B7.005 Prework 2: Local Anesthetics

Question 1

purpose of local anesthetics

Answer 1

block sensory transmission from a local area of the body to the CNS

Question 2

uses of local anesthetic

Answer 2

infiltration
field block
spinal block
topical application

Question 3

mechanism of local anesthetic

Answer 3

block nerve conduction by blocking voltage gates Na+ channels, thus preventing depolarization and conduction of the cation potential
also block K+ channels at higher concentrations

Question 4

how do local anesthetics interact with Na+ channels?

Answer 4

bind to specific receptors on the inner portion of the channel
tertiary amines penetrate membranes in the uncharged form
charged anesthetic molecules gain access to the channel when it is open

Question 5

efficacy of local anesthetics

Answer 5

use dependent and voltage dependent

higher frequency of stimulation > higher membrane potential > greater degree of anesthetic block

Question 6

clinical effects of local anesthetics

Answer 6

1. pain is blocked first, along with pre- and postganglionic sympathetic fibers (B, C, and A-delta)
2. cold, warmth, tough, and deep pressure (A delta) are blocked next
3. muscle tone, proprioception, and motor function are affected later (A-alpha, beta, gamma)

Question 7

absorption of local anesthetics

Answer 7

initially not an issue since agents are injected / applied at site of action
greater lipid solubility > greater potency, longer duration of action, and take longer to produce maximal effect
can readily diffuse from the site of injection leading to reduced nerve block an causing systemic effects
-more vascularization > shorter action, greater systemic absorption

Question 8

what can you co administer w local anesthetics to help improve efficacy

Answer 8

vasoconstrictor (epi)

increases duration and minimizes systemic absorption

Question 9

2 classes of local anesthetics

Answer 9

esters

amides

Question 10

examples of esters

Answer 10

procaine (prototype drug)
cocaine
benzocaine

Question 11

procaine

Answer 11

prototype drug
short acting
infiltration and spinal block only
used as a reference for comparing potency and duration of action

Question 12

cocaine

Answer 12

2x as potent as procaine
medium acting
penetrates membranes
vasoconstrictor
often used topically for nasal and ophthalmic procedures

Question 13

benzocaine

Answer 13

surface use only
VERY lipophilic
used for burns, bites, hemorrhoids, catheter, endoscope placement
variety of OTC preps
risk of methemoglobinemia, esp in kids <2

Question 14

examples of amides

Answer 14

lidocaine
mepivacaine
bupivacaine
articaine
mainly used for infiltration, and field or epidural block

Question 15

lidocaine

Answer 15

most widely used
4x as potent as procaine
medium acting (2 hours)
also used topically as a jelly, solution, patch, etc.

Question 16

mepivicaine

Answer 16

2x as potent as procaine
medium acting
tends to vasocontrict, so slightly longer acting than lidocaine

Question 17

bupivicaine

Answer 17

16x as potent as procaine
long acting (3-15 hr)
tends to be cardiotoxic at higher doses
often used for labor anesthesia and post of pain control

Question 18

articaine

Answer 18

popular dental anesthetic
medium acting
has both amide and ester groups, so short plasma t1/2

Question 19

CNS adverse effects of local anesthetics

Answer 19

low conc: excitation, sleepiness, light headedness, visual and auditory disturbances, restlessness, metallic taste
high conc: nystagmus and myoclonus, leading to tonic clonic seizures (esp esters)

Question 20

cauda equina syndrome

Answer 20

direct toxicity - esp lidocaine

tends to occur with continuous spinal anesthesia due to pooling in cauda equina

Question 21

transient neurologic symptoms (TNS) / post anesthesia pain syndrome

Answer 21

pain after spinal and epidural anesthesia, without loss of sensation, motor, or bowel function
can occur w even modest doses
most common with lidocaine, but also with procaine and mepivacaine
least common with bupivicaine

Question 22

CV toxicity

Answer 22

esp bupivacaine
due to depression of myocardial conduction
-hypotension
cocaine can cause hypertension, arrhythmias, and myocardial failure

Question 23

other adverse effects of local anesthetics

Answer 23

methemoglobinemia

allergic reaction - esters, due to p-aminobenzoic acid metabolites

Question 24

special considerations of local anesthetics

Answer 24

potency is affected by pH

• anesthetics are weak bases
• potency is decreased at low pH

Question 25

why are local anesthetics less effective at low pH?

Answer 25

cation interacts with Na+ channel, but the unprotonated form is required to penetrate the membrane to gain access to the channel