Local Anesthetics Flashcards
Local anesthetics cause a loss of sensation in a discrete region of the body (regional anesthesia), how is this done?
Local anesthetics cause a loss of sensation in a discrete region of the body (regional anesthesia)
caused by inability of nervous impulses to be transmitted to the brain*
Local anesthetics cause a loss of sensation in a discrete region of the body (regional anesthesia), what can mimic the LA?
may be mimicked by anoxia, trauma, hypothermia, or chemicals irritants (such as alcohol or phenol)
RMP (resting membrane potential) determined by what electrolyte concentration, will differential across nerve cell membrane (neurolemma)*
RMP (resting membrane potential) determined by K + concentration differential across nerve cell membrane (neurolemma)*
What is the resting membrane potential voltage inside the cell?
Inside of cell is -70mv relative to outside
If the neuron is stimulated, what happens? in terms of electrophysiology.
If neuron is stimulated (depolarized) (Na channels open, Na goes in the cell while K goes out)
- action potential is triggered
- Negative wave moves along outer cell membrane
- -Membrane becomes permeable to Na +
- This depolarizes the neurolemma
- -Na+ permeability gets turned off and K+ gets turned on to turn off depolarization (turns off to reset the RMP)
How fast can a nerve conduction happen?
One millisecond has elapsed (this happens quickly) during an actionpotential
Local anesthetics are poorly soluble in what solution?
Local anesthetics are poorly soluble in water.
Generally, local anesthetics with a chiral center, which enantiomers is less toxic?
Generally, local anesthetics with a chiral center, the S (Left) enantiomers are less toxic than the R.
What three chemical structures do local anesthetics consist of?
Aromatic residue:
Intermediate chain:
Amino group:
What is the Aromatic residue in the Local Anesthetics?
Aromatic residue: benzoic acid derivative; lipophilic*
What is the intermediate chain in the local anesthetics?
Intermediate chain: contains ester OR amide bond (this connects the the aromatic residue and the amino group)
What is the amino group in the local anesthetics?
Amino group: secondary or tertiary amine; hydrophilic*
What part of the local anesthetic chemical structure is hydrophilic?
Amino group: secondary or tertiary amine; hydrophilic*
What part of the local anesthetic chemical structure is lipophilic?
Aromatic residue: benzoic acid derivative; lipophilic*
What is the mechanism of action for local anesthetics?
Inhibits voltage-gated sodium channels*
Reversible blockade of neural tissue
Do local anesthetics work inside or outside the cell?
Must be delivered to inside of cell membrane to be effective* (non-ionized reaction will pass through membrane)
Un-charged base is needed to penetrate neurolemma
Charged salt is needed to bind to receptor site (from the inside)
How can local anesthetics block potassium channels??
Local anesthetics Can also block K+ channels (need to give a lot for this), higher concentrations needed to do so (we don’t go near this)
How are Local Anesthetics classified as?
Aminoesters (COO) or aminoamides (NHCO)
- -weakly basic compound
- -different metabolic pathways
What plasma proteins do local anesthetics bind to?
Bind to plasma proteins (a1-acid glycoprotein and serum albumin)
In marketed epi-containing solutions, the pH is even where? and almost no molecules exist in what form?
In marketed epi-containing solutions, the pH is even lower, and almost no molecules exist in the un-ionized form (the pH is even lower to preserve the epinephrine)
How does charged and uncharged portions of local anesthetics interact with cell membranes?
Uncharged species penetrates lipid membrane
Charged species interacts with receptor
Which chemical substitution will increase the the lipid solubility and potency of local anesthetics?
Increasing alkyl substitution increases lipid solubility and potency*
on aromatic ring or tertiary amine
lengthening chain increases potency and toxicity*
What determines the onset of a local anesthetics?
Onset is pKa (the more non-ionized the faster the onset)
What determines the potency of a local anesthetic?
Potency is length of chain and Lipophilicity (the longer the chain the more potent the drug, will also increase the toxicity)
What determines the duration of a local anesthetic?
Duration is Protein Binding (the more protein bond the longer it will last or the more it will hang around)
Compounds with asymmetrical carbons (chiral) have optical isomers (enantiomers), will this have a different properties than a racemic mixture?
Compounds with asymmetrical carbons (chiral) have optical isomers (enantiomers), MAY have different properties than racemic mixture
D(+) isomer has greater CV toxicity, prefer the S (Left) isomer**
State dependent blockade: block depends on the state of the sodium channel receptor, What are the three states in which the receptor can be in?
