07 - Local Anesthetics Flashcards
Local anesthetics work by causing the loss of
sensory
motor
autonomic function
The resting potential of a cell membrane is due to _______. Voltage gated Na and K channels depolarize the membrane, leading to
Na-K-ATPase
action potentials
LA enter the cell and bind to
Voltage-gated Na channels to prevent Na influx and slows conduction
May bind to other channels as well (K, Ca, NMDA)
How many types of nerve fibers are there? List them in order from smallest to largest.
3
C < B < A
What is a differential block?
= Autonomic (C and B) -> Sensory (C and A) -> Motor (A)
Nerve fibers are surrounded by ____ and bundled into ______ which are surrounded by
endoneurium
fascicles
perineurium
Groups of fascicles surrounded by epineureum?
Axons
Nerves can be surrounded by a
myelin sheath
LAs don’t fight their way through the myelin, they bind at ____ between the myelinated segments
Nodes
Myelinated fibers (B) are ____ to LA than nonmyelinated fibers (A, C)
more sensitive
The generalized notion that LA block the smallest fibers first or most is
CLEARLY WRONG
According to Miller
Small fibers (C) are ____ to LA than large fibers (A)
more susceptible
What is the sequence of a differential block?
B -> C -> A
What is the sequence of loss for sensory fibers?
Temp -> pain -> touch -> deep pressure -> proprioception
Local anesthetics are ____. How do they become charged?
Weak bases
Accepting a proton on the amine
The protonated species of a LA is
More potent
pKa of amides
7.6-8.1
pKa of esters
8.2-9.0
The ability of an LA to penetrate membranes depends on its
lipid solubility, charge, size
Adding HCO3 with an LA ____ onset, but can cause
speeds
precipitation (bupivicaine)
The density of a block depends on
concentration - need enough drug to saturate binding sites and prevent transmission of AP
volume - need to block a critical length of nerve fiber
The duration of action of a block correlates with
lipid solubility
LAs are rapidly absorbed through _____ but poorly through
Mucous membranes (conjunctiva, trachea, oropharynx) Intact skin
What is EMLA cream?
Stands for Eutectic Mixture of Local Anesthetics - eutectic mixture of equal quantities by weight of lidocaine and prilocaine.
What is the onset of EMLA cream and its depth of penetration?
1 hour
3-5 mm
EMLA cream can cause?
Methemoglobinemia
Systemic absorption of LA is proportional to ______ and
Vascularity, vasoconstriction
IV > tracheal > intercostal > caudal/epidural > peripheral nerves > subcutaneous
Avoid epi on
fingers, toes, penis, nose
The only local anesthetic that causes vasoconstriction is
COCAINE!!!
Vasoconstriction with LA causes increased ____ and decreased _____. This is less pronounced with
Neuronal uptake
Redistribution
Bupivicaine
Which LA are esters and which are amides?
Esters have one i and amides have two i
How are esters metabolized?
Ester hydrolysis by pseudocholinesterase
Excreted in urine
There is no pseudocholinesterase in
CSF
Procaine/benzocaine can cause formation of PABA (4-aminobenzoic acid) which can cause
allergic reaction
Which ester is partially metabolized in the liver?
COCAINE
Amides are metabolized in the
liver
T or F. Metabolization of esters is faster than amides.
True, ester hydrolysis is rapid
Toxicity of LA is treated by
Benzos (caution with thiopental, propofol)
Toxicity spectrum of LA
Circumoral/tongue numbess, dizziness, tinnitus, blurred vision
Restlessness, agitation, nervousness, slurred speech, drowsiness, LOC
Muscle twitching, seizures
Lidocaince can ____ CBF and minimize increase of ____ with intubation
decrease
ICP
A lidocaine infusion can decrease MAC by
40%
What are the effects of COCAINE?
euphoria, restlessness, tremors, convulsions, respiratory failure
Repeated doses of lidocaine and tetracaine can cause
Cauda equina syndrome
What can be used to treat ventricular arrhythmias?
IV lidocaine
LA can sometimes cause cardiac arrest. The cardiotoxicity is reversed by
Intralipid
This LA rapidly blocks Na channels and can be difficult to resuscitate
IV bupivicaine
Are esters or amides more likely to cause allergic reaction? Why?
Esters
They are derived from PABA
PABA = procaine and benzocaine allergy
How does cocaine work?
Prevent NE reuptake
Methemoglobinemia is commonly caused by ______. What do you use to treat it?
Prilocaine and benzocaine, sometimes lidocaine
Methylene blue
Met Hgb >10%
Met Hgb > 50% is severe
