Local Anesthetics Flashcards
In what order are perceptions lost with local anesthetics
- Pain
- Cold
- Warmth
- Touch
- Deep Pressure
The ability of an anesthetic to penetrate the axon membrane is determined by three properties:
Molecular size
Lipid solubility
degree of ionization at tissue pH
Where are esters metabolized?
in the blood by esterases
Where are amides metabolized?
In the liver
When absorbed in sufficient quantities, local anesthetics cause CNS ______ followed by ________
excitation followed by depression
In large quantities, how do systemic local anesthetics effect the heart?
Risk for decreased conduction: bradycardia, heart block, reduced EF
What effect do local anesthetics have on blood vessels?
Vasodilation
Which local anesthetic can cause methemoglobinemia?
Topical benzocaine (usually in children < 2 years)
Allergic reactions are much more common in response to ____ anesthetics than _____
more common in ester than amide
If a patient is allergic to Lidocaine, how likely is it that they are allergic to Tetracaine?
Very low. Cross-hypersensitivity between amides and esters is very low
If a patient is allergic to Lidocaine, how likely is it that they are allergic to Mepivicaine?
VERY VERY likely
If you’re allergic to one local anesthetic in a class, you’re allergic to all the others in that class
List the Amide LAs:
Lidocaine
Bupivicaine
Mepivicaine
Prilocaine
Ropivicaine
List the Ester LAs
Procaine
Chloroprocaine
Tetracaine
What was the first local anesthetic?
Cocaine (ester type)
Is procaine more effective topically or by injection?
Injection only. Not effective topically
What are five tips for avoiding LAST?
- Apply the smallest amount needed
- Avoid application to large areas
- Don’t put on broken or irritated skin
- Avoid strenuous exercise
- Avoid wrapping or anything that increases skin temperature
Conduction velocity is increased by:
myelination and diameter
What is the function of A alpha nerves?
Skeletal muscle MOTOR
AND
Proprioception
What is the function of A beta nerves?
Touch and Pressure
What is the function of A gamma nerves?
Skeletal muscle TONE
What is the function of A delta nerves?
Fast pain
AND
Temperature
AND
Touch
What is the function of B nerves?
PRE ganglionic ANS fibers
What is the function of C fibers?
POST ganglionic ANS
AND
Slow Pain/Temp/Touch
Which nerve fibers are heavily myelinated?
A alpha and beta
Which nerve fibers have medium myelination?
A gamma and delta
Which nerve fibers have light myelination?
B Fibers
Which nerve fibers have no myelination?
C Fibers
What is Cm?
The minimum effective concentration of a local anesthetic, the ED95 of LAs
In the clinical setting, LAs inhibit nerves in the following order:
B
C
A (g+d)
A (a+b)
What is the resting membrane potential in a peripheral nerve?
-70 mV
What is the threshold potential of a peripheral nerve?
-55 mV
What is the primary determinant of resting membrane potential?
Serum K
An increase in serum K ______ the RMP
decreases the RMP (makes it more positive)
A decrease in serum K ______ the RMP
increases the RMP (makes it more negative)
What is the primary determinant of threshold potential?
Serum Ca
An increase in serum Ca ______ the threshold potential
makes the threshold potential more positive
A decrease in serum Ca ______ the threshold potential
makes the threshold potential more negative
Where do local anesthetics bind?
On the INSIDE of the alpha subunit of sodium channels
When local anesthetics are placed in solution, they dissociate into:
an uncharged base and a conjugate acid
When local anesthetics are placed in the plasma, what happens?
Since all LAs are stored in acidic environments, they enter the plasma as conjugate acids (LA+).
In plasma, about 50% of it dissociates into an uncharged base (LA minus its hydrogen atom). This makes it lipid soluble.
How are bases stored in the vial?
Remember, polarized atoms are dissolvable in water. You want the weak base to be in an acidic environment to keep it water soluble. All LAs are stored in acidic environments
Once an uncharged base travels through the axolemma, what happens?
