Local Anaesthesia and Analgesia

Question 1

What is local analgesia?

Answer 1

Use of a drug to produce temporary loss of all sensation in a limited part of the body

Question 2

What is local anaesthesia?

Answer 2

Any technique to render part of the body insensitive to pain without affecting consciousness

Question 3

Which groups of drugs can be used to achieve local analgesia?

Answer 3

Opioids, local anaesthetics and alpha-2 agonists

Question 4

What are the characteristics of lidocaine?

Answer 4

Amide type local anaesthetic with good potency
Short 10-15 min onset of action with up to 2 hrs DOA
Causes vasodilation so often formulated with adrenaline, may cause SC and skin swelling in horses so rarely used

Question 5

What are the characteristics of procaine?

Answer 5

Ester with poor potency
Long onset of action 15-20min with 45-60min DOA
Only local anaesthetic licensed for food producing animals in EU

Question 6

What are the characteristics of mepicacaine?

Answer 6

Amide with good potency
Short onset of 10 mins and long DOA of 6-8 hrs
Commonly used for equine nerve blocks

Question 7

What are the characteristics of bupivacaine?

Answer 7

Amide with strong potency
Long onset of 30-40 mins and long DOA of 6-8 hours
High incidence of cardiotoxicity

Question 8

What are the characteristics of ropivacaine?

Answer 8

Similar to bupivacaine but less cardiotoxic and may be slightly less potent

Question 9

What are the characteristics of tetracaine and proparacaine?

Answer 9

Both lipid soluble used topically in the eye with short onset and duration

Question 10

What are the characteristics of prilocaine?

Answer 10

Lipid soluble for absorption across intact skin
Relatively high potential to cause methaemaglobinaemia
Formulated with lidocaine in EMLA cream

Question 11

How is EMLA cream used?

Answer 11

Applied to intact skin and left for 30 minutes to take effect, apply a light bandage over the ointment
Useful for IV or arterial catheterisation of nervous animals

Question 12

What is the dose for lidocaine?

Answer 12

5mg/kg with toxic dose at 10-20mg/kg

Question 13

What is the dose used for bupivacaine?

Answer 13

2mg/kg with toxic dose of 3.5-4.5mg/kg

Question 14

What is the dose used for ropivacaine?

Answer 14

1.5-3mg/kg with toxic dose of 5mg/kg

Question 15

What is the biochemical structure of local anaesthetics?

Answer 15

Weak bases consisting of a lipophilic ring, a link and a hydrophilic amine
Solubilised for injection as strong conjugate acidic hydrochloride salts

Question 16

What is the classification of amines or esters based on?

Answer 16

The link present between the lipophilic ring an hydrophilic amine

Question 17

How do local anaesthetics work?

Answer 17

Block the sodium channels from the inner surface of the axonal membrane preventing propagation of axonal action potentials

Question 18

What is the speed of onset of a local anaesthetic related to?

Answer 18

Dose and proportion of drug in the non-ionised lipid soluble form which is determined by the pKa and ambient pH

Question 19

What determines the potency of a local anaesthetic?

Answer 19

Lipid solubility with sensitivity of the axon determined by axonal diameter and degree of myelination

Question 20

What is the duration of action of a local anaesthetic determined by?

Answer 20

Degree of protein binding and speed of absorption from injection site

Question 21

How are ester local anaesthetics metabolised?

Answer 21

Hydrolysis by plasma cholinesterase

One of the break down products, p-aminobenzoic acid, can cause allergic reactions

Question 22

How are amide local anaesthetics metabolised?

Answer 22

In the liver

Question 23

Why are vasoconstrictors added to local anaesthetics?

Answer 23

To reduce local blood flow and therefore reduce absorption and prolong duration of action

Question 24

How is the caridovascular toxicity of local anaesthetics explained?

Answer 24

Related to reduced membrane conduction resulting in arrhythmias and reduced cardiac contractility

Question 25

How is the difference in severity depending on different local anaesthetic drugs used explained?

Answer 25

Relative protein binding Lidocaine binds less strongly than bupivacaine and is used as an anti-arrhythmic Bupivacaine binds strongly and is difficult to displace from cardiac conducting tissue and myocardium, it also blocks potassium channels

Question 26

What are the potential CNS toxicity reactions of local anaesthetics?

