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Anesthesiology > Local anesthetics > Flashcards

Local anesthetics Flashcards

1
Q

What is the mechanism of action of local anesthetics?

A

Blocks voltage-gated Na channels in neuronal cell membrane; slows rate of depolarization (threshold potential not reached; action potential not propagated)

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2
Q

True/False: in general, smaller diameter fibers and those with less myelination are blocked first.

A

True

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3
Q

What is the exception to the order of nerve blockade with local anesthetics?

A

Brachial plexus block. Motor fibers are blocked first

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4
Q

What is the order of nerve blockade when using local anesthetics?

A

1) B fibers (preganglionic sympathetic)
2) A-delta and C fibers (pain)
3) A-gamma (proprioception)
4) A-beta (touch, pressure)
5) A-alpha (motor)

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5
Q

True/False: all local anesthetics are weak acids.

A

False. All are weak bases; pK ~7.7-9

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6
Q

How does pK affect local anesthetics?

A

Lower pK (closer to tissue pH) –> faster onset

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7
Q

How does protein binding affect local anesthetics?

A

Higher protein binding –> longer duration of action

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8
Q

How does lipid solubility affect local anesthetics?

A

Higher lipid solubility –> higher potency

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9
Q

What are some commonly used amides?

A

Lidocaine, bupivacaine (Marcaine), mepivacaine (Carbocaine), ropivacaine, prilocaine (with lidocaine in EMLA cream)

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10
Q

True/False: all local anesthetics can be given IV or used for intravenous regional anesthesia (IVRA).

A

False. Lidocaine is the only LA that can be given IV or used for IVRA

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11
Q

What are the pharmacodynamics of lidocaine?

A

Fast onset, short duration of action (1-2 hours when used in loco-regional techniques

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12
Q

What are the effects of lidocaine when used systemically (IV)?

A

Anti-arrhythmic; decreases MAC (MAC-sparing); analgesic; free radical scavenger; improves GI motility

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13
Q

What are the pharmacodynamics and adverse effects of bupivacaine?

A

Intermediate onset; duration of 3-8 hours; has the highest CV toxicity

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14
Q

What are the pharmacodynamics and uses of mepivacaine?

A

Fast onset; duration of 1.5-3 hours; used for intra-articular nerve block (common in large animal)

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15
Q

What are the pharmacodynamics and adverse effects of ropivacaine?

A

Intermediate onset; duration of 3-8 hours; less cardiotoxic than bupivacaine

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16
Q

What are some commonly used esters?

A

Procaine (procaine penicillin G (PPG)); tetracaine and proparacaine (ophthalmic preparations); benzocaine (historically used in laryngeal spray for intubation)

17
Q

What blocks have the fastest absorption?

A

Intercostal blocks (highest peak plasma concentration)

18
Q

How does epinephrine affect LAs?

A

Added to prolong duration of block; causes vasoconstriction, SLOWING systemic absorption and increasing duration of action at local site (most LAs cause vasodilation on their own)

19
Q

What is another additive used with LAs?

A

Bicarbonate. Faster onset of action and prolonged duration; less sting on injection

20
Q

True/False: when combining two LAs, it is ok to use the max dose of both drugs.

A

False. Toxicity is additive; can’t use max dose of both agents.

21
Q

What are some types of toxicity caused by LAs?

A

Methemoglobinemia; neurotoxicity; chondrotoxicity; systemic toxicity

22
Q

What LAs cause methemoglobinemia?

A

Benzocaine and prilocaine; seen in many species

23
Q

Neurotoxicity caused by LAs

A

Concentration dependent; permanent nerve injury extremely rare with normal clinical use; spinal lidocaine seems to be worse than bupivacaine; preservative-free versions should be used for epidurals and spinal anesthesia

24
Q

Chondrotoxicity caused by LAs

A

Concentration and time dependent; bupivacaine most damaging (not administered IA anymore for this reason); mepivacaine (Carbocaine) leas damaging (used for equine lameness diagnosis IA)

25
Q

What causes systemic toxicity from LAs?

A

Due to excessive plasma concentrations (inadvertant intravascular injection; excessive dose, intolerant patient)

26
Q

What are the systemic toxicity signs of lidocaine?

A

Depression/sedation, twitching, seizures, then CV signs; not clinically apparent if animal is anesthetized

27
Q

What are the systemic toxicity signs of bupivacaine?

A

Supra ventricular tachycardia, ventricular tachycardia, AV block, wide QRS complexes; hypotension; first sign is CV collapse –> death

28
Q

How do LAs cause CV toxicity?

A

Block cardiac Na channels

29
Q

What will you see if epinephrine is included with LAs?

A

Increasing heart rate as first sign of intravascular injection; can alert to IV injection and prevent additional drug administration

30
Q

Risk factors of LAs

A

IV injection; increased absorption (intercostal > epidural > brachial plexus); patient factors (hypoproteinemia; pregnancy; Beta- or Ca channel-blockade; hypoxemia, acidosis

31
Q

How do you treat arrest secondary to LA?

A

Start CPR; low dose epinephrine; give 20% lipid emulsion (Intralipid); avoid lidocaine, Ca-channel blockers, Beta-blockers, vasopressin

32
Q

True/False: allergic reactions to LAs occur most commonly with procaine in PPG.

A

True. PPG metabolized to PABA which is a common allergen. Much less commonly occur with amides

33
Q

What are the goals of local anesthesia?

A

Provides analgesia before, during, and after procedure; may allow decreased dose of systemic drugs (MAC-sparing; decreased hypotension and other adverse effects of general anesthesia); prevents central sensitization; important part of multimodal anesthesia

34
Q

What the different types of local anesthesia?

A

Topical (epidermis, eye, laryngeal); subcutaneous (mass removal, wounds); line block (ventral midline, flank)

35
Q

What are the different types of regional anesthesia?

A

Intravenous regional (IVRA); peripheral (individual nerve; plexus; intercostal; paravertebral); central (epidural; spinal)

36
Q

How is an IVRA (“Bier block”) done?

A

Limb is wrapped tightly from distal to proximal and tourniquet placed; local anesthetic injected into peripheral vein, diffuses into target tissues; released into systemic circulation when tourniquet is removed (lidocaine ONLY); procedure time limit of 60-90 min

37
Q

What is epidural anesthesia used for?

A

Tail/perineum, hindlimb, abdominal/thoracic

38
Q

What are the contraindications for an epidural?

A

Coagulopathy; hypovolemia; infection at injection site; neoplasia at injection site; anatomy (inability to palpate landmarks); sepsis. CHINAS

39
Q

What are adverse effects of epidurals?

A

Depends on drug used (LAs, opioids, alpha-2 agonists); hypotension (block of sympathetic trunk –> decreased vascular tone); motor block/paralysis (not a big deal in small animal, but is for a horse); hematoma, infection, neurotoxicity

Anesthesiology (10 decks)

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