Analgesics notes Flashcards
How do opioids work?
- Modify transmission in dorsal horn
- Affects central/ emotional components of pain
- Natural (opiates) and synthetic
Discuss the receptors opioids act on
- Most domestic species have mu receptors
- Birds have kappa receptors
- Also act on delta and nociceptors
Describe the use of morphine
- Full mu/ delta/ kappa antagonist
- Po/ SC/CRI/IM/IV/Epidural
- Oral increases time of action
- Reduces MAC/ MIR
- Schedule 2 CD
- No license
Discuss the use of methadone
- Full mu agonist
- Affinity for ndma receptor
- Norepinephrine/ serotonin reuptake inhibitor
- IV
- 4hrs
- Premed for sedation
- CD schedule 2
- Dogs/ cats
Describe use of pethidine
- Short acting 1.5hrs
- Mu agonist/ some A2b
- Blocks Na channels
- Negative inotrope (incr HR)
- Histamine release so NOT IV
- Dogs/ cats/ horses
- Schedule 2
Discuss the use of fentanyl
- Short acting- 20 mins
- Mu agonist
- Lipid soluble
- Intra op bolus
- Induction with BZD to heelp with intubation
- CRI/ transdermal patches good for continuous release BUT ingestion risk for animal/ children
Describe tramadol
- Licensed in dogs
- Low potency partial mu agonist
- 5ht uptake inhibitor: may cause sedation instead of pain relief
Describe buprenorphine
- Partial agonist mu (mild kappa antagonism)
- Potent but not like pure opioids
- Sedation/ mild-mod pain
- Oral trans mucosal route
- Easily displaced
Describe butorphanol
- Kappa agonist/ mu antagonist
- Short to med duration
- Used with ACP for sedation of cardiac patients
- License with dogs/ cats/ dogs
Which pain pathways can be treated with which drugs?
- Peripherally: NSAIDs/ LA
- Peripheral nerve: LA block
- Dorsal horn: LA, Opioids, A2s
- Brain: Opioids/ A2s
What is post injury hypersensitivity?
- Red, inflamed areas hurt because of post injury hypersensitivity, makes pets more likely to nibble and chew wounds.
- Can treat with NSAIDs for example which causes a temporary decrease in pain
- Better to give analgesic before injury so hypersensitivity reaction doesn’t occur therefore much less pain.
List some side effects of opioids
CVS effects Vomitting Fever Pruritis Ileus
What receptor does ketamine act upon?
NMDA
List the receptors used by the different opioids
- Morphine: mu, delta, kapp (full)
- Methadone: mu( full), NMDA (partial)
- Pethidine: mu
- Fentanyl: mu
Which opioid can you NOT give IV?
Pethidine (histamine response)
Describe the LA method of action
- Unionised LA crosses cell membrane (active form). Its then ionised so it can block the Na+ channel.
- Results in the cell not being able to depolarise emaing it cannot transmit a pain signal.
Why is pH a problem in inflammed tissue?
-Inflammation causes the pH to decrease, becomes more acidic
-Results in LAs being ionised quicker so they cannot ente the cell so work less effectively.
-Will switch between ionised/ unionised so some level of efficacy just takes longer
-
What do LA drugs end in?
‘caine’
Describe LA toxicity
The toxic dose very close to effective dose= narrow window
- ALWAYS aspirate before injecting to ensure not IV as this causes toxicity
- CNS signs, CVS signs, methoaemoglobinaemia
Other than infiltrating somatic tissue, what other ways do we use LAs?
- Intubeaze (lidocaine spray)
- EMLA (Lidocaine/ prilocaine)
- Proparacaine (used for cornea)
- Direct into abd/ thoracic cavities (lidocaine/ bupivicaine)
- IV (LIDOCAINE ONLY)
What are the effects of IV lidocaine?
- Decrease MAC
- Never give IV to cats!!!
What are the different techniques for placing a nerve block?
- Nerve stimulator
- Ultrasound
Why would you do a phrenic block?
For forelimb surgery
What are soaker catheters?
- Useful for total ear canal ablations
- Large bore catheters with tiny holes
What is an IVRA and how do you perform one?
- Intra Venous Regional Anaesthesia
- Esmarch bandage applied and tourniquet proximal
- Bandage removed and lidicaine injected into a vein distal to tourniquet
- Tourniquet can remain for up to 90 minutes
Describe the action of NSAIDs
- Inhibit prostaglandins (COX/ LOX)
- NSAIDS inhibit cyclooxygenase (steroids inhibit phosphilipase so no arachidonic acid formed)
What does COX 1 do?
- PG synthesis along length of GIT (maintain blood flow, increase mucus production, vasodilation to the kidney)
- Found in neurons
- Present in fetus, amniotic/ uterine tissue
- Found in platelets
- ‘Housekeeping’ enzyme: keeps everything in balance
What effects does inhibiting COX 1 have?
- Prevents blood clotting as COX 1 converts arachidonic acid into TBX A2
- TBX A2 potent vasoconstrictor & aggregatory mediator
What effects does inhibiting COX 2 have?
Causes a reduction in rate of healing of gastric mucosa so in patients with IBD you make their symptoms worse
How does Grapiprant work?
Instead of inhibiting COX it inhibits PGs
PGE2 and EP4
In which patients are NSAIDs contraindicated?
- Renal
- Hypovolaemia
- Gastric ulceration
- Steroid patients
- Hepatic (less of a worry, must be advanced)
(i.e. be most wary of kidneys and stomach!!)
What are the subtypes of A2 receptors?
A
B
C1
C2
What are the effects of A2 agonists?
- Sedation
- Reduce MAC
- Analgesia
- Hyper/ hypotension
- Decreased CO/HR
- Respiratory depression
- Increase urine production
- Decreased GI motility
Why do you get a biphasic reaction with A2 agonists/
- Vasconstriction causing BP to increase
- Baroreceptors compensate for this causing decrease
What causes the NMDA receptor to be triggered?
Repeated trauma
Causes chronic sensitisation and upregulation of pain
How long post op should we provide pain relief?
24-72 hrs for routine procedures
