4. Week 4 Pain and Analgesia part 2 Flashcards
What is acute pain?
a. Associated with actual tissue damage (or threat of this) due to surgery, injury or disease
b. RAPIDLY alters animal’s behaviour
i. To minimise damage
ii. Optimise conditions for tissue healing
iii. Typically stops when healing completed
c. Varies in severity depending of different factors
i. Predominantly the size and magnitude of the noxious stimulus
d. Relatively easy to treat
What is chronic pain?
a. Persists beyond expected time frame of tissue healing (>3 months, arbituary time period)
b. May be associated with ongoing disease/ injury e.g. osteoarthritis
c. Maladaptive (no useful function)
i. Unlike acute pain serves no useful purpose
d. Often poorly responsive to treatment
What happens is acute pain isn’t adequately managed?
- Unstable GA
- Poor animal welfare – violation of 5 freedoms
- Prolonged hospitalisation/ delayed recovery
- Development of central sensitisation chronic/ long term pain
a. Ongoing welfare problem for animal and owner/ caregiver
i. Behavioural issues – 60% cases underlying pain
b. Long term animal behavioural issues
c. Expensive and difficult to treat
What are the stages of the nociceptive pathway?
- Transduction
- Transmission
- Modulation
- Perception
Look at notes info on each page 5
Why are the 4 stages of the nociceptive pathway important?
different drugs will target different stages of this pathway
What drugs do we have to manage acute pain?
NO PLAN
- NSAIDS
- Opioids
- Paracetamol
- Local anaesthetics
- Alpha 2 agonists
- NMDA antagonists
Tramadol
Gabapentin
Antidepressant medication
NSAIDS
- COX inhibitors
- Often first line analgesic treatment
- Reduce production of inflammatory mediators responsible for receptor sensitisation both peripherally and centrally, affecting: (3/4!)
– Transduction
– Modulation
– Perception
Stages of nociception pathway
More on doc
Opioids
o butorphanol
o buprenorphine
o methadone
o fentanyl
o (morphine)
Pethidine
Acts on 3/4 stages of nociceptive pathway (TTMP)
• Commonly included in pre-anaesthetic medication
• Act centrally and peripheral analgesic
– Centrally – opioid receptors expressed in dorsal horn neurones; endogenous opioids stimulate descending inhibitory pathways and reduce neurotransmitter release
– Peripherally - receptor expression upregulated in inflamed tissue
• Principally affect modulation and perception of nociceptive pain pathway and transduction to a lesser degree
What opioid tends to be our first choice for efficacious analgesia in dogs and cats and why
METHADONE (opioid)
C&D
- methadone tends to be first choice for efficacious analgesia.
a. Licensed
b. Convenient dosing interval (approx. every 4 hours)
c. Admin several diff routes
d. Rarely causes emesis (vomiting), few side effects
e. Can cause panting when used pre med, especially if OD or not painful. Can be issue for brachycephalic. Methadone resets thermoregulatory set point in hypo so animal thinks it is hotter than it is.
Severely painful animal what type of opioid do we wanna use
FULL mu agonist opioid Pethidine Morphone Methadone Fentanyl
Pethidine is licensed in D and C and horses so why not used as much as methadone?
- Licensed D&C&H
- Usually only works 1 hr
- Regular repeat doses therefore
- NEVER IV as causes histamine release
- Painful injection
- Not realistically best to use as painful injection every hour☹
- Only full mu agonist in horses but as said above not sued v frequently
- Vet prep not on market at moment
Fentanyl
OPIOID
Full mu agonist
a. Rapid onset
b. Short duration of action = rarely used (20 mins)
c. Can be useful to admin as top up or rescue during GA procedure
d. Can be used nicely as constant rate infusion
e. Transdermal fentanyl patches used in humans can be used in animal but pose risk to human
Morphine
- Not licensed in any veterinary species
- Used because available as preservative free prep so can be used epidurally
- Epi morphine can last 18-24 hrs – prolonged post operative analgesia (poor lipid solubility)
- Systemic IV in horses theoretically should use pethidine for full mu but many implications to this so then go to methadone but horses more twitchy with methadone so we cascade to morphine
What si a possible side effect of all opioids in horses
locomotor excitement is a possible side effect of any opioid in horses
Why wouldn’t we use butorphanol in horse leg radiograph?
Predominantly sedative SA sometimes horses, not analgesic as mu antagonist. Locomotor excitement in horses – annoying if radiographs of legs and moving legs!!! Antitussive properties so good when coughing
Paracetamol
• NOT a typical NSAID, often incorrectly classified as one
• POOR anti-inflammatory properties compared to NSAIDs
• Analgesic mechanisms not fully understood
– Weak PG synthesis inhibitor (better centrally than peripherally)
– Stimulates descending serotonergic inhibitory pathways
– Inhibits reuptake of endogenous cannabinoids
– Acts centrally at modulation stage of nociceptive pathway
• Affect Modulation stage of nociceptive pathway
NEVER IN CATS AS TOXIC
Local anaesthetic
Acts on 2/4 stages of the nociceptive pathway (transduction and Transmission)
• Sodium channel blockers
– Prevent generation and propagation of action potentials inhibit transmission stage of nociceptive stimulus in all nerve types
– TRUE analgesics
• Prevent generation and propagation of action potentials
– Transmission in sensory, motor, somatic, visceral and autonomic nerves
– Peripherally and centrally
• Agents differ in terms of speed of onset and duration of effect (+/- adrenaline)
Alpha 2 agonist
Xylazine
detomidine,
romifidine
medetomidine, dexmedetomidine
• Common premedication agents
– All provide analgesia, sedation and muscle relaxation
– Variable α2 receptor selectivity
– Variable duration of effect
• Analgesia mediated by spinal and supraspinal effects
– Modulation; activate descending serotoneregic and noradrenergic inhibitory pathways
– Analgesia synergistic with opioids
• Sedation ≠ analgesia
• Somatic analgesia ≠ visceral analgesia
• All shown to affect GI motility
• All have profound cardiovascular side effects
• Analgesia peaks later than sedation and duration is shorter
• Antagonising sedation will antagonise analgesia
Tramadol
Acts on modulation and perception
• Multiple mechanisms of action
– μ opioid activity (mainly due to M1 metabolite)
– Inhibition of noradrenaline and serotonin uptake
– Increases serotonin release
– α2 receptor activity
– Licensed for use in dogs; has been described in cats and horses
• Caution if used with TCAs and serotonin reuptake inhibitors
Adjunctive therapies
– Physiotherapy – Massage – Acupuncture – Bandaging – Psychological aspects – Good nursing care • TLC • Feeding • Comfortable, safe environment • Defaecation/urination