State dependent blockade: block depends on the state of the sodium channel receptor: resting vs. activated vs. inactivated
Frequency and voltage-dependence of Na-channel block, when do local anesthetic have a
higher affinity when channels are (what state) or which state of the cell (in terms of voltage)?
Frequency and voltage-dependence of Na-channel block
higher affinity when channels are inactivated or when cell is hyper-polarized
Degree of blockade by local anesthetic depends upon how much the nerve has been stimulated, what state of the nerve will have a less sensitive to a block?
resting nerve less sensitive to blockade than one repeatedly stimulated
Degree of blockade by local anesthetic depends upon how much the nerve has been stimulated, what does a higher frequency of stimulation mean in terms the degree of blockade?
higher frequency stimulation = greater degree of blockade
What are the 5 things the LA block depends on in terms of susceptibility?
- Length of nerve fiber exposed to LA
- Presence of myelin (makes the block better)
- Size of fiber
- Position of fiber in nerve bundle (we inject outside the bundle) (the center of the bundle will take longer to block)
- Specific LA (onset)
What are the types of afferent fibers, in order, that will get block first from a local anesthetic?
- —(first) B: pre-ganglionic myelinated, vasocontriction, skin temp (when you block this, you will have vasodilation and feel hot)
- –(2nd) C: post-ganglionic un-myelinated. Nociceptors of paleo-spinothalamic tract (slow pain).
- –(third) A-delta (Aδ): free nerve endings of touch and pressure. Cold thermoreceptors. Nociceptors of neo-spinothalamic tract (fast pain).
- —A-gamma (Ay) fibers (pressure?)
- -Aβ: cutaneous mechanorecptors
- -Aα: muscle spindle (also motor)
Toxicity (meaning this has to hit the blood stream) is based on dose and rate of absorption (depends where you put it, if you put it IV its going to be the fastest), this can be decreased by what?
Toxicity (meaning this has to hit the blood stream) is based on dose and rate of absorption (depends where you put it, if you put it IV its going to be the fastest)
decreased by vasoconstrictors (slow down absorption)
Toxicity for a local anesthetic is related to non-bound or bound drug???
Toxicity of a local anesthetic is related to non-bound drug concentration (unbond drug is what will cause effect)
Esters hydrolyzed by?
Esters hydrolyzed by plasma esterases, probably cholinesterase
Local anesthetics are bound by what two things?
Bound by plasma proteins and lung tissue
Amide-linked local anesthetics are degraded by what organs?
Amide-linked local anesthetics are degraded by hepatic endoplasmic reticulum
- –N-dealkylation and hydrolysis (makes more water soluble)
- –lung uptake may also be important
Amide-linked local anesthetics are
bound extensively to plasma protein?
Amide-linked local anesthetics are
bound extensively to a1-acid glycoprotein (binds to weak bases)
LA’s decrease electrical activity of all excitable tissues (including cardiac) At low doses LA’s are effective at what?
LA’s decrease electrical activity of all excitable tissues (including cardiac), At low doses LA’s are effective anticonvulsants by raising the seizure threshold (B/C it blocks Na channel)
As plasma levels rise, local anesthetics cause what? (in terms of toxicity s/s)
As plasma levels rise, local anesthetics cause light-headedness, dizziness, paresthesias (usually get ringing in the ears), Further increase cause disorientation, LOC, and tonic-clonic seizures
LA’s depress electrical conduction in myocardial tissues
greater affinity for sodium channels when channel is in what position?
LA’s depress electrical conduction in myocardial tissues
greater affinity for sodium channels when channel is in open position (frequency dependent blockade) tachycardia potentiates cardiac toxicity*
What kind of kinetics does Bupivacaine display?
Bupivacaine displays fast-in slow-out kinetics (this is not good)
- -compared with lidocaine
- -potentiated by hyperkalemia
- -Progesterone may increase susceptibility (pregnancy makes it worst)
What is the name of the preservative that local anesthetics contain?
Methylparaben
What is the purpose of the preservative in local anesthetics?
Methylparaben is used to inhibit microbial growth
What is one side effect of methylparaben?
Methylparaben can be a potent allergen
Preseverative containing local anesthetics should not be used for which type of blocks?
Preservative containing local anesthetics should NOT be used for neuraxial blockade (spinal, epidural or caudal)*
What does Procaine get rapidly metabolized into? why is this bad?
Rapidly metabolized to PABA (this is an allergenic agent?), prob by plasma cholinesterase
What is the concentration of 2-chloroprocaine?
2-3%
What is the onset of 2-chloroprocaine?
5-15min
What is the duration of 2-chloroprocaine?
30-45-min
30-90 w/ Epi