Local anesthetics

Question 1

What is the mechanism of action of local anesthetics?

Answer 1

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

Question 2

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

Answer 2

True

Question 3

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

Answer 3

Brachial plexus block. Motor fibers are blocked first

Question 4

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

Answer 4

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)

Question 5

True/False: all local anesthetics are weak acids.

Answer 5

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

Question 6

How does pK affect local anesthetics?

Answer 6

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

Question 7

How does protein binding affect local anesthetics?

Answer 7

Higher protein binding –> longer duration of action

Question 8

How does lipid solubility affect local anesthetics?

Answer 8

Higher lipid solubility –> higher potency

Question 9

What are some commonly used amides?

Answer 9

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

Question 10

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

Answer 10

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

Question 11

What are the pharmacodynamics of lidocaine?

Answer 11

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

Question 12

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

Answer 12

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

Question 13

What are the pharmacodynamics and adverse effects of bupivacaine?

Answer 13

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

Question 14

What are the pharmacodynamics and uses of mepivacaine?

Answer 14

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

Question 15

What are the pharmacodynamics and adverse effects of ropivacaine?

Answer 15

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

Question 16

What are some commonly used esters?

Answer 16

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

Question 17

What blocks have the fastest absorption?

Answer 17

Intercostal blocks (highest peak plasma concentration)

Question 18

How does epinephrine affect LAs?

Answer 18

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)

Question 19

What is another additive used with LAs?

Answer 19

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

Question 20

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

Answer 20

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

Question 21

What are some types of toxicity caused by LAs?

Answer 21

Methemoglobinemia; neurotoxicity; chondrotoxicity; systemic toxicity

Question 22

What LAs cause methemoglobinemia?

Answer 22

Benzocaine and prilocaine; seen in many species

Question 23

Neurotoxicity caused by LAs

Answer 23

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

Question 24

Chondrotoxicity caused by LAs

Answer 24

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

Question 25

What causes systemic toxicity from LAs?

Answer 25

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

Question 26

What are the systemic toxicity signs of lidocaine?

Answer 26

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

Question 27

What are the systemic toxicity signs of bupivacaine?

Answer 27

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

Question 28

How do LAs cause CV toxicity?

Answer 28

Block cardiac Na channels

Question 29

What will you see if epinephrine is included with LAs?

Answer 29

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

Question 30

Risk factors of LAs

Answer 30

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

Question 31

How do you treat arrest secondary to LA?

Answer 31

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

Question 32

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

Answer 32

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

Question 33

What are the goals of local anesthesia?

Answer 33

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

Question 34

What the different types of local anesthesia?

Answer 34

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

Question 35

What are the different types of regional anesthesia?

Answer 35

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

Question 36

How is an IVRA ("Bier block") done?

Answer 36

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

Question 37

What is epidural anesthesia used for?

Answer 37

Tail/perineum, hindlimb, abdominal/thoracic

Question 38

What are the contraindications for an epidural?

Answer 38

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

Question 39

What are adverse effects of epidurals?

Answer 39

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