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Flashcards in Local Anesthetics Deck (93)
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Local Anesthetics:
what is the determinant for the speed of onset?

amount of LA in NON-Ionized form
AKA-- the pKa


Local Anesthetics: Onset (pKa)
agents w/ ______ pKa are MORE NON-Ionized at a pH of 7.4. (the more non-ionized the faster it penetates the lipid bilayer.



Local Anesthetics: Onset (pKa)
Agents w/ a high pKa have a _______ onset

slower onset


Local Anesthetics: Onset (pKa)
Agents with a low pKa have a _____ onset



Local Anesthetics: Onset (pKa)
why is it that a lower pKa has a faster onset

bc if you remember the line and place the pKa on it it will be more non-ionized.
Also if pH is 7.4 and pKa is 9 then obviously less than 50% is non-ionized, but if pKa is 7.4 then 50% is non-ionized


Local Anesthetics: Onset (pKa)
place these in order from slowest onset to fastest onset
Drug pKa
Procaine 8.9
Mepivacaine 7.6
Lidocaine 7.7
Tetracaine 8.6
Etidocaine 7.7
Ropivacaine 8.1
Bupivacaine 8.1
Chloroprocaine 9.1

onset % non-ionized (just to visualize)
Procaine 8.9 3
Tetricaine 8.6 14
Bupivacaine 8.1 17
Ropivacaine 8.1 17
Chloraprocaine 9.1 2 (the one exception) fastest
Lidocaine 7.7 24
Etidocaine 7.7 33
Mepivacaine 7.6 39


Local Anesthetics: Onset (pKa)
in general, the LOWER the pKa of the LA, the greater the proportion of LA in the NON-IONIZED form at pH=7.4, and the _______ the onset of the conduction block.



LA block nerve conduction by blocking (impairing) propagation of the action potential along axons. the block is accomplished by directly acting on _______ channels and inhibiting ____ influx

Sodium Channels


the ____ of the LA determines the speed of onset?



A LA that has a HIGH lipid solubility is very ____. Lipophilic LA more readily cross nerve membranes



Local anesthetics the are highly ____ ____ will have a prolonged duration of action.

Protein Bound


pKa relates the what?
Lipid solubility relates to what?
Protein binding relates to what?

pKa - onset (low pKa fast onset)
Lipid solubility - potency (more lipid soluble more potent)
Protein binding - duration (more bound longer duration)


what is the determinant for potency?

lipid solubility


Local Anesthetics: Lipid Solubility (potency)
what is a good measure of lipid solubility?

oil:water partition coefficient


Local Anesthetics: Lipid Solubility (potency)
the greater the oil:water partition coefficient the ____ the lipid solubility



Local Anesthetics: Lipid Solubility (potency)
place in order from least to most potent
LA Oil: water partition Coefficient
Etidocaine 140
Chloroprocaine 1
Mepivacaine 1
Tetracaine 80
Lidocaine 4.0

Drug least potent to most potent
Chloroprocaine 1
Mepivacaine 1
Lidocaine 4
Tetracaine 80
Bupivacaine 30


Local Anesthetics: Lipid Solubility (potency)
explain the potency as it applies to our anesthetics like Lidocaine and Bupivacaine

Lido is less potent oil:water is 4.0. we need large amounts when we use it for ex usually comes in 1%, 2%, 4% etc
Bupivacaine is more potent, oil:water is 30. we need small amounts when we use it for ex usually comes in 0.25%, 0.5%, 0.75%


Local Anesthetics: Lipid Solubility (potency)
In general the more lipid soluble the LA the greater it's _____



what determines the duration of action of a LA

both protein binding and lipid solubility (but primarily we are concerned w/ protein binding)


Local Anesthetics: Duration of action
what is the MOST important factor in determining the duration of action of a LA

Protein binding


Local Anesthetics: Duration of action
the greater the protein binding the _____ the duration of action



Local Anesthetics: Duration of action
how does protein binding control the duration of a LA

immediately after injection of LA, much of the agent binds to proteins in the vicinity of the injection site. as the unbound anesthetic diffuses from the injection site, some protein bound LA is released and becomes available to diffuse to nerve axons. thus proteins serve as storage depots for the LA


Local Anesthetics: Duration of action
so we already discussed that duration is primarily dependent on Protein binding, but what is the other factor that plays a role in duration of action?

Lipid solubility


Local Anesthetics: Duration of action
agents w/ greater lipid solubility tend to have ____ durations of action



Local Anesthetics: Duration of action
How does lipid solubility control duration of action of a LA?

After the LA is injected some of it will dissolve in lipids in and around the site of injection. Agents w/ higher lipid solubility will dissolve to a greater extent in surrounding tissue (lipids). thus the lipid act as a reservoir for lipid soluble agents, just as proteins act as a reservior for agents that bind to proteins. As the LA diffuses away from the site of injection, LA will diffuse out of the lipid compartment down the concentration gradient and will act on the nerve to maintain he nerve block.


Local Anesthetics: Duration of action
what single change in property of a LA will result in more POTENT and LONGER acting agent?

an increase Lipid solubility. An Increase in lipid solubility will increase the duration of action and the potency


Weak Bases ( Benzos, LA, Opioids, Ketamine) bind to what protein?

Alpha 1 - glyco protein


Weak acids (thiopental, Propofol, barbiturates) bind to what protein?



Local Anesthetics: Duration of action
the duration of action is greatest for LA that exhibit the greatest _______ and highest _____ . However ______ is more important when it comes to duration of action

Protein Binding and Lipid solubility
Protein binding


Determinants of blood concentration of LA:
blood concentration is determined by what 4 things

pressance or absence of vasoconstrictor
tissue blood flow
concentration of injection
number and frequency of injections

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