Answer 26

Mild changes in peripheral sensation to sedation/coma/convulsions and death

Question 27

How does prilocaine cause methaemoglobinaemia?

Answer 27

Its o-toluidine metabolite can oxidise haemoglobin to methaemaglobin

Question 28

What are the different routes of local anaesthetic administration?

Answer 28

``` Topical Infiltrative Regional nerve block Intravenous Regional Analgesia (Bier's Block) Intra-articular analgesia Epidurals ```

Question 29

Which local anaesthetic methods are considered topical?

Answer 29

Ophthalmic use, laryngeal prior to intubation, cutaneous and splash blocks

Question 30

How is local anaesthetic used infiltratively?

Answer 30

Injection of local anaesthetic into the tissues around the surgical site and is commonly referred to as a line/field block

Question 31

How are local anaesthetics used with soaker catheters?

Answer 31

Soaker catheters are inserted intraperitoneal/interpleural/ intra-articular and slowly release local anaesthetic

Question 32

Give an example of soaker catheter use

Answer 32

Installation of bupivacaine through chest drains has been shown to improve respiratory function after thoracotomy although pain on initial installation may be seen

Question 33

What are the indications for intravenous regional analgesia (IVRA)?

Answer 33

Surgery of the distal limb

Question 34

What are the contraindications for IVRA?

Answer 34

Tumour removal, sensitivity to lidocaine, surgery lasting longer than 2 hours

Question 35

What is the theory behind IVRA?

Answer 35

The limb is exsanguinated and isolated from the circulation with an Esmarch's bandage and tourniquet Local anaesthetic is injected into a vein distal to the tourniquet and bathes the dependent tissues resulting in intense analgesia as long as the tourniquet is in place

Question 36

What drugs can be used for IVRA?

Answer 36

Lidocaine only as bupivacaine and ropivacaine are CI due to their cardiotoxicity

Question 37

What dose of lidocaine is used for IVRA?

Answer 37

Dogs and cats with mid-radial tourniquet = 0.5-2ml 2% lidocaine

Question 38

What are the benefits of using intra-articular analgesia?

Answer 38

Produced profound analgesia of the particular joint with minimal systemic side effects

Question 39

What must be considered when performing intra-articular analgesia?

Answer 39

Drugs must be preservative free and a sterile injection technique is essential

Question 40

Which surgeries can epidurals be used with?

Answer 40

Hindlimb, pelvic or abdominal surgery

Question 41

How are epidural used?

Answer 41

In combination with general anaesthesia or as a sole anaesthetic/analgesic technique

Question 42

How is an epidural performed?

Answer 42

Analgesic drugs are injected into the epidural space beneath the ligamentum flavum but above the dura mater SA = lumbosacral space LA = sacro-coccygeal space between Co1-Co2

Question 43

What are the contraindications for performing an epidural?

Answer 43

Sepsis or local infection, coagulopathy, hypovolaemia or lumbosacral fractures

Question 44

Why is there a greater chance of puncturing the dura mater in cats?

Answer 44

The subaracnoid space extends further caudally than in dogs

Question 45

What combination of drugs are typically used for an epidural?

Answer 45

Local anaesthetic (lidocaine/ropivacaine) and an opioid (morphine) but alpha-2 agonists (xylazine) are used in large animals

Question 46

What are the potential complications of an epidural?

Answer 46

Failure of block, incompete analgesia, impaired locomotion post-operatively, urinary retention and hypotension

Question 47

What are the onset and duration of action of some of the drugs commonly used in epidurals?

Answer 47

Lidocaine 2% rapid onset, 2h DOA Bupivacaine 0.5% 20-30 min onset, 4-8h DOA Levobupivacaine 0.75% similar to bupivacaine Ropivacaine 0.75% 15-20 min onset, 4-6h DOA less motor Morphine 0.1mg/kg onset 1 hour, DOA 12-24 hrs, sensory effects only Methadone 0.1mg/kg Alpha-2 agonists xylazine and detomidine have been used but sedation and ataxia at high doses