Local Anesthetics Flashcards
what are the two types of local anesthetics? which has a longer duration of action?
esters (ROR) and amides (RNR); amides have a longer duration of action
all new local anesthetics are
amides (RNR)
what do local anesthetics do
block pain without causing unconsciousness
what is the MOA of local anesthetics
block Na channels -> inhibits APs in all neurons, as well as myocytes
what is the order in which local anesthetics block
small fibres (ex. pain) are blocked before large fibres (ex. motor)
where is the receptor for local anesthetics and what does this imply
on the cytoplasmic surface; must diffuse in the cell to produce effect
why are local anesthetics slower to produce an effect in inflammed or infected tissues
because they are weak bases, so they are protonated and ionized in an acidic environment, which prevents them from diffusing into the cell to block the Na receptor and cause effect
what combination with local anesthetics can produce necrosis
when lidocaine and epinephrine are used SQ
why might someone use lidocaine with epinephrine in practice
because lidocaine works until it diffuses away; by combining with a vasoconstrictor (E) we slow its removal and therefore prolong its action
what are some considerations if you were to use lidocaine with epinephrine
- it slows the healing
- do not use in end-artery supplied organs (ears, teats, digits, nose, penis…)
what are the first signs of local anesthetic overdose
drowsiness and muscle twitching
as plasma concentration of local anesthetics rises what happens
convulsions, unconsciousness, coma
what is a potential CVS side effect of LAs
blocks excitability of myocardium: AV block and arrhythmia
what are some examples of lidocaine formulation
- injectible +/- epinephrine
- gel
- spray (ex. for intubating cats)
what is a consideration before reaching for lidocaine spray in difficult to intubate cats
they are more susceptible to CVS effects and seizures
what type of LA is lidocaine
an amide (RNR)
what LA is used for equine nerve blocks
mepivicaine
why does mepivicaine diffuse better in tissues and what is a consequence of this?
it has a lower pKa -> favours B form (unionized); greater toxicity compared to lidocaine
what are 3 characteristics of bupivicaine
- slower onset (higher pKa) but longer action
- 4x as potent as lidocaine
- greater CVS toxicity so not available as IV
how does levobupivicaine relate to bupivicaine
it is the s(-) enantiomer
why is levobupivicaine safer than bupivicaine
does not distribute as well to the CNS or heart
what is the least irritating LA for eyes
proparacaine
long term use of proparacaine is related to
corneal sloughing and delayed healing
what is a “use-dependent block”
lidocaine blocks highly active Na channels more than resting Na channels because, in order to bind to the cytoplasmic part of the Na receptor, it needs to be open (vs. closed, resting sodium channels)
how does lidocaine have an anti-arrhythmic effect in low concentrations
arrythmias are related to unusual and frequent depolarizations of Na channels in damaged cardiac muscle; lidocaine blocks these random and unusual depolarizations but preserves the
less frequent, normally functioning signals via the use-dependent block effect
what should you never use if giving lidocaine systemically
products containing epinephrine (will impact HR and BP)
are esters or amides more resistant to enzymatic activity
amides
T/F amide LAs are more stable under sterilization and temperature extremes
T
LA enters the nerve fiber as a _________ base and the (ionized/unionized) form blocks conduction by interacting at _______ surface of the _____ channel
neutral/free base; ionized; inner; Na+
local anesthetics with a (higher/lower) pKa have more rapid onset of action
lower
describe what determines the following chemical properties of LAs:
- potency
- duration
- onset of action
potency: lipid-solubility
duration: protein binding
onset of action: pKa
lidocaine vs bupivicaine:
- which has higher potency
- which has higher duration of action
- which has higher onset of action
bupivicaine for all 3
describe the channel type based on the following affinity of Na:
- low
- intermediate
- high
- low: closed
- intermediate: open
- high: inactivated
T/F LAs block sensory, motor and autonomic pathways
