EXAM 3 -Local Anesthetics- COMP Flashcards
Comment on desirable characteristics of LA as far as OA, DOA, toxicity degree and reaction
Desirable characteristics are:
Very fast onset of action
Long duration of action
NO tachyphylaxis
What was the first Local anesthetics
Cocaine
What was the first synthetic ESTER
Procaine
Gold standard of local anesthetics? (amide or ester)
Lidocaine ; amide
• Local anesthetics and water solubility ? are they acids or bases?
Poorly water soluble
They are BASES
When mixed with Hydrochloric acid
Become water soluble
Because local anesthetics are poorly water soluble they are supplied as
Hydrochloric salts
Local anesthetics since they are supplied as hydrochloric salts become
Acidic ph 6.0
How does the fact that they are acidic affect Local anesthetics
More painful on injection
Slower onset of action (because they become ionized and only unionized can penetrate)
Benefits of Alkalization of local anesthetic solution
Makes onset of action shorter
Less pain when you inject
Some providers may add ________to alkalinized
Sodium bicarbonate
What is the Functional unit of peripheral Nerve?
Axon
Extension of Centrally located neuron
Axon
Function of schwann cells
Insulation and Support
In unmyelinated cells, one schwann cell for
multiple axons
In LARGER cells, one schwann cell cover___________ with several layers of ______ (lipid)
one axon with several layers of MYELIN
Small segment of Axon without myelin
Node of Ranvier
What does the Node of ranvier has making it able to generate impulse? how does the impulse travel? what is the term use for that?
Contains large number of Sodium channles
Impulse travel from node to node
Saltatory conduction
What is the PRIMARY area where LOCAL anesthetics EXERT THEIR ACTIONS?
Node of RANVIER
Which one is more difficult to block with local anesthetics, myelinated or unmyelinated?
MYELINATED
Myelinated fibers are _____and conducts impulses _____
larger; faster
How many nodes of ranvier must be blocked for prevention of nerve conduction?
2-3 Nodes of ranvier
Bundle of axons together is called
Fasciculi
What are the 3 layers of the Fasciculi?
Endoneurium
Perineurium
Epineurium
Role of epineurium
Act as a barrier that Local anesthetics must get through to work.
Resting membrane potential or peripheral nerve? Cause by?
-70 to -90 ; ionic imbalance as accomplished by Na-k+ ATPAse
Local anesthetics exert their effects by
Blocking the Na+ Channel
Local anesthetics have greater affinity for Na+ channels in _____and _____states. Known as the ________hypothesis (extra)
Active
Inactive
Guarded receptor
Some receptors have affinity for both the ionized and UN-ionized form of the local anesthetics? Which form can penetrate the membrane
UNIONIZED
MOA of benzocaine______only occur in the ______Form
interact with ion channel within the membrane
uncharged form
Define minimum blocking concentration (Cm)
The lowest concentration of drugs needed for blocking impulse propagation. think of it like MAC for local anesthetics
Because the ______form of the molecule crosses
the cell membrane, compounds that are more lipophilic have a _____onset of blockade
nonionized ; faster onset
Can local anesthetic bind when Na+ in resting state?
NO
The more frequently the sodium channel depolarized the
the more time for LA have to bind, the faster the drug action will be to block
Effect of : Increased nerve fiber diameter on Cm
Increase Cm
Effect of Increased myelination on Cm
Increased Cm
Greater distance between nodes of Ranvier on Cm
Increased Cm
Summary of factors that increase Cm
Increased nerve fiber diameter
Greater distance between node of ranvier
Increased myelination
Effect of “increased tissue pH” on Cm:
Decreased Cm
Effect of “ high frequency of nerve stimulator” on Cm
Decreased Cm
Effect of “ Pregnancy” on Cm
Decreased Cm
Effect of “ elevated tempature” on Cm
Decreased Cm
Summary of factors that decrease Cm
Increased tissue ph
high frequency of nerve stimulator
Pregnancy
Elevated temperature
Nerve fibers classified based on
Diameter and myelination
Conduction faster on
Wider diameter and myelination
What are 3 classes of PN fibers?
