Local anesthetics Dr. Maney Flashcards
What are the 2 types of local anesthetics
Esters
Amides
What is the MoA of local anesthetics
Blocks voltage-gated Na channels in neuronal cell membrane
slows rate of depolarization
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
What are the fibers that sense pain
A-δ and C fibers
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
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
Common Amide LAs
Lidocaine
bupivacaine
Mepivacaine
Ropivacaine
Prilocaine
- w/ Lidocaine in EMLA cream
“Eutectic Mixture of Local Anesthetics”
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
Systemic (IV) lidocaine effects
Anti arrhythmic
Decr MAC
Analgesic
Free radical scavenger
Bupivacaine
Intermed onset
DoA: 3-8 h
HIGHEST CV toxicity!
Mepivacaine
Fast onset
DoA: 1.5-3 h
used for nerve block, intraarticular (large animal)- lameness dx in horses
Ropivacaine
Intermed. onset
DoA: 3-8 h
less cardiotoxic than bupivacaine
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
What does systemic absorption of LA depend on
Site of inj.
Dosage
characteristic of drugs
used w/ epi or not
How are ester LAs metabolized
Hydrolysis by plasma cholinesterase (no liver metab.) to PABA
can cause subsequent allergic reactions
How are amide LAs mtabolized
Microsomal liver enzymes
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!
Why would bicarbonate be added to LAs
Faster onset & prolonged DoA
Less sting on inj.
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!)
What are some toxicities of LAs
Methemoglobinemia
Neurotoxicity
Chondrotoxicity
Systemic
Methemoglobinemia
Benzocaine & prilocaine in multiple spp
Severe methemoglobinemia seen in cats when Cetacaine (benzocaine + tetracaine) was used
NO LONGER USED IN VET MED!
Neurotoxicity
concentration dependent
spinal lidocaine seems worse than bupivacaine
Preservative-free version should be used for epidurals & spinal anesthesia!
Chondrotoxicity
Conc & time depend.
Bupivacaine- most damaging (not IA anymore)
Mepivacaine - least damaging used in horse lameness dx.
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!*
Risk factors when using LAs
IV inject.
incr absorption
intercostal>epidural>brachial plexus
Pt factors
hypoprotenemia
pregnancy
Beta or Ca channel blockade
Hypoxemia, acidosis
Tx of arrest 2° to LA
CPR
low dose epi
20% lipid emulsion
avoid lidocaine, Ca channel blockers, Beta blockers & vasopressin
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.
3 types of LA use
Topical
SQ
Line block
3 types of regional anesthesia
Intravenous regionall (IVRA)
Peripheral
Central (neuraxial)
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!)
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
where does SC end
Dogs: L6-L7
Cats: L7-S3
Horses, ruminants, pigs: mid sacrum
if CSF encountered decr dose 50%
epidural contraindications
(CHINAS)
- Coagulopathy
- Hypovolemia
- Infection at inj site
- Neopasia at inj site
- Anatomy (inability to palpate landmarks)
- Sepsis
Adverse effect of epidural
Hypotension
block of sypathetic trunk → decr vascular tone
Motor block/ paralysis
Not big deal w/ SA but HORSE BIG Deal!
