Local Anesthetics

Question 1

What are the properties that anesthesiologists look for in the ideal anesthetic?

Answer 1

1) short onset, long duration of action
2) remains at injected site (minimal absorption and distribution)
3) acts predictably and reversibly without causing tissue damage
4) possess a large margin of safety

Question 2

What are the 3 components of local anesthetics and what does each component determine?

Answer 2

• Aromatic ring/lipophilic portion: determines potency and duration of action
• Intermediate linkage: determines class
• Terminal amine/hydrophilic portion: determines onset of action

Question 3

What are the two classes of local anesthetics and how can you quickly tell what class each drug is in?

Answer 3

• esters (metabolized by PLASMA esterases)
• amides (metabolized by HEPATIC amidases)
• if there’s an i before the -caine, it’s an amide

Question 4

You’re doing a anesthesia rotation on the ob/gyn floor and it was just determined that an expectant mother needs a c-section. What can you do to modify your epidural anesthetic to make the block onset quicker?

Answer 4

• mix with sodium bicarbonate to make the environment more basic!

Question 5

Which form of the anesthetic is diffusible and which is active?

Answer 5

diffusible - unionized

active - ionized

Question 6

What can you expect when trying to numb up an abscess (or any acidic environment)?

Answer 6

• onset of action will be prolonged

Question 7

What channel do local anesthetics work on?

Answer 7

Na channel

• bind to and block INTRACELLULAR portion of INACTIVATED voltage gated sodium channels, which slows repolarization and prevents propagation of further action potentials

Question 8

What qualities of the nerve fiber determine the onset of the block?

Answer 8

• size (smaller diameters blocked first - I guess it does matter)
• degree of myelination (myelinated blocked quicker)
• firing frequency (faster firing blocked first)

Question 9

Arrange these fibers in order of block, from first to last:

Aα, B, C, Aβ, Aδ, Aγ

Answer 9

B > C > Aδ > Aγ > Aβ > Aα

Question 10

Arrange these actions in order of block, from first to last:
pain, temperature, motor, sympathetic tone, light touch

Answer 10

sympathetic tone > temp > pain > light touch > motor

Question 11

You’re numbing up an area with lidocaine and the attending asks you if you added epinephrine to the mixture. What’s the reason that you would? What’s an amide that epi has little effect on?

Answer 11

• epi is a vasoconstrictor that will decrease absorption, increase neuronal uptake, and prolong duration of action
• little effect on bupivicaine, whose long duration of action is due to a high degree of protein binding

Question 12

What are the side effects of CNS toxicity of local anesthetics (both early and then late)?

Answer 12

• Early: decreased inhibition, sensory disturbances, restlessness, tremor, tinnitus
• Late: lethargy, hypotension, bradycardia, decreased respiratory rate, seizures

Question 13

What are the cardiovascular side effects related to local anesthetics and how do you treat them?

Answer 13

• reduction of cardiac conductivity, excitability, and contractility
• if gets bad enough for cardiovascular collapse, use ACLS and administer lipid emulsion

Question 14

You’re administering a local anesthetic in high doses and notice the pulse ox is down to 87%. What do you suspect happened and how do you treat it?

Answer 14

• methemoglobinemia, treat with methylene blue

Question 15

What are some local toxicities that can occur with local anesthetics?

Answer 15

• transient neurologic symptoms (pain), neuronal injury, allergy (is this really a true allergy?)

Question 16

When giving a local anesthetic, which broad class is more likely to cause an allergic reaction and why?

Answer 16

Esters - derivatives of PABA, a known allergen