Term
List the nerve fibers in order of decreasing susceptibility to local anesthetic block.
In general what is rule on which nerves are blocked first and why? |
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Definition
Small, Myelinated (A gamma motor, A delta sensory)
Large, Myelinated (A alpha, and A beta)
Small, non-myelinated (C)
small mylinated fibers
b/c these fire the fastest (less time for LA to dissociate from Na channel) |
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Term
| What are situations where the rate of fire is high, or abnormal repetitive firing occurs? |
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Definition
Tissue injury or trauma
Various disease states such as neuropathic pain |
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Term
| How may neuropathic pain be treated w LA compared to "normal" nerves? |
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Definition
| May often be treated with local anesthetic concentrations lower than necessary to block normal nerves. |
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Term
| What are the determinants of local anesthetic onset, duration and potency? |
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Definition
pKa
Lipid Solubility
Protein binding
Concentration
Vasoconstrictive or vasodilatory properties |
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Term
lipid solubility & onset
How does incr lipid solubility affect penetration of the lipid membrane?
What can happen to highly lipid soluble drugs?
Net result? |
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Definition
incr Lipid Solubility = rapid penetration of the lipid membrane
but...may also result in sequestration of the drug in myelin and other lipid-soluble compartments, including the axonal membrane itself
Net result of increased lipid solubility is delayed onset of action |
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Term
| What is the net result of increased lipid solubility? |
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Definition
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Term
| What is the affect of increased lipid solubility on duration and why? |
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Definition
Increased duration
due to slow release of drug from the lipid depot |
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Term
potency _____ lipid solubility
_____ Na channel receptor affinity
Ability to alter _____ of Na channel through ___ membrane altering effects
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Definition
Potency parallels lipid solubility
Higher Na+ channel receptor affinity
Ability to alter conformation of Na+ channel through direct membrane altering effects |
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Term
Local Anesthetic Mixtures
Mixed results have been obtained depending on what 3 things? |
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Definition
Which drugs are combined
Ratio of drugs to each other
Site of injection |
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Term
| When mixing local anesthetics, toxicity should be presumed to be...? |
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Definition
Additive
Don't think you can give maximum doses of multiple drugs |
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Term
| What are 3 effects of alkalinization on Local Anesthetic Block? |
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Definition
Speeds onset of the block
Provides a greater depth of block
Increases the spread of epidural block
**also decreases pain with SQ inj** |
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Term
| What is the mechanism for the effects of alkalinization on local anesthetic block? |
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Definition
| Increases the percentage of drug existing in the neutral form which may rapidly diffuse across the neural membrane. |
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Term
| How am I going to acheive this alkalinization of which you speak? |
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Definition
| Add 1 ml of Sodium Bicarbonate to 10ml of LA. |
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Term
| When is the addition of bicarb particulary valuable? |
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Definition
| When used with local prepackaged with epi - the pH of these is lower to extend shelf life of epiniphrine |
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Term
| What is the effect of the addition of vasoconstrictors to LA? |
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Definition
Decreases the rate of vascular absorption
Prolongs duration |
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Term
| Why do we get a denser block with vasoconstrictors? |
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Definition
More drug reaches site of action
May act through a2-adrenergic receptors to produce analgesia |
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Term
| Why do we get a prolonged duration with vasoconstrictors? |
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Definition
| Decreased clearance of drug from site of injection |
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Term
What does the effectiveness of the addition of vasoconstrictors depend on?
When are VC most effective? |
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Definition
Individual drug and site of injection
Most effective for infiltration and peripheral nerve blocks with drugs of short to moderate duration |
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Term
What vasoconstrictor is most commonly added to LA? Concentration?
What other 2 can also be used? |
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Definition
Epi - 1:200,000 = 5mcg/ml
Also Phenylephrine, Norepinephrine |
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Term
| When would we want to avoid the use of vasoconstrictors? |
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Definition
| Avoid in sites which may experience vascular compromise |
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Term
| What is relative potency? |
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Definition
Potency of a given local anesthetic varies with application
This may make certain drugs more appropriate for certain blocks |
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Term
| What is an example of considering relative potency when choosing a particular drug for a particular block? |
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Definition
Lidocaine vs Mepivacaine for a peripheral block
Mepivacaine has a higher potency than Lidocaine for peripheral nerves |
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Term
What is a bier block?
LA diffuses out of the blood vessles to its ____?
What is the most commonly used drug for bier blocks? Do you use epi?
Which LAs have the potential for thrombophlebitis & thus are not used with a bier block? |
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Definition
Exsanguination of the limb followed by interruption of blood flow w/ a double tourniquet
LA diffuses out of the blood vessels to it's effect site
Lido without epi!
ester LAs |
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Term
A minor peripheral nerve block might involve...?
A major peripheral nerve block would involve...? |
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Definition
Single nerves
Two or more nerves (ankle block, 3-n-1 block)
or
A nerve plexus (Brachial plexus - interscalene, supraclavicular, axillary) |
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Term
| What determines drug choice? |
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Definition
| Required duration of block |
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Term
The most rapid onset is with drugs of...?
Name 2 |
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Definition
Intermediate potency
(Lidocaine, Mepivacaine) |
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Term
| When does the addition of epinephrine have the greatest benefit? |
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Definition
| When used with drugs with shorter duration of action |
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Term
How are esters metabolized?
Fastest?
Slowest? |
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Definition
Hydrolysis of the ester bond by plasma cholinesterase
Chloroprocaine fastest
Tetracaine slowest |
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Term
How are amides metabolized?
What happens to metabolites? |
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Definition
Enzymatic degradation in the liver
Renal excretion of metabolites |
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Term
| What is the enzymatic degradation in the liver dependent on for amides? |
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Definition
Hepatic extraction
Hepatic perfusion
Hepatic metabolism
Protein binding |
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Term
What is the clearance and half life of Lidocaine?
of mepivicaine? |
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Definition
Lido
0.85 L/kg/hr
T1/2 1.6 hours
Mepivicaine
0.67 L/kg/hr
T1/2 1.9 hrs |
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Term
What is the initial step in metabolism of lido?
Amides have decr clearance under what 3 conditions? |
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Definition
oxidative dealkylation
hepatic dysfcn
presence of IAs
PIH |
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Term
| What is the absorption of LA dependent on? (5) |
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Definition
Vascularity of injection site
Fat content of injection site
Dose of local anesthetic
Physicochemical properties of the drug
Presence of a vasocontrictor |
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Term
| How does dose affect absorption? |
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Definition
| Nearly linear relationship with systemic absorption and peak plasma levels |
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Term
| How do the physicochemical properties of the drug affect absorption? |
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Definition
| More potent agents have higher lipid solubility and protein binding resulting in less systemic absoprtion |
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