Analgesia Flashcards
What is COX and when is it activated?
- Cyclooxygenase enzyme
- Activated after cell damage
- Stimulates the release of prostaglandins
Important functions of prostaglandins
- Maintaining the lining of the stomach
- Maintaining blood flow to the kidneys
- Supporting platelet function
Describe the action of NSAIDs
- NSAIDs are COX inhibitors
- NSAIDs also target descending pain pathways (PAG=peri-aqueductal grey, NRM=nucleus raphe magnus)
Commonly used small animal NSAIDs
- Meloxicam
- Carprofen
- Robenacoxib
- Firocoxib
Commonly used equine NSAIDs
- Phenylbutazone
- Meloxicam
- Fluxinin meglumine
Commonly used farm animal NSAIDs
- Meloxicam
- Ketoprofen
For which common procedures should farm animals receive NSAIDs
- Castration
- Disbudding
- Assisted calving
- Foot care
When are NSAIDs contraindicated?
In patients with:
- Renal/hepatic insufficiency
- Hypovolaemia
- Congestive heart failure
- Pulmonary disease
- Coagulopathies, active haemorrhage
- Spinal injuries
- Gastric ulceration
- Shock, trauma
- Pregnancy
- Concurrent use of steroids
Sometimes NSAIDs must be used for welfare even when there is a comorbidity; need to weight up benefit vs. risk.
What are some likely side effects of NSAIDs?
- Vomiting
- Diarrhoea
- Renal problems
- Clotting dysfunction
Describe the action of Gapriprant compared to traditional NSAIDs
- Gapriprant is a more selective, new class of NSAID
- It does not block COX; instead it works lowers down the inflammatory pathway by blocking some activity of a prostaglandin (at the EP4 receptor)
- Mode of action targets canine oesteoarthritic pain and inflammation while reducing the impact on gastric, renal and hepatic homeostasis
- This is a selective drug that allows the kidney and gut to be spared from damage
Describe the action of paracetamol
- Blocks central and peripheral COX
- Has CB1 and TRPV1 receptor action
- It is antipyretic
Which species can paracetamol be used in?
- Paracetamol is licensed in pigs
- Can be used IV in dogs and horses (off-licence)
- Pardale-V (paracetamol + codeine) is licensed in dogs
Which species should paracetamol never be used in?
- Cats
Differentiate between opioids and opiates
- Opioids: naturally occurring or synthetic substances with morphine-like activity
- Opiates: drugs derived directly from opium e.g. morphine, codeine
True/false: some opioids have both agonist and antagonistic activity but at different receptors.
True
e.g. Buprenorphine is an agonist at mu but an antagonist at kappa
Butorphanol is the reverse of this
Where are opioid receptors found and what do they do?
- Found throughout the CNS in somatic and visceral sensory neurones
- Found in the periphery (GI tract and joints)
- Present especially after inflammation
- All mediate inhibition of pain
Agonists of which opioid receptor are recommended for moderate to severe pain/procedures where noxious stimulation will be high?
Mu (µ) receptor agonists
e.g. Methadone is a full agonist at mu, kappa and delta receptors
Birds and reptiles have an increased number of which opioid receptors compared to dogs and cats?
Kappa
This means for optimal pain management, we may opt for kappa-agonist opioids in these species
Which species is morphine licensed in?
None
But it can be used on cascade in certain situations e.g. epidurals
Which species is pethidine licensed for?
Dogs
Horses
Which species is fentanyl licensed for?
Dogs
What scenarios/kind of pain are opioids suitable for?
Wide variety - opioids are effective and very versatile. For example:
- Management of acute trauma patients
- Patients undergoing surgical/diagnostic procedures
- Patients suffering from chronic pain (e.g. cancer)
True/false: butorphanol is a potent analgesic.
False
It is not very potent
What types of chemicals are local anaesthetics? Where are they metabolised?
- Typically amides e.g. lidocaine or esters
- Esters are hydrolysed in blood plasma by cholinesterases
- Amides are metabolised in the liver
Describe the mechanism of action of local anaesthetics
- They exist in equilibrium between ionised and non-ionised form
- The receptor they interact with is located within the pore of the sodium channel
- Only the ionised form can interact with the receptor
- The local anaesthetic is injected close to the nerve and leads to a neural blockade
What is a nerve locator and how does it work?
- Nerve locator = hollow needle through which current can be passed
- When close to the nerve, we see a twitching of the muscle mass
- Switch off the current and inject the local anaesthetic at this location
Describe the mode of action of alpha-2 agonists and their effects
- They interact with alpha-2 receptors at the presynaptic membrane
- They provide potent but short-lived analgesia (~1 hr)
- They also provide sedation, and decrease HR
Describe the mode of action of ketamine as an analgesic
- NMDA receptor antagonist
- May also interact with other receptors
What are some example of opioids?
Methadone Fentanyl Pethidine Buprenorphine Butorphanol Morphine (not licensed)
