Local Anesthetics Flashcards
What is the mechanism of local anesthetics?
LA block voltage gated sodium channels preventing the influx of sodium ions, thus preventing an action potential
Local anesthetics bind Na channels in the _______ state
activated
How do local anesthetics affect resting and threshold potentials?
not altered
Which nerves are more easily blocked? Myelinated or unmyelinated? small or large fibers? resting or rapidly firing channels? proximal or distal?
Easier: Myelinated Small rapidly firing proximal
What are the three groups in the LA molecule?
Lipophilic group (benzene ring) and hdyrophilic group (tertiary amine) linked together with either an ester or amide
Name the amide LAs
Lidocaine mepivacaine Prilocaine Bupivacaine Ropivacaine Etidocaine i before the caine
Name the ester linked LAs
Procaine Chloroprocaine tetrocaine cocaine benzocaine
Where are the amides metabolized?
liver
Where are esters metabolized? Exception?
esters are metabolized by plasma cholinesterases, except cocaine which is met in liver
Which are more potent, esters or amides? What determines potency?
Esters are more potent. Potency determind by lipid solubility
what properties determine onset?
lipid solubility and pKa
What is the measure of local potency for LA that is similar to MAC?
Cm
What factors determine the absorption of LA?
Dosage Site of injection Drug-tissue binding Local blood flow vasoconstrictors
What side effect can metabolites of prilocaine and benzocaine have?
methemoglobinemia (ortholuidine)
What is the metabolite of esters that may cause an allergic rxn?
para aminobenzoic acid (PABA)
What is the physiologic effect of methemoglobinemia?
decreases O2 and CO2 carrying capacity of hemoglobin
Which group of pts have a higher risk for methemoglobinemia?
neonates due to fetal hemoglobin
What is normal methemoglobinemia?
<1%
What are the S & S of methemoglobinemia?
SOB
cyanosis
chocolatey-brown arterial blood
pulse ox that always reads 85%
What is the treatment for methemoglobinemia?
1-2 mg/kg methylene blue
Does addition of vasoconstrictors affect onset of LA?
no
Vasoconstrictors _________ duration of LA by limiting systemic _________ and maintaining the drug concentration at the _______.
increases
absorption
nerve
Addition of epi (increases/decreases) the chance of systemic toxicity?
decreases
What is concentration of epi used with LA?
1:200,000 or 5 mcg/ml
How much does epi prolong the effect of bupivicaine?
epi has very little effect with bupivicaine
what factors influence the possibility of absorption leading to systemic toxicity?
Dose
Vascularity of site
presence of epi
properties of the drug
Subcutaneous, intravenous, sciatic, tracheal, intercostal, brachial plexus, caudal, epidural paracervical. Arrange from most to least vascular
intravenous>tracheal>intercostal>caudal>paracervical>epidural>brachial plexus>sciatic>subcutaneous
What effects does systemic toxicity have on CNS?
Circumoral numbness
Restlessness
slurred speech
siezure
What effects does systemic toxicity have on the cv system?
Hypotension
Decreased cardiac conduction
Ventricular arrythmias
Which has worse effects on the CV system? lidocaine or bupivicaine?
bupivicaine
What is the limit for dosages for lidocaine?
5mg/kg/ without epi
7mg/kg with epi
What is the limit of the dose of bupivicaine?
2.5 mg/kg
What is the method of action of cocaine?
aside from its local effect cocaine also blocks reuptake of dopamine and norepi
How is cocaine used clinically?
mostly in ENT surgery for the nose due to its vasoconstrictive and local effects
What is the peak time of intranasal cocaine?
30-45 minutes
What are the adverse effects of cocaine?
increased HR, ventricular arrhythmias, and coronary vasoconstriction
How do you treat the toxicity of cocaine?
Supportive treatment
nitroglycerine
Is ephedrine a good choice for a chronic cocaine user?
No. Norepi stores will be depleted so ephedrine won’t work
How long does lidocaine last in a spinal?
<1.5 hours
What is the side effect of lidocaine in a spinal?
transient neurological symptoms
What is a usual concentration of lidocaine for a spinal?
5% with 7.5% dextrose
How long does bupivicaine last in a spinal?
2-2.5 hours
What is a usual concentration of bupivicaine used for a spinal?
0.5% and 0.75% with or without dextrose
How long does tetracaine last in a spinal?
2-3 hours up to 5 with epi
What is the usual concentration of tetracaine used for a spinal?
1% solution, can be mixed with 10% dextrose
When and how does epidural anesthesia during deliveray become a problem for the neonate?
When the baby is in distress, it will become acidotic which traps the drug. toxicity may become a problem, esp with lido
What is the differences in volume used for epidural vs spinal?
Spinal usually uses 1-2 ml (up to 3-4 with bupivicaine according to the chart in the notes) while epidurals require 15-30ml
How is the duration of a bier block determined?
tourniquet time
How much 5% lidocaine or prilocaine would you use for a bier block?
50 cc
Which has a lower plasma concentration in a bier block at the same dose, prilocaine or lidocaine?
prilocaine
How much bupivicaine do you use for a bier block?
don’t use bupivicaine for a bier block, more likely to cause cardiac effects when tourniquet is deflated
The _______ of the drug determines the amount of local anesthetic that exists in the nonionized form in the tissue
pKa
Which drugs are used for topical anesthesia?
prilocaine,cocaine, lidocaine (w/ oxymetazoline
What is local infiltration?
Extravascular placement of local anesthetic in the area to be anesthetized
What anesthetic is in hurricane spray?
benzocaine
Name a procedure where nebulized lidocaine may be used?
awake bronchoscopy
What is emla?
eutectic mixture of local anesthetic (a cream that is put on the skin and held there with a tegaderm)
What is the impediment that makes it difficult for local anesthesia to cross skin
keratin in the skin
How long is the onset and duration of emla?
1 hour, duration ~ 2 hrs
What is the dose for emla?
1-2 gram/10cm2 area of skin
Which patient is contraindicated for emla?
<1mo
Can a patient use an emla if they are susceptible to methemoglobin?
yes, just don’t use prilocaine or benzocaine in the preparation
How much EMLA do you use for broken skin?
Don’t use on broken skin
Which drugs are considered short acting for infiltration anesthesia?
procaine-20-30 min
Chloroprocaine 15-30 min
Which drugs are considered moderate duration for infiltration anesthesia?
Lidocaine-30-60 min
Mepivacaine 45-90 min
prilocaine 30-90
Which drugs are long acting for infiltration anesthesia?
Bupivacaine 120-240 min
Endocaine 120-180 min
What are the first 4 symptoms of local anesthetic toxicity?
circumoral numbness
tinnitius
lightheadedness
visual disturbances
What are the last 5 symptoms of LA toxicity?
Muscle twitching Unconsciousness convulsions Respiratory depression cardiovascular collapse