A, B, and C
What are the subtypes of PN A fibers
Alpha
Beta
Gamma
Delta
Largest fibers
A- Alpha
Fastest conduction velocity
A-Alpha (60-120m/s)
Most myelinated
A-alpha
A-ALPHA RESPONSIBLE FOR (MP)
MOTOR FUNCTION
PROPRIOCEPTION
what is the fiber last blocked by LOCAL anesthetics?
A-ALPHA
A-BETA CONDUCTION velocity is
30-70m/s
A-BETA responsible for
Touch
Pressure
A -GAMMA responsible for
Skeletal muscle tone
Reflexes
A- gamma CV
15-35 m/s
A- Delta responsible for
Pain, temperature and Touch
The fibers constitutes pre-ganglionic autonomic nerves
B fibers
Fibers that local anesthetics BLOCK FIRST
B fibers
Which are the ONLY UNMYELINATED FIBERS?
C fibers
Slowest conduction out of all fibers
C fibers (0.5 m/s)
2nd fibers to be BLOCKED is ____ along with _____Fibers
C fibers
A-delta fibers
C fibers conduct
Pain , temperature, touch and post ganglionic symp neurons
Clinically , sequence of Differential blockade is
***Autonomic function BLOCKED FIRST by B fibers
Pain, touch and Temperature by A delta and C fibers)
Motor and proprioception BLOCKED LAST by A-alpha, beta and gamma
Blocked first in differential blockade
Autonomic function
Blocked last in differential blockade
Motor and proprioception
Best example of differential blockade is
Bupivacaine
0.125 % blocks autonomic function some pain and touch BUT NOT motor and proprioception, same with 0.25%
However, 0.5% block them all
NEURONS blocking order
Blocking order
1st –> B fibers, autonomic function block
2nd –> A-delta fast pain and temp; C slow pain
3rd –> A-gamma, muscle tone and motor
4th –> A-beta, sensory touch and pressure
5th –> A-alpha, motor and , skeletal muscle
Block throbbing pain and temperature
C fibers
Para and sympathetic PREGANGLIONIC Neurons are______ but POST GANGLIONIC neurons are _____
B fibers; C fibers
Block SHARP, prickling pain and temperature
A-delta
What are the 3 characteristics segments of Local anesthetics?
Aromatic ring (lipophillic) Intermediate Carbon group (ester or amide) Tertiary amine (hydrophillic)
All local anesthetics are (acids/bases)
weak bases
***Best determinant of POTENCY for LOCAL anesthetics is
Lipid Solubility (Oil water partition is m
The more lipid soluble the agent
Onset of action determines by
the more drug enters the axon
Ionization
***The duration of action of local anesthetics is related to
PROTEIN BINDING
Linkage characterizes drugs as either
Ester or amide
How can you tell ester from amide?
Ester “one i”
amide “ two i’s)
Ester are Hydrolized by
Plasma cholinesterase (blood)
Hydrolysis of Ester results in which metabolites? and what is it associated with>
PABA ( Paraaminobenzoic acid); asscociated with allergic reactions
Amides are metabolized by
Liver
Amides allergic reaction are RARE and are due to
Methylparaben- structurally similar to PABA
Systemic toxicity ( more likely or less likely with agents)
Amide (more likely )
Ester (less likely )
Which one is chemically stable in solution
Amide
Onset of action of amide vs ester
Amide (mod to fast)
Ester (slow)
pKa of Ester compared to amide
Ester 8.5-8.9
Amide close to ph 7.4 which is 7.6 - 8.1
Distribution of local anesthetics is dependent on________and there is _________> what is a major factor?
Blood flow
High initial uptake by lung
Redistribution
Relative clinical potency and chloroprocaine; Oil/ water partition
1; 1
Relative clinical potency and Mepivacaine; Oil/ water partition
2; 1
Relative clinical potency and Lidocaine Oil/ water partition
2;4