Opioids

Question 1

Opioids

Answer 1

• Fast acting and potent analgesic
• Prevent transmission of pain signals to nociceptors
• Drug molecules with a high affinity (stronger ability to bind to a receptor) may exhibit a greater effect than drugs with low affinity
• Intrinsic activity = ability of a drug molecule to produce a cellular effect after binding with a receptor
• Opioids vary in intrinsic activity

Question 2

Agonist

Answer 2

• Good affinity for the receptor and produces maximum cellular stimulation

Question 3

Antagonist

Answer 3

• Good affinity for the receptor but has little to no cellular stimulation
• I.e. the molecule binds to the receptor and blocks (antagonises) other molecules (e.g agonists) from binding to and stimulating the receptor

Question 4

Partial Agonist

Answer 4

• Good affinity for the receptor but produces a lower degree (25%) of intrinsic activity than an agonist
• May partially reverse or inhibit effects of a pure agonist (if partial given after a pure agonist, it would reduce the effects of the stronger drug while still providing its weak effects)
• Has a ceiling effect (giving higher concentrations does not produce a greater effect)

Question 5

Mixed Agonist/Antagonist

Answer 5

• Combines multiple types of receptors, stimulating them simultaneously
• May be solely mixed agonist, a mixed antagonist or a mixed agonist/antagonist

Question 6

Opioid Receptors

Answer 6

• Mu = strongest degree of analgesia
• Kappa = milder degree of analgesia
• Delta = minimal degree of analgesia

Question 7

Morphine

Answer 7

• Pure Mu receptor agonist
• IV, IM, SQ, CRI, Epidural, PO (dogs)
• 30mins to 3h duration
• Moderate to severe pain
• Insufficient analgesic for surgery when used alone due to its maximum analgesic effect

Question 8

Methadone

Answer 8

• Pure Mu receptor agonist
• IV, IM, SQ
• 4h to 6h duration
• Moderate to severe pain

Question 9

Butorphanol

Answer 9

• Mixed agonist/antagonist (Kappa agonist/partial Mu antagonist)
• IV, IM, SQ
• Up to 1h duration
• Mild pain

Question 10

Buprenorphine

Answer 10

• Partial Mu agonist
• IV, IM , SQ, OTM (cats)
• 6h to 12h duration
• Mild to moderate pain

Question 11

Fentanyl

Answer 11

• Pure Mu agonist
• IV, transdermal patch
• 15mins to 30mins injectable duration
• Constant delivery of patch for 72h
• Moderate to severe pain

Question 12

Naloxone

Answer 12

• Pure Mu antagonist
• IV
• 30mins to 60mins duration
• No analgesic effect
• Classed as reversal agent for opioids
• High affinity for Mu receptors but no intrinsic activity so will block and reverse effects of any Mu receptor drug molecule

Question 13

Morphine Side Effects

Answer 13

• CNS depression or excitement
• Dysphoria (more common in cats)
• Restlessness in dogs if no pain present
• Temp changes (hypo in dogs, hyper in cats)
• Pupil changes (miosis in dogs, mydriasis in cats)
• Resp depression or panting (dogs)
• Bradycardia
• Urinary retention
• Vomiting and salivation
• Histamine release if given rapidly IV
• Increased intracranial pressure

Question 14

Methadone Side Effects

Answer 14

• Profound sedation
• Dysphoria
• Bradycardia
• Resp depression
• CNS depression or excitement
• Thermoregulation effects
• Reduced gut motility
• Urinary retention
• Mydriasis (cats)
• Increased intracranial pressure
• Decreased tear production

Question 15

Butorphanol Side Effects

Answer 15

• Cardiac depression (much less than other opioids)
• Decreased blood pressure (much less than other opioids)
• Resp depression
• Sedation
• Ataxia
• Mydriasis
• Hypersalivation
• Decreased tear production
• Mucus accumulation

Question 16

Butorphanol #2

Answer 16

• Mostly utilised for sedation
• Useful as a reversal for negative effects produced by pure Mu opioids (morphine/methadone) such as CNS and respiratory depression
• Antitussive and antiemetic effects
• Ceiling effects noted

Question 17

Buprenorphine #2

Answer 17

• Difficult to reverse and reported to block effects of pure Mu agonists if administered after
• Ceiling effect limits analgesic effectiveness
• Useful analgesic in cats when given OTM in cats

Question 18

Buprenorphine Side Effects

Answer 18

• Resp depression
• Bradycardia
• Miosis (dogs) and mydriasis (cats)
• Behavioural changes (cats)
• Hypersalivation

Question 19

Fentanyl #2

Answer 19

• Considered one of the most potent analgesics
• Rapid onset and short duration of action
• Typically administered via CRI after an IV loading dose
• Can be given via a transdermal patch (set dose per hour)

Question 20

Fentanyl Side Effects

Answer 20

• Bradycardia
• Profound sedation
• Resp depression
• Panting (dogs)
• Increased sound sensitivity
• Dysphoria
• Mydriasis (cats)

Question 21

Fentanyl Patch

Answer 21

• Do not have immediate onset, can take 12h to 24h to reach therapeutic blood levels
• Must clip hair first
• Do not cut or fold, may cause erratic release of drug
• Patients must be normothermic prior to placement
• Hyperthermia/heating device may cause excessive release
• Hypothermia may cause slowed release
• Must be labeled when placed (patch size, date of placement, activeness and expiry)
• Do not let patient lick or chew patch

Question 22

Opioid Adverse Effects

Answer 22

• Are generally related to areas in which opioids exert their effects (CNS, gastro, urinary, smooth muscle)
• Often is dose dependant and may vary between species and patient