LA

Question 1

List the 2 types of LA with some examples of drugs in each class

Answer 1

Ester: Procaine, Tetracaine
Amide: Lidocaine, Mepivacaine, Bupivacaine

Question 2

List the factors affecting action of LA

Answer 2

1. Lipid solubility
2. Myelination
3. Size
4. pH

Question 3

How does lipid solubility affect LA action?

Answer 3

More lipid solubility = more hydrophobic and able to pass through the lipid bilayer membrane of axons more easily

Question 4

How does myelination affect LA action?

Answer 4

With myelination, LA works better and faster

Question 5

How does pH affect LA action?

Answer 5

LA are weak bases. At alkaline pH, they will be unionized and can pass through the axonal membrane more easily. At acidic pH, they will be ionized and unable to pass through the membrane.

Hence LA are usually given in unionized form and remain in that form since physiological pH is the highest alkaline pH that a body can achieve.

Question 6

How does size affect LA action?

Answer 6

Small axon has greater and faster LA activity than larger axons

Question 7

What is the mechanism of LA?

Answer 7

Inhibits function of voltage-gated sodium channels by binding to their inactivated form from the inside of the axon. This hence blocks conduction of action potential by preventing influx of sodium ions.

Question 8

Bupivacaine is more _ (adverse effect)

Answer 8

Cardiotoxic

Question 9

Describe the metabolism and elimination of LA

Answer 9

Ester LAs: via blood esterases to PABA derivatives that occasionally but rarely cause allergic reactions
Amide LAs: via hepatic enzymes

Question 10

Compare the duration of action of lidocaine and bupivacaine

Answer 10

Lidocaine (short)
Bupivacaine (long)

Question 11

List the drugs that are used in labour delivery

Answer 11

1. Epidural medications: bupivacaine, ropivacaine, lidocaine 9used less)
2. Anaesthetic adjuncts: fentanyl
3. Opioids: Pethidine, Morphine (not recommended because of its long onset)