Local Anesthetics Flashcards
Local anesthetics are poorly __ soluble unless prepared as HCl salts
water
What is the Gold Standard of Local Anesthetics?
Lidocaine
HCl salts contribute to local anesthetic __
stability
LA’s containing epi may have pH lowered to __ with sodium bisulfate
4.0
Lower pH’s prevent oxidative decomposition of __
epinephrine
the hydrophilic portion of LA’s is a tertiary __
amine
the lipophilic portion is an unsaturated aromatic ring like __
PABA
is the lipo or hydrophilic portion essential for anesthetic activity?
lipo-philic, because it can pass the lipid bilayer
what does an ester linkage look like?
-CO-
what does an amide linkage look like?
-NHC-
Name all the esters
the one eyed ester bunny
Chloe drank four coke’s like a pro
Chloro Tetra Cocaine Procaine
Pipecoloxylidides like mep, bup, and rop are __ molecules that have assymetric carbon atoms and result in both left and right handed configurations.
chiral
“S” enantiomers are __ than “R” enantiomers which are __
“S”safer
“R”risky
How do LA’s work?
inhibit passage of sodium ions through channels in nerve membranes
LA binding to sodium channels is __ specific
stereo
sodium channels are __ when activated AND closing
open
sodium channels are __ when OPENING and RESTING
CLOSED
resting is an equilibrium between __-closed and __-closed
rested-closed and inactivated-closed
LA’s work on sodium channels in the __ open and __ closing states
final open
intermediate closing
Sodium channels partially recover from LA blockade BETWEEN action potentials, but LA’s strengthen their blockade each time sodium channels open DURING subsequent action potentials
recover between AP’s but
deeper block with subsequent AP’s
the more actively firing a nerve, the easier it is to block. again, LA’s have easiest access to sodium channels in the __ open and __ closing states
final open
intermediate closing
Cm = MAC for local anesthetics
minimum concentration needed to block
bigger nerves have __ Cm
bigger
Acidic tissues have __ Cm
higher
how does pKa affect Cm?
the further the pKa from physiologic pH, the more Cm you’re gonna need.
Faster firing nerves have __ Cm’s
lower
because there are more opportunities for the LA to block when the channels are constantly opening and closing.
motor fibers have 2x the Cm of sensory fibers so…
you’re gonna need twice the LA to block motor as you do to block sensory
how many nodes of ranvier must be blocked for adequate conduction blockade?
2-3
pain fibers (A delta and C) require similar Cm’s for blockade, but B fibers (sympathethic) require less
so you’re gonna knock out the sympathetic nervous system before you block the pain signals
In order, what tissues are LA’s absorbed and metabolized into first to last
lungs VRG skeletal muscle fat metabolism elimination
the greater the protein binding, the __ the plasma drug concentrations
lower, because all of the drug is going to be bound up and none will be available.
who has greater systemic absorption, amides or esters?
amides
what do you use for bier blocks? lidocaine which is an amide for better absorption
the greater the protein binding, the __ can transfer through the placenta
less, because it’s all bound up and too big to fit through the placenta.
you don’t even have to worry about esters crossing the placenta because__
hydrolyze very quick, ester hydrolysis.
how do amides metabolize?
hepatic microsomal enzymes
prilocaine fast
lido/mepi middle
etido, bup, rop slooooow
is systemic toxicity more likely with amides or esters?
amides, because esters under quick ester hydrolysis
what’s so special about prilocaine?
metabolite orthotoulidine can convert hemoglobin to methemoglobin
-give methylene blue
what is the most slooooowly eliminated amide?
dibucaine
that’s why we use a dibucaine number to define a person with atypical plasma cholinesterase
why should you consider tetracaine if you want a really loooong acting spinal?
CSF lacks cholinesterase, so the block will last until it is systemically absorbed.
what is the cause of most allergic reactions to LA’s
the ester bunny’s papa
ester metabolite PABA
patient allergic to esters may receive amides
and vice versa
what is the number one cause of systemic toxicity?
intravascular injection - duh
systemic absorption from greatest to least
intercostal
epidural
brachial plexus
ribs, spine, arms
ieb - in emergency break glass
the lower the PaCO2, the higher the seizure threshold, so_
hyperventilate your patient
hyperkalemia (as in end stage renal pt in for shunt revision) has a higher probability of seizing.
don’t try a block with a K of 7.0 and a biphasic qrs
transient radicular irritation symptoms usually disappear in one __
week
__ has a lower incidence of transient radicular irritation than lido
bup
hyperbaric 5% lido is the LA implicated in the horse’s tail
cauda equina from 5% lido
high doses of LA’s produce hypotension due to __ and __ by blocking sodium channels and decreasing cAMP
arteriolar relaxation and myocardial depression
limit epidural bup concentrations to ___% due to cardiac toxicity
0.5% bup, pretty standard concentration of bup
can you use EMLA cream on someone with methemoglobinemia?
Nope, cause it’s prilo and lido
LA’s diffuse from the mantle (outside) to the core (center). Are fibers in the mantle proximal or distal
proximal, they are added to the top layer as the nerve ascends to the spinal cord, thus anesthesia develops proximally and spreads distally
what LA’s can you use for bier block
lido or prilo, same as in EMLA cream
during epidural, the motor blockade may extend 4 segments __ the block
below
for spinals, the SNS block may extend 2 segments __ than the sensory block, but the motor block may extend 2 segments __ than the sensory block
SNS 2 segments higher
motor 2 segments lower
even though capacitance vessel dilation does occur below the level of spinal anesthesia, compensatory vasoconstriction occurs in the upper extremities and DOES NOT involve the central vasculature
DOES NOT
__ circulation alterations are the most important physiologic events occurring with spinals
venous
because unlike arterioles, they cannot maintain intrinsic tone. SO tank em up first with a liter or two
where are the cardiac accelerators?
T1-T4
what is the bainbridge reflex?
decreased venous return = lower heart rate
I know it doesn’t make any sense, that just the way it is.
