LA Review Flashcards
How do local anesthe-cs (LA) work? ••
LA work by preven-ng voltage-gated fast channels in the nerve axons from opening. LA bind to Na channels inside the neuron when they are in the inac-vated state.
• What part of the nerve cell is affected by LA? •
The nerve axon
What is the primary determinant of LA potency? •
Lipid solubility
Which LA is the standard for comparison of potency?
Lidocaine
What term defines the lowest concentra-on of LA that blocks impulse conduc-on along a give nerve fiber?
Minimum blocking concentration
What three LA are effec-ve when applied topically? 2
Tetracaine, lidocaine, cocaine
• How are nerve fibers classified?
Three groups of nerve fibers A, B, and C. Separated on basis of diameter. A fibers further divided into alpha, beta, gamma, and delta.
• Which fibers are blocked last by LA?
A-alpha
Are the largest 15-20um
A-alpha
The most myelinated, fastest conduction velocity,
A-Alpha
Responsible for motor function and proprioception.
A-alpha
The only unmyelinated fibers
C fibers-
Pain and temperature sensation.
Delta
are smallest in diameter 3-4um and provide pain and temp sensation
Delta fibers
Motor function, touch, and pressure sensation.
A beta and GAMMA
Slowest conduction
C fibers
Responsible for reflexes
GAMMA
What LA has the longest half–me elimination? • •
Bupivicaine
What 6 factors determine the absorption of a LA into surrounding vasculature and therefore influence blood levels of LA?
1) Total dose
2) Tissue blood flow
3) The presence of epinephrine or phenylephrine
4) Lipid solubility
5) Protein binding
6) pH
• The onset and extent of diffusion of a LA to its site of ac-on depends primarily on what 3 factors? •
1) pKa of the drug
2) Concentration
3) Lipid solubility •
Why does LA that is absorbed into the venous circulation initially have limited effects in systemic circula-on? •
Substantial uptake into lungs
What property of LA most determines duration of ac-on of LA? What other proper-es will determine duration of ac-on?
Protein binding; Lipid solubility (greater lipid solubility, the greater the dura-on of ac-on).
What property of LA most determines onset of ac-on of LA?
Ioniza-on LA with higher pKa will be more ionized that LA with lower pKa and therefore have a slower onset of ac-on. Bupivicaine (pKa of 8.1) will have a slower onset of ac-on than lidocaine (pKa 7.9)
Why is epinephrine added to LA solutions?
1) Decrease the absorption of LA and prolong the effects,
2) decrease systemic toxicity
3) permit higher doses of LA
What drug would you add to a LA for vasoconstriction if cardiac accelera-on is not desired?
Phenylephrine •
What is the maximum dose of epinephine when used with a LA for prolonging infiltration, brachial plexus, epidural, caudal, or intrapleural anesthesia
200-250mcg (3-5mcg/kg)
What is the optimal concentration of epinephrine in local anesthetic solutions? •
1:200,000 (5mcg per ml)
How many ml of 1:100,000 epinephrine can be safely injected into adult pa-ents?
200-250mcg or 20-25ml of 1:100,000 ml solution (1:100,000 = 10mcg per ml)
What LA can cause methemoglobinemia?
Prilocaine
What is the only LA that produces vasoconstriction?
Cocaine
What LA is an ester of Benzoic acid?
Cocaine
• What is the maximum safe dose of Cocaine applied nasally or topically?
1.5mg/kg
• Which amide LA is most cardiotoxic?
• Bupivicaine
• Compare the frequency of hypersensitivity reac-ons with esters compared to amides?
Rare with esters, even rarer with amides •
• List 4 manifestations of hypersensi-vity from LA?
1) Localized edema
2) urticaria
3) bronchospsasm
4) anaphylaxis
• What are most potent and most toxic LA?
Tetracaine, etidocaine, and bupivacaine are equipotent. Etidocaine causes seizures at the lowest serum concentra-on among LA. Etidocaine and bupivacaine have the lowest maximum dosage ranges
• What are the 3 most cardiotoxic LA?
• Bupivacaine> e-docaine> ropivacaine
Plasma concentrations of ester LA may be elevated in patients with what problem?
• Deficient or atypical plasma cholinesterases • . • • • • No
• Name 4 an-cholinesterase agents that can prolong the ac-on of ester LA? •
1) Echothiophate (IRREVERSIBLE) 2) neos-gmine 3) pyridos-gmine 4) edrophonium
Name 4 condi-ons that decrease the concentra-on and hence prolong the ac-on of ester LA?
Pregnancy
Liver disease
1st 6 months of life
Atypical plasa cholinesterases
When using amide LA, which drugs would you want to avoid to minimize the likelihood of cardiac toxicity? •
Beta-blockers
CCB’s
Digoxin, These agents decrease impulse propagation.
What preservatives are added to LA? What are potential problems associated with their use?
Parabens. Allergic reactions
Should multiuse vials be used for spinal, epidural, or intravenous regional anesthesia?
No
Identify agents that potentiate amide toxicity?
Agents that inhibit the hepatic cytochrome p450 system or decrease hepatic blood flow (Volatile anesthetics, propranolol, cimetidine).
• How are amides eliminated•
Liver metabolism •
How are amides eliminated•
Liver metabolism
Which of the amide LA is least toxic?
Prilocaine
What are 3 least potent amide LA?
Lidocaine, mepivacaine, prilocaine
•• What is the maximum dose of bupivacaine?
In general, maximum single doses of BUPIVACAINE in healthy adults should not exceed 175 milligrams without EPINEPHRINE and 225 milligrams with EPINEPHRINE 1:200,000.
What is the maximum dose of bupivacaine?
In general, maximum single doses of BUPIVACAINE in healthy adults should not exceed 175 milligrams without EPINEPHRINE and 225 milligrams with EPINEPHRINE 1:200,000.
What are ester local anesthetics?
Procaine, chloroprocaine, tetracaine, and cocaine • • •
How are they eliminated?
Plasama cholinesterases with exception of cocaine (liver).
Why is tetracaine the most toxic ester LA
Tetracaine is eliminated by plasma cholinetserases the slowest.
What is the least potent LA
Procaine
What LA is the least toxic? Why?
Chloroprocaine because it is eliminated faster than all other LA
When should procaine, chloroprocaine, and tetracaine avoided?
In patients with atypical plasma cholinesterases and in patients with a history of allergy to an ester LA •
Compare Bupivacaine amd Ropivacaine
Ropivacaine is a long acting LA similar to bupivacaine in pKa, potency and protein binding but is less cardiotoxic and produces less motor blockade.
Why does injecting a LA with Bicarbonate increase the speed of onset?
The LA becomes mostly non-ionized with higher pH of surrounding solution
Define pKa?
The pH at which 50% of a weak acid or weak base ionized
LA are prepared as what kind of salts?
HCl
What is the range for pKas for LA?
7.6 to 9.1
What LA are most ionized at physiologic pH?
The LA with the highest pKas are most ionized at physiological pH ( procaine 8.9, chloroprocaine 8.7, and tetracaine 8.5)
What LA are most un-ionized at physiological pH? •
The LA with the lowest pKas are the most unionized at physiological pH (mepivacaine 7.6, and e-docaine 7.7)
The HCl salt solu-on of LA is acidified to what pH range to favor the existence of the water soluble ionized form to avoid precipita-on?
• pH 4.4 to 6.4
If LA is injected into an area of local infection, will the onset of action be hastened or slowed?
Slowed
