.5 – 1 hr
  • Uses:
    • Topical
  • Adverse effects:
    • Could accelerate coronary atherosclerosis, tachycardia
    • Seizure, CNS depression or excitation, anxiety
  • Comments:
    • Still popular due to vasoconstrictive properties
    • Used primarily in the eye and as part of a combination with tetracaine and epinephrine
24
Q

Tetracaine

A
  • Potency:
    • Low
  • Duration:
    • 1.5 – 6 hrs
  • Uses:
    • Spinal, topical
  • Adverse effects:
    • Cardiac or respiratory arrest and hypotension
    • CNS depression or excitation
  • Comments:
    • Slow onset due to high pKa
25
Q

Procaine

A
  • Potency:
    • Low
  • Duration:
    • 0.5 – 1 hrs
  • Uses:
    • Infl., (dental procedures)
  • Adverse effects:
    • Same as tetracaine
  • Comments:
    • A short acting analogue 2-chloroprocaine is used in obstetrics
26
Q

Benzocaine

A
  • Potency:
    • Low
  • Duration:
    • 0.5 -1 hr
  • Uses:
    • Topical
  • Adverse effects:
    • Methemoglobinemia
  • Comments:
    • Used on mucous membranes
27
Q

Ester Drugs

A

4. HDS Multisystem (43 decks)

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