Local anaesthetics

Question 1

Difference between local analgesia and local anaesthesia

Answer 1

Local analgesia: use of a drug to produce temporary loss of all sensation in a limited part of the body.
Local anaesthesia: Use of ANY TECHNIQUE to render part of the body insensitive to pain without affecting consciousness

Question 2

Lidocaine

Answer 2

Licenced in dogs.
Amide
Onset: Short (10-15min)
Duration: Short-medium (up to 2 hrs)
Potency good.
May cause vasodilation so often formulated with adrenaline
RARELY USED IN HORSES

Question 3

Only local anaesthetic licenced for food producing animals in EU

Answer 3

Procaine: Ester.
Onset: Long: 15-20 minutes
Duration: Short: 45-60 min
Poor potency

Question 4

Which local anaesethic is not commonly used in horses

Answer 4

Lidocaine (onset short, duration short to medium)

May cause sub-cutaneous and skin swelling in horses.

Question 5

Mepivacine

Answer 5

Amide
Onset: Short (10 min)
Duration: Medium (2 hours)
Potency: Good
Commonly used for equine diagnostic nerve blocks

Question 6

Bupivacine

Answer 6

Amide
Onset: long (30-40 min)
Durating: Long (6-8 hours)
Potency: Strong
HIGH INCIDENCE OF CARDIOTOXICITY

Question 7

Which local anesthetic has a) long onset, short duration

Answer 7

Procaine (only licenced in food producing)
Onset: Long (15-20min)
Duration: Short (45-60 min)
Potency Strong

Question 8

Ropivacine

Answer 8

Similar to Bupivacine (amide) (long onset, long action- 6/8hrs) but less cardiotoxic.
May be slightly less potent.

Question 9

Examples of topical local anaesthetic for eye

Answer 9

Tetracaine, Proparacine (Proxymetacaine)
Both lipid soluble
Short onset/duration.

Question 10

EMLA cream ingredients and side effect

Answer 10

Prilocaine formulated with Lidocaine - lipid sol for absorption across intact skin.
Relatively high potential to cause methaemoglobinaemia.

Question 11

Pharmacology of Local Anaesthetics

- how are they made soluble

Answer 11

Local anaesthetics are weak bases and consist of a lipophilic ring, a link and a hydrophilic amine.
As weak bases, they are solublised for infection as a strong conjugate acidic hypocholride salt.

Question 12

Local anaesethics below to what chemical class

Answer 12

Weak bases. 
Either Esters (Procaine) or Amides (Lidocaine, Mepivacine, Bupuvacine)

Question 13

How do local anesthetics work

Answer 13

Block sodium channels from inner surface of axonal membrane and prevent propagation of axonal action potentials.

Question 14

Speed of onset of local anaesthetic is related to…

Answer 14

The dose and proportion of the drug in the non-ionised lipid sol form (determined by the dissociation constant, pKA)

Question 15

For Intravenous Regional Analgesia (IVRA) which is the only local anaesthetic that should be used

Answer 15

Only Lidocaine
Lidocaine-esther (onset short, duration short to medium)
May cause sub-cutaneous and skin swelling in horses.

Question 16

Where is the local analgesic injected during an epidural

Answer 16

Injected into Epidural space (aka Extradural) beneath the ligamentum flavum but above the dura mater. Small animals: Lumbosacral space
Large animals: First 2 cocygeal vetebrae or sacrococcygeal space

Question 17

When using epidural anaesthetia to prevent straining during assisted parturition, why would the first 2 coccygeal vetebrae be used over the lumbosacral space

Answer 17

First 2 coccygeal vetebrae are more caudal than the lumbosacral space therefore reduce tracking up the spinal cord. Less hindlimb weakness= less risk of animal going down.

Question 18

Most common combination of drugs used as a epidural in a) small animal b) large animal

Answer 18

a) Small animal: combination of local anaesthetic+ opiod (normally morphine)
b) Large animal: Normally a2 agonists (xylazine)
All drugs should be preservative free.