Analgesia Flashcards
Define hyperalgesia
A decrease in pain threshold/increase in perceived pain for a given noxious stimulus
Define appropriate pain
Pain while injury is still there
Define chronic/pathologic pain
Pain that extends beyond the healing process
Describe the effect of post-injury analgesia on post-injury hypersensitivity
- Analgesia will decrease hypersensitivity and pain
- Effects only last as long as analgesic action
- Once wears off, go back up to hypersensitivity that would have been present without analgesia
- Repeated doses until patient will no longer experience pain
Explain post-injury hypersensitivity
- Following injury, will have a hypersensitive period where contact to the site is painful
- Pain may continue after the injury has healed
Describe the effect of pre-emptive analgesia on post-injury hypersensitivity
- Analgesia is in effect at the time of injury (surgery)
- No hypersensitivity occurs after
- No chronic pain
What is IVRA and what is it used for?
- Intravenous regional anaesthesia
- Good for providing anaesthesia and analgesia in an area where it is needed, especially in cattle e.g. for claw removal
Name different classes of analgesic drugs and state whether they act peripherally or centrally
- Opioids (central)
- Alpha2 agonists (central)
- Local anaesthetics (peripheral)
- NSAIDs (peripheral)
Where can analgesic drugs act?
- May act at site of injury, decrease pain associated with inflammatory reaction
- May alter nerve conduction
- May modify transmission in dorsal horn
- May affect central component and emotional aspects of pain
Name the opioid receptors and state which is the most important
- Mu (most important)
- Delta
- Kappa
- Nociceptin
What is nociceptin?
A pro-nociceptive receptor
Name the opioids used in analgesia
- Morphine
- methadone
- pethidine
- Fentanyl
- Buprenorphine
- Butorphanol
Outline the key features of morphine (duration of action, routes of administration, licensing, effect on MAC/MIR)
- Most efficacious opioid at relieving pain
- Significantly reduces MAC and MIR
- Can be given PO, SC, IM, IV, CRI, epidural
- Duration of action approx 4 hours
- Not licensed, schedule 2 controlled drug
Outline the key features of methadone (site of action, administation, side effects, uses, licensing, duration of action)
- Action on NMDA receptor, involved in chronic pain
- Norepinephrine and serotoning reuptake inhibitor
- 4 hours duration of action
- Used as premed, sedative, introp, in recovery, CRI
- IV administration, poor oral availability
- Licensed for dogs and cats
Describe the key features of pethidine (duration of action, administration, licensing)
- Significant negative inotropic effects, increase heart rate with no cardiac output change
- Controlled drug Schedule 2, licensed for dogs, cats and horses
- Duration: 1.5hours
Describe the key features of fentanyl (uses, administration, drug schedule)
- Intraop as bolus, at induction with BZD
- IV, CRI, transdermal patches, spot ons
- Controlled drug, Schedule 2
Describe the key features of buprenorphine (administration, licensing, drug schedule, uses)
- High potency, lower efficacy
- Used for mild-moderate pain, good sedative, long duration of action
- IV administraiton, oral transmucosal route
- Peak effect after 45-60mins
- Licensed for dogs, cats, horses
- Schedule II drug but does not need recording
Describe the receptor affinity of buprenorphine
Partial agonist, strong affinity for mu, kappa antagonist
Describe the key features of butorphanol (adminstration, uses, licensing)
- Oral, IV
- Useful in combination with ACP for sedation of cardiac patients
- Antitussive
- Licensed fo rdogs, cats and horses
Describe the receptor actions of butorphanol
- Kappa agonist
- Mu antagonist
List potential unwanted effects of opioids
- CVS effects
- Pruritus
- Urinary retention
- Ileus
- Pancreatic duct
- Pyrexia
- Miosis
- Mydriasis
- Vomiting and nausea
Describe the receptor actions of morphine
- Full agonist
- Mu, delta and kappa receptors
What is the main side effect seen with morphine?
Vomiting
Describe the receptor action of methadone
- Full mu agonist
- Affinity for NMDA receptor
What is the main side effect seen with methadone?
Less vomiting vs morphine but may still occur
Describe the receptor interactions of pethidine
- Short acting mu agonist
- Sodium channel blocker
- Alpha2B subtype receptor agonist
What are the contraindications for pethidine?
- Causes histamine release so should not be administered IV
- Should not be administered to dogs receiving selegiline
- Avoid combination with Monoamine oxidase inhibitors to prevent serotonin syndrome occurring
Describe the receptor interactions of fentanyl
- Mu agonist
- Short acting
What are the main side effects seen with fentanyl?
- Bradycardia
- Respiration may slow or stop following adminsitration of a bolus
Outline the use of codeine in veterinary medicine
- Often with paracetamol
- Mild-moderate pain post-op
- No evidence for efficacy
What is naloxone used for?
- Is a total antagonist of pure mu opioids
- Only used in overdose situation as will also block action of endogenous opioids
Outline the use of alfentanil, sufentanil, remifentanil
- Pure mu agonist opioids
- Used in anaesthesia to blunt sympathetic stimulation
- Short half lives
- Low dose for analgesia or high dose as part of TIVA
- Reduces MAC in dogs and cats
- Not licensed for use in dogs and cats
When might remifentanil be used and why?
- In dogs with no liver function
- Metabolised by plasma esterases so no liver function required
- Administered CRI
Outline the use of tramadol
- Popular, but lack of supporting data
- Is a pro-drug and so required metabolism, only 5-10% of dogs are able to
- Weak opioid action
- Increases serotoning of patient and so hides behaviour indicative of pain
Outline the use of partial and mixed agonist/antagonist opioids
- e.g. buprenorphine and butorphanol
- Buprenorphine has higher affinity for receptors and will displace full agonists, weaker analgesic effect
- Butorphanol has very short duration of action (10 mins)
What are the different modes of action of local anaesthetic agents?
- Affect cell membrane directly
- Block sodium channel function
Explain how local anaesthetics may block channel function
- Local anaesthetic initially unionised
- Enters cell, becomes ionised
- Ionised form blocks channels
Give examples of uses of local anaesthetics
- Lidocaine spray (intubeaze)
- Local nerve blocks
- Lidocaine jelly
- EMLA cream
- Proparacaine/butacaine for corneal block
- Instillations of lidocaine/ropivicaine/bupivicaine into wounds
- Epidural blocks
Describe the use of lidocaine IV
- More beneficial in neuropathic pain
- Good anti-ventricular dysrhythmia drug
- Can be used in dogs and horses as an infusion
- MAC decreased, reduced opioid requirement
- Useful post-op
- Low toxic dose in cats
- CRI in horses with laminitis
List commonly used local anaesthetic agents
- Lidocaine
- Prilocaine
- Bupivicaine
- Mepivicaine
- Ropivicaine
- Etidocaine
- Amethocaine
- Proparacaine
- Cocaine
What are intercostal blocks used for?
Analgesia for whole chest wall
What is the main factor that affects the onset of action of local anaesthetics?
- Speed of onset inversely related to degree of ionisation
- Are weak bases, largely ionised at physiological pH
What is the effect of inflamed tissue on local anaesthetic onset of action?
- Inflamed tissue is acidic
- Longer time to onset of action in inflamed tissue
What does the potency of local anaesthetics relate to?
Their lipid solubility
What is the duration of action of local anaesthetics related to?
Their degree of protein binding