CAL - pain and analgesia

Question 1

What drugs can be used for local analgesia?

Answer 1

• local anaesthetics
• opioids
• alpha 2 agonists
  (opioids and alpha 2s are generally used in association with an local anaestheic agent)

Question 2

Action - local anaesthetic

Answer 2

reversibly blocks sodium channels preventing the transmission of action potentials along an axon. Unionized lipid-soluble drug passes through the phospholipid membrane where, in the axoplasm it is protonated. In this ionized form it binds to the internal surface of the sodium channel, preventing it from leaving the inactive state. The degree of blockade in vitro is proportional to the rate of stimulation (‘frequency dependence’) due to the attraction of local anaesthetic to ‘open’ sodium channels

Question 3

What does the intrinsic vasodilator activity influence?

Answer 3

• potency

- duration of action

Question 4

T/F: local anaesthetics cause vasodilation at low concentrations adn vasoconstriction at higher concentrations

Answer 4

True

Question 5

How does cocaine have solely vasoconstrictor actions?

Answer 5

by inhibiting neuroal uptake of catecholamines and inhibiting monoamine oxidase (MAO)

Question 6

Which local anaesthetics may be formulated with adrenaline/epinephrine

Answer 6

Procaine and lidocaine (lignocaine): both cause vasodilation and adrenalise helps reduce systemic absorption and increase duration of action

Question 7

What is the advantage of ropivacaine compared to bupivacaine?

Answer 7

Ropivacaine has less cardiac toxicity compared to bupivacaine which dissociates very slowly from sodium channels and may cause persistent myocardial depression. It can also cause ventricular arrhythmias and ventricular fibrillation. Life-threatening arrhythmias may also reflect disruption of Ca and K channels. Ropivacaine dissociates more rapidly from cardiac sodium channels and produces less myocardial depression. However, it has a slightly shorter duration of action and is slightly less potent than bupivacaine, resulting in a slightly larger dose requirement for an equivalent block.

Question 8

What might you use to do a mandibular nn block?

Answer 8

lidocaine

Question 9

In what circumstances would you NOT perform epidural anaesthesia/analgesia?

Answer 9

• Infection at site of needle insertion
• Coagulopathies
• Spinal/pelvic deformities
• Bacteraemia
• Raised ICP

Question 10

What anatomical landmarks do you need to perform a brachial plexus block in the dog? 5

Answer 10

• jugular vein
• cranial border of greater tubercle
• acromion
• first rib
• brachial plexus located at the ventral border of the scalenus mm

Question 11

Why shouldn’t you ever give pethidine IV?

Answer 11

causes histamine release

Question 12

What is a good choice analgesic for orthopaedic sz?

Answer 12

morphine

Question 13

Does pethidine or buprenorphine have a long duration of action?

Answer 13

• PETHIDINE: short duration of action
• BUPRENORPHINE - long duration of action so lasts into post-op period, however it is a partial agonist so the analgesia may not be as profound as with a full agonist

Question 14

Does butorphanol provide good analgesia for sx?

Answer 14

No because it is a mu-opioid receptor antagonist

Question 15

Is midazolam an analgesic?

Answer 15

No

Question 16

Does nitrous oxide provide analgesia?

Answer 16

Can provide good intra-operative analgesia but remember this wears off soon after its administration is discontinued at end of anaesthesia

Question 17

Should carprofen be given as a top up intra-operatively?

Answer 17

No - should be given every 12-24 hours so will not be required intra-operatively

Question 18

If morphine is given in the epidural injection during sx, how long may this provide analgesia for?

Answer 18

up to 24 hours

Question 19

What are the vital signs? 4

Answer 19

Temperature
Pulse
Respiration
PAIN!

Question 20

Signs a cat is in pain

Answer 20

Reluctant to move; growling, hissing, vocalising when touched; tachycardia, tachypnoea; dilated pupils. She may look ‘distant’ and may try to hide.

Question 21

What other factors might produce similar signs to pain in cats?

Answer 21

Anxiety, stress, unfamiliar surrounding (cage, people, other animals etc.). Cats do not like bandages and you should try to differentiate between pain and the dislike of restrictive bandages or dressings.

Question 22

Can methadone be used to control pain in cats?

Answer 22

• a pure mu-opioid receptor agonist and appears to be efficacious for controlling pain in cats. Unlike morphine, it rarely appears to cause vomiting. May cause dysphoria at higher doses.

Question 23

Can buprenorphine be used to control pain in cats?

Answer 23

it is a partial mu-opioid receptor agonist - i.e. its maximal effect is less than pure mu-agonists. In clinical studies, buprenorphine has produced better analgesia than morphine and pethidine. It rarely causes vomiting or dysphoria. It can also be given by the OTM route with almost 100% bioavailability.

Question 24

Can NSAIDs be used to provide pain relief in cats?

Answer 24

Until recently, NSAIDs were not widely used in cats because of fears about toxicity. Carprofen is effective and is licensed as a once-only dose for perioperative analgesia. Meloxicam is also licensed for perioperative use and provides analgesia of similar effect and duration to carprofen.

Question 25

Why use epidural analgesia?

Answer 25

• A successful epidural with a local anaesthetic will block all sensory input to the spinal cord during surgery and also help to reduce the amount of volatile anaesthetic needed to maintain anaesthesia. Morphine (the preservative-free preparation) can also be incorporated and will provide postoperative analgesia for up to 24 hours.

Question 26

Where to place epidural analgesia in cats versus dogs?

Answer 26

In cats, the meninges end further caudally (between L7-S1) than in dogs and it is therefore possible that the needle may penetrate the dura and arachnoid and enter the CSF. If this does occur, the amount of local anaesthetic injected should be reduced by at least 50%.

Question 27

T/F: pain is an emotional experience

Answer 27

True - a cat that is cold, frightened, hungry, thirsty or unable to empty its bladder will experience greater pain. Exposing cats to the noise and smell of dogs may also be stressful. Cats should also be given a box or covered bed so that they can hide and feel safe.

Question 28

If you want to suture a wound on a distal horse limb, what should you use?

Answer 28

Suitable ‘nerve block’ of lower limb with mepivacaine. Mepivacaine is a local anaesthetic licensed for use in the horse and will provide good local analgesia for suturing.