Local Anesthetics Flashcards
Does a person lose memory with local anesthesia
No - pt remains conscious - drug doesn’t make it to the brain - so no memory loss
What is a local anesthetic (LA)
drug that when applied locally can inhibit perception of sensation and prevent movement
primary mechanism of action for LA
blocking sodium channels (can bind to other receptors)
can block Na+ channels on any type of fiber - including in heart
Nerve bundle
sensory nerves don’t travel on their own but as a bundle
What does blocking peripheral sodium channels do
prevents propagation of AP -> prevents release of glutamate and substance p -> prevents ascending pathway = no pain
What is a nerve block
inject LA near location of nerve where you want to block sensation. Inject in periphery -> if you inject too near/in spinal cord, it can bind to sensory fibers in spinal cord - will be epidural/spinal block then.
Uses/administration of LA
oral nerve block
peripheral nerve block
epidural block
spinal (intrathecal) block
Oral nerve block
inject into nerve bundle rather than individual nerve to get stronger block -> will numb entire area where bundle runs
Peripheral nerve block
block nerve bundle in periphery
Epidural
route of administration
drug released outside of dura mater
drug has to cross dura mater, enter CSF, and come into contact with nerve cord fibers
Most common method -> NOT puncturing dura mater so less chance of infection
Spinal (intrathecal)
puncture dura mater, drug directly into CSF
slightly more potent than epidural
Can be used to treat central pain at cell body in spinal cord (intrathecal catheter/pump)
What drugs can you give via intrathecal
opioids, alpha-2 agonists, etc
LA bind to sodium channels that are
open or inactive (refractory)
Degree of inhibition of sodium channels is dependent upon
frequency of action potentials - infrequent AP won’t be inhibited as much as frequent AP
Why do you want LA to bind to open or refractory confirmation
b/c once channel is closed, it will reopen and fire an AP. If you can block channel when it’s open or refractory, it prevents closing and thus prevents AP
Tonic inhibition
occurs in low frequency firing of AP -> long time btwn AP so LA will dissociate and not block the AP -> limited nerve fiber inhibition
example of tonic nerve fiber
nerve bundles in mouth -> this is why oral LA take a while to work
Phasic inhibition
occurs in high frequency AP. LA constantly bound to sodium channel b/c it does not have a chance to dissociate before another AP is fired.
Does phasic firing of AP indicate pain
Yes -> very active nerve fiber, this is why epidurals in active labor work so well.
What can lead to increase in frequency of AP firing
tissue injury or trauma -> LA will block nociceptive fibers or CNS axons better, leading to greater pain inhibition compared to tonic state
Do you want a strong, moderate, or weakly hydrophobic LA
moderate is best (wide range of moderate)
Why are poorly hydrophobic drugs not effective
cannot cross cell membrane and enter intracellular space, which is where binding site/ sodium channel is
why are strongly hydrophobic drugs not effective
will get trapped in lipid bilayer and not cross into intracellular space
Moderately hydrophobic LA
procain tetracain cocaine lidocaine prilocaine bupivacaine
Procaine
Novocain
short acting, low hydrophobicity -> thus rapidly removed and not sequestered in tissue for long
common in dental procedures
Tetracaine
long-acting, highly potent
highly hydrophobic
able to remain in tissue near injection for extended period of time
hydrophobicity responsible for high potency
used for epidural and spinal anesthesia.
also used topically
Cocaine
medium acting, medium potency
low hydrophobicity
can block catecholamine transporters and act as vasoconstrictor at local site
rarely used in certain eye procedures
Lidocaine and Prilocaine
medium duration, moderately potent agen
moderate hydrophobicity
can be used with epinephrine to increase duration of action
MOST commonly used LA
often paired with Epi -> vasoconstricts -> keeps drug at site longer
Bupivacaine
highly hydrophobic, long duration of action
epidural, spinal, and peripheral blocks
can be cardiotoxic at high dose
drug can move towards lower end of spinal cord causing “short fusing” effect -> pt has sudden urge to walk around
encourage pt to move around after injection to prevent