T
what is the order of blockage for LAs:
β < C < Αδ< Aα < Aβ
T/F all fibers can be blocked by a LA, but some fibers may NOT be blocked
T
T/F cats are more sensitive to LA toxicity compared to dogs
T
put sites in order of fastest to slowest rate of absorption:
- SQ
- epidural
- spinal
- intercostal
- major nerve block
intercostal > spinal > epidural > major nerve block > SQ
what should you keep a dose below for the following LAs:
- lidocaine
- bupivicaine
lidocaine: below 8 mg/kg
bupivicaine: below 2 mg/kg
T/F most blocks are like IM/SQ injection
T
With LA toxicity, we see ______ signs before ______ signs, but anesthesia masks _____ signs. Thus, you should watch ____ signs when a patient is anesthetized and ____ signs when a patient is awake
CNS; CVS; CNS; CVS; CNS
in terms of LAs, you can only give ____________ intravenously
lidocaine
LAs: __ blood loss, __ thromboembolism risk, __ perfusion
decrease; decrease; increase
what is the general rule for the dose of LA (mL/kg)
0.2 mL/kg
what are the 8 ways to clinically apply local anesthetics
1) topical
2) regional
3) catheter
4) splash
5) IVRA
6) neuraxial
7) infiltration
8) IV
what is topical administration of local anesthetic
direct application to eyes, skin, mucous membranes, wounds
what are three examples of topical LAs
1) lidocaine spray: facilitates intubation
2) skin: EMLA cream
3) proparacaine for direct application to cornea and conjunctiva
what is a splash block
direct application to surgical site/cavity before closure
what is infiltration
injected into tissue without anatomical concern regarding innervation
what are examples of infiltration
incisional block for laparotomy, ring block
what amount (mL) do you give for infiltration in the following species:
- horse/cow
- dog/sheep/goat
- cat
- horse/cow: 100-200 mL
- dog/sheep/goat: 5-15 mL
- cat: 1-2 mL
testicular block is an example of
infiltration
inverted L block is an example of
infiltration
where is the LA for a testicular block deposited
spermatic cord and testicular tissue
what is a regional block
when LA is injected adjacent to sensory neurons
what are some examples of regional blocks
- brachial plexus block
- sciatic block
- RUMM block
- retrobulbar block
- head block
- paravertebral block
what nerves does the retrobulbar block stop
CN II, III, IV, V, VI and the ciliary ganglion
what is the risk with retrobulbar blocks and what do we take from it
can cause proptosis, blindness, globe perforation, retrobulbar hemorrhage; should only use if you are enucleating
what 4 head blocks are performed in small animals every day
1) mental
2) infraorbital
3) maxillary
4) mandibular (inferior alveolar)
what nerves does the brachial plexus block stop
C6-8, T1
what is blocked by the paravertebral thoracolumbar block
T13, L1, L2, L3 dorsal and ventral branches
what 3 nerves do we block in a cornual nerve block and what is the additional nerve if an adult
All:
- cornual
- infratrochlear
- supraorbital
Adult:
- branches from C2
what is another name for the cornual nerve
zygomaticotemporal
dehorning in goats is different because they receive dual supply from
zygomaticotemporal and infratrochlear
what is IVRA
intra-venous regional anesthesia: IV in a distal limb that has had its blood supply isolated using a tourniquet
what are the two most common neuraxial LAs and what is used more often in vetmed
epidural and subarachnoid: epidural more common in vetmed
what are the 2 epidurals we use
caudal and lumbosacral
what does a lumbosacral epidural block
hindlimb and caudal abdomen as well as motor fibres
how do we commonly administer a lumbosacral block (what drugs)
opioid and lidocaine or bupivacaine
what does a sacrococygeal epidural block
anus, vulva, vagina, perineum, rectum, tail
what is a useful block for feline urethral obstruction
sacrococygeal
what do we commonly use for sacrococygeal block and why
lidocaine: faster onset
what are some applications of IV lidocaine
- analgesia/ MAC reduction
- antiarrhythmic
- antiinflammatory
- anti-endotoxic
- good for GI motility
- neuropathic pain
because local anesthetics can be cardiotoxic, it is important to monitor (2)
ECG and blood pressure
what are some contraindications for local anesthetics
- infection
- allergic reacton
- tumors
- coagulopathies
- cardiac disease
