Local Anesthetics

Question 1

Rank the sensitivity of each nerve fiber type to the physiologic effects of local anesthetics: A alpha fibers, A delta fibers, B fibers, C fibers

Answer 1

1. B fibers
2. C fibers
3. A delta (smaller diameter)
4. A alpha (larger diameter)

Question 2

The primary mechanism of action for local anesthetic blockade is:

Answer 2

voltage-gated sodium channels

Question 3

What subunit of the voltage-gated sodium channels do local anesthetics reversibly bind to?

Answer 3

Alpha subunit

Question 4

Saltatory conduction does NOT occur in:

Answer 4

C fibers (they lack myelination)

Question 5

What part of a local anesthetic determines allergic potential?

Answer 5

Intermediate chain

Question 6

What part of a local anesthetic determines lipophilicity?

Answer 6

Benzene ring (determines class- amide or ester)

Question 7

What part of a local anesthetic makes it a weak base?

Answer 7

Tertiary amine

Question 8

Local anesthetic potency best correlates with:

Answer 8

lipid solubility

Question 9

Local anesthetic onset of action best correlates with:

Answer 9

pKa

Question 10

Local anesthetic duration of action best correlates with:

Answer 10

protein binding

Question 11

What is the definition of pKa?

Answer 11

the pH where 50% of a drug exists as the uncharged base (unionized) and 50% of the drug exists as the conjugate acid (ionized)

Question 12

Rank the following local anesthetics according to their degrees of ionization at physiologic pH: lidocaine, tetracaine, bupivicaine, chloroprocaine

Answer 12

1. Chloroprocaine (most ionized)
2. Tetracaine
3. Bupivicaine
4. Lidocaine (least ionized)

Question 13

What is the pKa of Lidocaine?

Answer 13

7.8

Question 14

What is the pKa of Bupivicaine?

Answer 14

8.1

Question 15

What is the pKa of Tetracaine?

Answer 15

8.4

Question 16

What is the pKa of Chloroprocaine?

Answer 16

8.7-9.1

Question 17

Which local anesthetic is unionized at physiologic pH?

Answer 17

Benzocaine (pKa 3.5); for all other LA, the charged species is required to produce the anesthetic effect

Question 18

Local anesthetics are ___ that are ___ at physiologic pH. (with the exception of benzocaine)

Answer 18

weak bases, ionized (charged)

Question 19

What is a side effect of benzocaine administration?

Answer 19

Metheglobinemia

Question 20

What is subarachnoid chloroprocaine negatively associated with?

Answer 20

Neurotoxicity (Chloro is not used for spinals- its preservative bisulfite might be the cause of the toxicity)

Question 21

Which local anesthetic is least protein bound in the plasma?

Answer 21

Chloroprocaine

Question 22

Rank the speed of local anesthetic uptake after injection for the following sites: intercostal, caudal, epidural, interpleural, brachial plexus, subcutaneous, sciatic, femoral

Answer 22

1. Interpleural
2. Intercostal
3. Caudal
4. Epidural
5. Brachial plexus
6. Femoral
7. Sciatic
8. Subcutaneous

Question 23

What is the initial bolus dose of 20% lipid emulsion therapy for a patient suffering from LAST?

Answer 23

1.5 mL/kg (lean body mass) over 1 minute followed by an infusion of 0.25 mL/kg/min.

Question 24

What factors increase the the risk of local anesthetic toxicity?

Answer 24

hypercarbia/acidosis, local anesthetic potency, hyperkalemia

Question 25

What metabolite of ester-type local anesthetics is MOST likely to cause a type-1 hypersensitivity reaction?

Answer 25

Para-aminobenzoic acid

Question 26

What is the makeup of 5% EMLA cream?

Answer 26

50/50 combo of 2.5% Lidocaine and 2.5% Prilocaine

Question 27

How long does is take for EMLA cream to produce analgesia?

Answer 27

1 hr; achieves max effect in 2-3 hrs