Local anesthetics Dr. Maney

Question 1

What are the 2 types of local anesthetics

Answer 1

Esters

Amides

Question 2

What is the MoA of local anesthetics

Answer 2

Blocks voltage-gated Na channels in neuronal cell membrane

slows rate of depolarization

Question 3

What is the order of nerve blockade

Answer 3

small diam fiber & those w/ less myelination are blocked first

• Sympathetic
• Pain

Exception is the Brachial plexus block

Motor fibers blocked first - motor on periphery of n. trunck & sensory in center

Question 4

What are the fibers that sense pain

Answer 4

A-δ and C fibers

Question 5

What is the structure of local anes

What can modify their action

Answer 5

All are weak bases ≈ pK 7.7-9, not lipid soluble

Acidic tissue (infection) will incr ionized portion therefore making drug inactive

Question 6

How does pK, protein binding & lipid solubillty affect potency, onset & duration of LA

Answer 6

Lower pK (closer to tissue pH) ⇒ faster onset

Higher protein binding ⇒ longer DoA

Higher lipid solubility ⇒ higher potency

Question 7

Common Amide LAs

Answer 7

Lidocaine

bupivacaine

Mepivacaine

Ropivacaine

Prilocaine

• w/ Lidocaine in EMLA cream

“Eutectic Mixture of Local Anesthetics”

Question 8

Lidocaine

Answer 8

Only LA that may be given IV or used for intravenous regional anesthesia (IVRA)

Fast onset

Short duration - 1-2 h used in locoregional technique

Available as a patch

Question 9

Systemic (IV) lidocaine effects

Answer 9

Anti arrhythmic

Decr MAC

Analgesic

Free radical scavenger

Question 10

Bupivacaine

Answer 10

Intermed onset

DoA: 3-8 h

HIGHEST CV toxicity!

Question 11

Mepivacaine

Answer 11

Fast onset

DoA: 1.5-3 h

used for nerve block, intraarticular (large animal)- lameness dx in horses

Question 12

Ropivacaine

Answer 12

Intermed. onset

DoA: 3-8 h

less cardiotoxic than bupivacaine

Question 13

Common Ester LAs

Answer 13

Procaine - commonly used to decr sting of Pen G

Tetracaine & Proparacaine - ophthalmic anes.

Benzocaine - laryngeal spray for intubation, not used any more

Question 14

What does systemic absorption of LA depend on

Answer 14

Site of inj.

Dosage

characteristic of drugs

used w/ epi or not

Question 15

How are ester LAs metabolized

Answer 15

Hydrolysis by plasma cholinesterase (no liver metab.) to PABA

can cause subsequent allergic reactions

Question 16

How are amide LAs mtabolized

Answer 16

Microsomal liver enzymes

Question 17

What is the use of epinephrine w/ LAs do

Answer 17

Prolongs duration of block

causes vasoconstriction, SLOWING systemic absorbtion → incr. DoA at local site

Caution when using in distal limbs due to vasoconstrictive effects!

Question 18

Why would bicarbonate be added to LAs

Answer 18

Faster onset & prolonged DoA

Less sting on inj.

Question 19

What effect does combining LAs have

Answer 19

May shorten onset & lenghten DoA when lidocaine & bupivacaine are combined

Toxicity is additive (can’t use max dose of both agents!)

Question 20

What are some toxicities of LAs

Answer 20

Methemoglobinemia

Neurotoxicity

Chondrotoxicity

Systemic

Question 21

Methemoglobinemia

Answer 21

Benzocaine & prilocaine in multiple spp

Severe methemoglobinemia seen in cats when Cetacaine (benzocaine + tetracaine) was used

NO LONGER USED IN VET MED!

Question 22

Neurotoxicity

Answer 22

concentration dependent

spinal lidocaine seems worse than bupivacaine

Preservative-free version should be used for epidurals & spinal anesthesia!

Question 23

Chondrotoxicity

Answer 23

Conc & time depend.

Bupivacaine- most damaging (not IA anymore)

Mepivacaine - least damaging used in horse lameness dx.

Question 24

Systemic toxicity

Answer 24

Lidocaine:

depression/sedation, twitching, seizures THEN CV signs

which will NOT be clinically apparent if anesthetized

with bupivacaine : 1st sign is CV collapse ⇒ death!

• Blocks cardiac Na channels*
• bupivacaine most cardiotoxic*
• if used w/ epi may see incr HR as first clue of IV injection!*

Question 26

Risk factors when using LAs

Answer 26

IV inject.

incr absorption

intercostal>epidural>brachial plexus

Pt factors

hypoprotenemia

pregnancy

Beta or Ca channel blockade

Hypoxemia, acidosis

Question 27

Tx of arrest 2° to LA

Answer 27

CPR

low dose epi

20% lipid emulsion

avoid lidocaine, Ca channel blockers, Beta blockers & vasopressin

Question 28

when is allergic reaction seen

Tx?

Answer 28

most common w/ procaine & pen G

much less common w/ amide LA

Tx as any allergic reaction:

Epi, fluids & airway maint.

Question 29

3 types of LA use

Answer 29

Topical

SQ

Line block

Question 30

3 types of regional anesthesia

Answer 30

Intravenous regionall (IVRA)

Peripheral

Central (neuraxial)

Question 31

IVRA aka

Answer 31

Bier block

• limb wrapped tightly (Esmarch bandage) distal to proximal & tourniquet placed
• LA injected into peripheral vein, diffusing into target tissue
• Procedure limit 60-90 m
• LA becomes systemic when tourniquet removed (thats why LIDOCAINE ONLY!)

Question 32

Epidural anesthesia

Answer 32

epidural space is between dura & vertebra

different from spinal anes (space between dura & SC, containing CSF)

used for tail/perineum, hindlimb, abdminal/thoracic

Question 33

where does SC end

Answer 33

Dogs: L6-L7

Cats: L7-S3

Horses, ruminants, pigs: mid sacrum

if CSF encountered decr dose 50%

Question 34

epidural contraindications

(CHINAS)

Answer 34

• Coagulopathy
• Hypovolemia
• Infection at inj site
• Neopasia at inj site
• Anatomy (inability to palpate landmarks)
• Sepsis

Question 35

Adverse effect of epidural

Answer 35

Hypotension

block of sypathetic trunk → decr vascular tone

Motor block/ paralysis

Not big deal w/ SA but HORSE BIG Deal!