What are the 2 types of local anesthetics
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
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
- w/ Lidocaine in EMLA cream
"Eutectic Mixture of Local Anesthetics"
Only LA that may be given IV or used for intravenous regional anesthesia (IVRA)
Short duration - 1-2 h used in locoregional technique
Available as a patch
Systemic (IV) lidocaine effects
Free radical scavenger
DoA: 3-8 h
HIGHEST CV toxicity!
DoA: 1.5-3 h
used for nerve block, intraarticular (large animal)- lameness dx in horses
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.
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
Benzocaine & prilocaine in multiple spp
Severe methemoglobinemia seen in cats when Cetacaine (benzocaine + tetracaine) was used
NO LONGER USED IN VET MED!
spinal lidocaine seems worse than bupivacaine
Preservative-free version should be used for epidurals & spinal anesthesia!
Conc & time depend.
Bupivacaine- most damaging (not IA anymore)
Mepivacaine - least damaging used in horse lameness dx.
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
Beta or Ca channel blockade
Tx of arrest 2° to LA
low dose epi
20% lipid emulsion
avoid lidocaine, Ca channel blockers, Beta blockers & vasopressin
when is allergic reaction seen
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
3 types of regional anesthesia
Intravenous regionall (IVRA)
- 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 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
Horses, ruminants, pigs: mid sacrum
if CSF encountered decr dose 50%
Infection at inj site
Neopasia at inj site
Anatomy (inability to palpate landmarks)
Adverse effect of epidural
block of sypathetic trunk → decr vascular tone
Motor block/ paralysis
Not big deal w/ SA but HORSE BIG Deal!