Local anesthetics Dr. Maney Flashcards Preview

My 2016 RUSVM Anesthesia > Local anesthetics Dr. Maney > Flashcards

Flashcards in Local anesthetics Dr. Maney Deck (36):
1

What are the 2 types of local anesthetics

Esters

Amides

2

What is the MoA of local anesthetics

Blocks voltage-gated Na channels in neuronal cell membrane

slows rate of depolarization

3

What is the order of nerve blockade

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

4

What are the fibers that sense pain

A-δ and C fibers

5

What is the structure of local anes

What can modify their action

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

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

 

6

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

Lower pK (closer to tissue pH) ⇒ faster onset

Higher protein binding ⇒ longer DoA

Higher lipid solubility ⇒ higher potency

7

Common Amide LAs

Lidocaine

bupivacaine

Mepivacaine

Ropivacaine

Prilocaine

  • w/ Lidocaine in EMLA cream

"Eutectic Mixture of Local Anesthetics"

8

Lidocaine

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

 

9

Systemic (IV) lidocaine effects

Anti arrhythmic

Decr MAC

Analgesic

Free radical scavenger

10

Bupivacaine

Intermed onset

DoA: 3-8 h

HIGHEST CV toxicity!

11

Mepivacaine

Fast onset

DoA: 1.5-3 h

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

12

Ropivacaine

Intermed. onset

DoA: 3-8 h

less cardiotoxic than bupivacaine

13

Common Ester LAs

Procaine - commonly used to decr sting of Pen G

Tetracaine & Proparacaine - ophthalmic anes.

Benzocaine - laryngeal spray for intubation, not used any more

14

What does systemic absorption of LA depend on

Site of inj.

Dosage

characteristic of drugs

used w/ epi or not

15

How are ester LAs metabolized

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

can cause subsequent allergic reactions

16

How are amide LAs mtabolized

Microsomal liver enzymes

17

What is the use of epinephrine w/ LAs do

Prolongs duration of block

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

Caution when using in distal limbs due to vasoconstrictive effects!

18

Why would bicarbonate be added to LAs

Faster onset & prolonged DoA

Less sting on inj.

19

What effect does combining LAs have

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

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

20

What are some toxicities of LAs

Methemoglobinemia

Neurotoxicity

Chondrotoxicity

Systemic

21

Methemoglobinemia

Benzocaine & prilocaine in multiple spp

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

NO LONGER USED IN VET MED!

22

Neurotoxicity

concentration dependent

spinal lidocaine seems worse than bupivacaine

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

23

Chondrotoxicity

Conc & time depend.

Bupivacaine-  most damaging (not IA anymore)

Mepivacaine - least damaging used in horse lameness dx.

24

Systemic toxicity

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!

25

26

Risk factors when using LAs

IV inject.

incr absorption

intercostal>epidural>brachial plexus

Pt factors

hypoprotenemia

pregnancy

Beta or Ca channel blockade

Hypoxemia, acidosis

27

Tx of arrest 2° to LA

CPR

low dose epi

20% lipid emulsion

avoid lidocaine, Ca channel blockers, Beta blockers & vasopressin

28

when is allergic reaction seen

Tx?

most common w/ procaine & pen G

much less common w/ amide LA

Tx as any allergic reaction:

Epi, fluids & airway maint.

29

3 types of LA use

Topical

SQ

Line block

30

3 types of regional anesthesia

Intravenous regionall (IVRA)

Peripheral

Central (neuraxial)

31

IVRA aka

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!)

32

Epidural anesthesia

epidural space is between dura & vertebra

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

used for tail/perineum, hindlimb, abdminal/thoracic

33

where does SC end

Dogs: L6-L7

Cats: L7-S3

Horses, ruminants, pigs: mid sacrum

if CSF encountered decr dose 50%

34

epidural contraindications

(CHINAS)

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

35

Adverse effect of epidural

Hypotension

block of sypathetic trunk → decr vascular tone

Motor block/ paralysis

Not big deal w/ SA but HORSE BIG Deal!

36