Local Anesthetics

Question 1

How do local anesthetics generally work?

Answer 1

Bind to sodium channels in nerves to block nerve transmission

Question 2

What kind of fibers are easiest to block?

Answer 2

Small myelinated fibers

Question 3

How does myelination affect blocking by local anesthetic?

Answer 3

Unmyelinated fiberes are more difficult to blokc whereas myelinated fibers only need to block 3 nodes of Ranvier

Question 4

How is the propagation of block onset?

Answer 4

Nerve block -> Sciatic NErve -> leg will get numb from proximal to distal

Question 5

What are the 3 fiber types?

Answer 5

A, B ,C

Question 6

Which fiber types are myelinated?

Answer 6

A and B

Question 7

Which fiber type is unmyelinated?

Answer 7

C

Question 8

What are the receptor subtypes of fiber A?

Answer 8

Aa: motor neurons; hardest to block in A fiber group due to large size
Ab: Tactile/Proprioception
Ay: Reflexes;
Adelta: pain cold temperature, easiest to block due to small size

Question 9

What are B fibers?

Answer 9

Preganglionic sympathetics

Very small size and easiset to block with local anesthetics

Question 10

What are C fibers?

Answer 10

Visceral pain nerve unmyelinated

Hardest to block -> more dull slow pain

Question 11

What is the hierearchy of nerve blocks from easiest to hardest?

Answer 11

Sympathectomy (Fiber B) -> Sensory and Pain fibers -> Motor Fibers

Question 12

What states of the sodium channel do local anesthetics bind to?

Answer 12

Inactivated or Open states rather than resting states

Question 13

Where does hte local anesthetic bind on the sodium channel?

Answer 13

R site, which is on the intracellular site

Question 14

What property is important for the local anesthetic to be able to bind to the sodium channel?

Answer 14

Lipophilic

Question 15

What is frequency dependent blockade?

Answer 15

In very active nerves, sodium channels will be in either activated or inactivated states mostly -> nerves will more quickly be blocked by local anesthetics

Question 16

Which form of hte local anesthetic can cross the membrane? Which form is active?

Answer 16

Uncharged form can pass membrane

Charged form is the active form

Question 17

What is the primary determinant of hte potency of a local anesthetic?

Answer 17

Lipid solubility : more potent drug is more lipid soluble

Question 18

What determines the duraiton of action of a local anesthetic?

Answer 18

Local tissue protien binding

Question 19

What can be added to the local anesthetic mixture to tspeed hte onset time of a block?

Answer 19

Sodium bicarbonate -> generates more uncharged anesthetics to drive cells through

Question 20

Why is epinephrine given in adjunct to a local anesthetic?

Answer 20

For vasoconstriction -> keeps anesthetic near the nerve and increases duration of a nerve block

Keeps it out of systemic system : Inject with epi -> if HR inc -> indicate need to move needle

This is not as effective for drugs that are highly protein bound

Question 21

What are indications for neuraxial anesthesia?

Answer 21

Surgery below the chest

Question 22

What are the differences between a spinal and epidural?

Answer 22

spinal: inject around cauda equina -> most wil stay there and below (up to T10 blockade and below)

Epidural: site of injection is nerve roots exiting neural foramina -> thus can be at any level

Question 23

What are hte respiratory effects of neuraxial anesthesia at the thoracic level?

Answer 23

Normal tidal volume
Loss of proprioception -> pts cant feel themselves breathing
Respiratory arrest possible due to blocking B type sympathetic fibers (NOT due to paralyzed phrenic nerves)

Question 24

What are hte cardiovascular effects of neuraxial anesthesia?

Answer 24

B type fibers get blocked -> arterial vasodilation and venodilation -> dec venous return and BP

Question 25

What is the Bezold JArisch Reflex?

Answer 25

Unopposed vagal stimulation -> decreased venous return -> hear thinks it needs to slow down to allow filling -> bradycardia

Question 26

How are local anesthetics eliminated?

Answer 26

Esters: Plasma Cholinesterase
Amides: Liver

Question 27

What is the max recommended dose of bupivicaine?

Answer 27

3mg/kg for adults

2.5 mg/kg for neonates

Question 28

What is the max recommended dose for lidocaine?

Answer 28

5 mgkg plain

7 mg/kg with epi

Question 29

What are some neuro toxicity concerns of local anesthetic?

Answer 29

Inhibit inhibitory neurons -> activate excitatory neurons

Acidoses -> dec protein binding in blood -> inc free fraction of drug to brain

Treat with hyperventilation, succinylcoline, benzo, etc

Question 30

What are some caridovascular toxicity concerns?

Answer 30

Usually occurs at higher conc than for neurotoxicity
Cardiac sodium channel blockade
Vasodilation
Inhibition of sympathetic effects
Slurred QRS, V-fib, Vtach

Question 31

How is LA toxiicty treated?

Answer 31

Prevention: monitory, use of epi for intravascular marker

Supportive: oxygenation and ventilation, treat seizures and arrhythmias (epi, atropine etc)

Question 32

What is intralipid?

Answer 32

Specific treatment therapy for local anesthetic toxiciity

Very lipophilic and absorbs LA and sequesters them

Question 33

What are some complications of neuraxial anesthsia?

Answer 33

Postdural Puncture Headaache (PDPH): leakage of CSF
Transient Neurologic Sx (TNS): sensory disturbances or pain in back of lower extremities; No permanent damage
Backache

Question 34

What factors increase incidience of PDPH after neuraxial anesthesia?

Answer 34

Younger, female, larger needle, pregnant, multiple punctures

Question 35

How do we treat PDPH?

Answer 35

Prevention
Fluids
Analgesics
Blood Patch is treatment of choice

Question 36

What is the blood patch treatment for PDPH?

Answer 36

Take pt’s own blood -> put in epidural sac -> plug dural hole and preven CSF leakage