Local Anaesthesia

Question 1

Describe the Mechanism of Action of Local Anaesthesia.

Answer 1

It stops axonal conduction by blocking sodium channels in the axonal membrane when applied locally in appropriate concentration –> Prevent sodium ion entry –> slow down or bring conduction to a halt

Question 2

Describe the Use Dependency of Local Anaesthesia.

Answer 2

Depth of LA nerve block increases with action potential frequency because LA molecules

• gain access to the channel more readily when the channel is open
• have higher affinity for the inactivated than for the resting (closed) channels

Question 3

How is selectivity in LA achieved?

Answer 3

By administering LA to a limited area

Question 4

Name the Factors affecting LA action.

Answer 4

1. Greater lipid solubility –> More potency and longer acting

Nerves
2. Diameter of nerve: Smaller nerve&raquo_space; bigger nerve

3. Frequency of firing: high (sensory) > low (motor)
4. Position: circumferential > deep (large nerve trunk)
5. Myelination: myelinated > nonmyelinated
6. pH dependency: Alkaline pH - increased LA activity
   Acidic pH - decreased LA activity
   (Since LA molecules are weak bases pKa 8-9)

Question 5

Rank the following nerves in order of LA action (which is acted on first > which is acted on last).

small non-myelinated axons, small myelinated axons, large myelinated axons

Answer 5

Small myelinated axons > small non-myelinated axons > large myelinated axons

(Nociceptive and Sympathetic transmission is blocked first)

Question 6

Name the two subtypes of Local Anaesthesia.

Answer 6

Ester-type and Amide-type

Question 7

Compare and contrast Ester-type and Amide-type Local Anaesthesia.

Answer 7

1. Ester type LA contains ester bond, while amide type contains amide bond.
2. Ester type LA has a higher incidence of allergic reaction than amide type. (both are low)
3. Ester type LA is metabolised by plasma/tissues non-specific esterases (kidney related), amide-type LA is metabolised by hepatic CYP450 (liver related).

Question 8

Name 1 example of Ester-type LA

Answer 8

Procaine

Question 9

Name 1 example of Amide type LA

Answer 9

Lidocaine

Question 10

Which LA is cardiotoxic?

Answer 10

Bupivacaine

Question 11

Should you prescribe LA with epinephrine?

Answer 11

Yes. LA can combine with epinephrine and cause vasoconstriction, to prevent LA systemic distribution from the site of action
(Limits LA exposure systemically)

Question 12

Which LA can cause hypertension and vasoconstriction?

Answer 12

Cocaine

Question 13

Which LA can cause the patient to turn blue?

Answer 13

Prilocaine
Its metabolised into O-toluidine that causes methaemoglobin

We give iv methylene blue/ ascorbic acid to reverse this.

Question 14

Which 2 LAs are used as Epidural Anaeasthetics?

Answer 14

Lidocaine and Bupivacaine
(we may combine with fentanyl to reduce LA dose)

Question 15

Which 2 LAs are used as Dental Anaesthetics?

Answer 15

Lidocaine and Bupivacaine
(may combine with epinephrine to ensure vasoconstriction and control bleeding)