Local Anesthetics and Euthanasia

What are the main types of local anesthetics?

lipophilic group connected to hydrophilic groups via ester or amide bond

What are some properties of esters?

shorter duration and increased systemic toxicity

Local anesthetics are weak ____ that are predominately ______ at physiologic pH

bases, predominately ionized

Local anesthetics need to be ________ to cross membrane and once inside become ______ and bind to _______

need to be non-ionized

ionized and bind Na+ channel

What is the MOA of local anesthetics?

block Na+ w/ high affinity (more effective in rapidly firing axons) and inhibit neuronal firing

Duration of action of local anesthetics is dependent on?

time at site of action (exert effect then rapidly diffuse away)

Extent of block due to local anesthetics is dependent on?

voltage and time dependent (increases excitation threshold and slows impulse conduction)

What affects absorption?

dosage

site of inj (vascular vs fat)

drug tissue bidning

chemical properties

local blood flow

epinephrine (vasoconstricting agents)

How are amides metabolized/excreted?

liver metabolism from CYP450

toxicity more likely w/ hepatic dz or reduced hepatic BF

excreted via renal

How are esters metabolized/excretion?

butyrylcholinesterase in plasma

excreted via renal

What are local AE of local anesthetics?

tissue damage (rare)

neurotox and myotox

What are systemic AE of local anesthetics?

relative tox closely follows anesthetic potency

methemoglobinemia (benzocaine in cats/rabbits)

What are CNS AE effects of local anesthetics?

skeletal muscle twitches and tonic-clonic seizures firs signs of toxicity

What are CV AE of local anesthetics?

toxicity usually occurs at higher plasma concentrations than CNS tox

prolongation of PR and QRS intervals (due to slowed impulse control)

hypotension

decreased myocardial strength

What are some indications of lidocaine?

labeled infiltrations/nerve block/epidural

moderate topical activity (minimal local irritation)

correction of V fib and tachycardia (dogs)

prokinetic (horses)

What are AE of lidocaine?

profound cardio-depression

greater CNS

esp. cats

What local anesthetics are amides?

lidocaine, bupivacaine, mepivacaine, ropivacaine

What are indications of bupivacaine?

spinal anessthesia, infiltration blocks, epidural

analgesia post-op

cranial cruciate lig sx (liposome formulation)

What are some pharmacodynamics of bupivacaine?

longer duration of action than lidocaine (binds more resting cardiac Na, dissociates from channel slower)

more potent sensory block than motor

What are AE of bupivacaine?

cardiotoxicity

What are some indications of mepivacaine?

epidural and ventriculoectomy sx

some limb blocks (less post-inj. edema)

more potent/toxic than lidocaine

What are some indications of ropivacaine?

epidural, infiltration blocks, local blocks

slower onset and longer duration than lidocaine

What local anesthetics are esters?

benzocaine, proparacaine, tetracaine

What are some indications of benzocaine?

fish anesthesia/euthanasia

lipid soluble, topically absorbed

What are some indications of proparacaine?

topical use in ophthalmologic dx procedures

fast onset and short duration

2-3 repeat dosings can increase duration

What are some indications of tetracaine?

ophthalmological procedures needing deeper plane/longer duration than proparacaine

slow onset action, long duration

What are some AE of tetracaine?

burning/itching, chemosis (delayed)

as analgesic - diffuse epithelial keratitis that progresses to iritis, descemetitis, diffuse stormal edema

suppresses blink reflex (need eye lube, risks corneal opacification and keratitis)

What is the order of physiologic function loss in euthanasia to prevent fear/disress?

rapid loss of consciousness

loss of motor function

arrest of resp and cardiac function

permanent brain function loss

What is an acceptable method of euthanasia?

consistently produce humane death when used solely for euthanasia

What is an acceptable method with conditions of euthanasia?

may require certain conditions to be consistently humane death

may have greater potential for operator error/safety hazards

not well documented

may require secondary method to ensure death

What are some factors to consider when selecting euthanasia procedure?

reliability

time required to induce consciousness loss

compatibility w/ pt, intended animal use, purpose

emotional effect

drug availability, abuse potential

legal requirements

What is the MOA for barbiturates?

depress CNs in descending order (cerebral cortex start)

w/ OD deep anesthesia progresses to apnea due to respiratory center depression and following cardiac arrest

Barbiturates should never be used with what animals?

entering food chain

What is proper disposal of animals that have been euthanized with barbiturates?

carcass cannot contaminate food chain

burial only if carcass cannot be scavenged

consider cremation of companion animals!

What is the MOA of dissociative agents and a2 adrenergic agonists for euthanasia?

rapid loss of consciousness/muscle relaxation pre-op

may be given prior to euthanasia agents to minimize distress/fear/facilitate restraint and provide more aesthetic environment for O

How does epinephrine affect local anesthetic absorption?

a1 agonist that causes vasoconstriction

decreases diffusion of drug and prolongs duration

decreases systemic absorption and risk of systemic toxicity