ICF is actually MORE acidic than ECF. So even though the uncharged base didn’t ionize in the ECF, it does when it hits the ICF. It adopts a hydrogen atom, becoming LA+, which makes it able to bind to the alpha subunit
Local anesthetic molecules have three components:
Which portion of a LA is liphophilic?
The aromatic ring
Which portion of the LA is hydrophilic?
The tertiary amine
The intermediate chain of a LA determines:
Drug Class
Metabolism
Allergic Potential
Onset of action is determined by:
pKA
Duration of action is determined by:
protein binding
Potency is determined by:
Lipid solubility
What is the pKa of Bupivacaine and Ropivacaine?
8.1
What LA has the highest degree of protein binding?
Bupivacaine at 96%
Follow by Ropivacaine at 94%
Which LA has the highest pKa?
Procaine at 8.9, followed closely by chloroprocaine and tetracaine
All of the esters have higher pKa than the amides
Which LA does not undergo protein binding?
Chloroprocaine
There is only one LA with a pKa well below 7. Which one is it?
Benzocaine, with a pKa of 3.5!
Which sites have the highest uptake of LA?
The effect of added epinephrine is greatest with which LAs?
The ones that have the highest degree of intrinsic vasodilating activity, such as Lidocaine
If a patient receives exparel, what are the rules for subsequent lidocaine administration?
If they get exparel, they can’t have any other lidocaine for at least 96 hours
If the surgeon uses lidocaine infiltration intraoperatively, how long until exparel can be administered?
At least 20 minutes
LAs preferentially bind to which serum protein?
Alpha-1-acid Glycoprotein
What is the maximum dose of exparel?
2 vials
What is the maximum allowable dose of lidocaine?
4.5 mg/kg, never more than 300mg
What is the maximum allowable dose of Bupivacaine?
2.5 mg/kg, never more than 175 mg
What is the maximum allowable dose of Ropivacaine?
3 mg/kg, never more than 200 mg
Risk of LAST is increased by:
Hypoxia
Hypercarbia
Acidosis
HYPERkalemia
What is the initial intralipid bolus dose?
1.5ml/kg
If a patient in LAST is having dysrhythmias, what is the drug of choice?
Amiodarone
DO NOT use calcium channel blockers or beta blockers
Which drugs should be avoided in LAST resuscitations?
Vasopressin
AND
Epi (decreases effectiveness of intralipid)
If a patient is over 70kg, what should their initial bolus of intralipid be?
Just start with 100ml and move on to a drip of 250ml over 15-20 minutes
What is the maximum dose of intralipid therapy?
12mg/kg
What is the maximum recommended dose of lidocaine in tumescent solutions?
55 mg/kg
Which anesthetics are most likely to produce methemeglobinemia?
EMLA
Benzocaine
Cetacaine
What is methemeglobinemia?
When the Fe2+ on hemoglobin is oxidized to Fe3+, which does not allow hemoglobin to bind oxygen
What are the s/s of methemoglobinemia?
Chocolate blood!
Cyanosis
Refractory hypoxia
Tachycardia
Tachypnea
Altered LOC
What is the treatment for methemoglobinemia?
1-2 mg/kg methylene blue of 2-5 minutes
Methemoglobinemia causes the p50 to shift:
To the left
What drugs besides LAs can cause methemoglobinemia?
Nipride and Nitroglycerine
EMLA cream is a combination of:
2.5% Lidocaine
2.5% Prilocaine
Drugs that prolong the duration of LAs include:
Epinephrine
Decadron
Dextran
Drugs that provide supplemental analgesia with LAs include:
Epinephrine
Clonidine
Opioids
How much bicarb can be added to LA?
You can add 1ml of 8.4% bicarb for every 10ml of LA
What is the effect of adding hyaluronidase to LA?
Improves LA diffusion through tissues
As pKa approaches 7.4, does the speed of onset increase or decrease?
It increases, because a larger fraction of the solution will exist as lipid soluble, uncharged solution
