Opioids Flashcards
what do opioids do for level of wakeness and pain
General anesthetics produce unconsciousness but do not inhibit pain signal generation or propagation
why do patients not feel mildly painful surgical manipulations
Patients do not feel mildly painful surgical manipulations (e.g., a skin incision) simply because the small numbers of pain signals, which are transmitted to the Reticular Activating System (RAS) in the medulla, are insufficient to awaken the patient
how do we prevent the RAS from being stimulated to the point that an anesthetized patient wakes up?
We can prevent this by administering an analgesic beforehand to inhibit pain signal generation
is it safe to administer analgesic before anesthetic? why?
This is safer than trying to maintain unconsciousness using higher doses of general anesthetic (which inhibits respiration & CVS function in a dose dependent manner)
four things opioid drugs are used for
-analgesia; management of moderate to severe pain
-sedation
-cough suppressant (antitussive)
-treatment of diarrhea (constipation is a major side effect)
what three opioid receptor families mediate analgesia
mu (MOR)
delta (DOR)
kappa (KOR)
what one of the opioid receptor families produces dysphoria
sigma
what is the mechanism of opioid receptors
Opioids stimulate opioid receptors in pain pathways
K+ channels open which causes hyperpolarization and then inhibition of APs
also inhibition of calcium entry which inhibits NT release
main effect of MOR stimulation, and others
analgesia; full stimulation causes intense analgesia effects
-euphoria
-miosis or mydriasis
-respiratory depression
main effect of KOR stimulation, and others
analgesia; moderate effect, and primarily visceral
-negligable respiratory depressive effects
what opioid receptor mediated respiratory depression
Mu receptor
difference between full vs partial agonists
full = opioid maximally stimulates opioid receptor
partially = opioid weakly stimulates opioid receptor
what makes a mixed agonist/antagonist
some opioids are agonists at some opioid receptors and antagonists at others
what type of agonist is fentanyl and what effect does it have
Fentanyl is a full mu agonist = produces an intense analgesic
effect via MOR
what type of agonist is butorphenol and what effect does it have
Butorphanol is a kappa agonist
with mild to moderate analgesic
effect
It also very weakly stimulates Mu
receptors, acting as a competitive
antagonist to full Mu agonists
what is the analgesic effect of butorphenol and fentanyl combined
weaker analgesia than with fentanyl alone
primary effect of
what drugs have a mu receptor and what are their interactions
codeine, morphine, hydromorphone, meperidine, fentanyl (all agonists)
buprenorphine and butorphenol (partial agonist)
what drugs have a kappa receptor and what are their interactions
buprenorphine and butorphenol (full agonists
what drugs have analgesia as their primary effect
codeine, morphine, hydromorphone, meperidine, fentanyl, buprenorphine and butorphenol
what drug is good to use for cough suppression
butorphenol
what drug is good to use for constipation
loperamide
what drug is good to use for emesis in dogs
apomorphine
what is the relative duration of analgesia for buprenorphine
4-8 hours
what is the relative duration of analgesia for hydromorphone
3-6 hours
what is the analgesic potentcy of fentanyl relative to morphine
100x
how are opioids normally administered and why
Phase I & Phase II metabolism with high first-pass effect = most opioids are therefore administered parenterally
do dogs convert codeine into morphine
only at low or negligable amounts. not nearly the same as humans do
what can high doses of morphine cause?
excitement
-sham rage (cats)
-typically not seen with hydromorphone
physiological effects of full Mu agonists on the CNS
-3-6 hours of sedation/analgesia
-sedation is most likely in dogs, excitement is most likely in other species
physiological effects of full Mu agonists on the cardiovascular system
Usually little effect on CVS at the dosages normally administered
what can morphine trigger
may trigger histamine release = vasodilation = hypotension
physiological effects of full Mu agonists on the respiratory system
- Dose-dependent depression
- More intense effect when combined with a general anesthetic
- Death from overdose is due to respiratory arrest – a Mu effect
- Use naloxone to reverse
- Opioids suppress the cough reflex
physiological effects of full Mu agonists on the gastrointestinal system
- Increase segmentation but reduce propulsion in large bowel = stool becomes dehydrated = constipation
- Bile duct sphincter constriction = increase in gall bladder pressure
= biliary colic - Nausea & vomiting (chemoreceptor trigger zone MOR stimulation)
what do opioids do to the bladder
Urinary bladder sphincter tone increased & detrusor muscle tone
increased = urgency to urinate but difficult
what do we specifically need to worry about related to bladders and uterus when giving opioids
- Dogs may have difficulty urinating post-op
- Problems with blocked cats
- Opioids also slow uterine contractions, inhibit birth reflexes, depress neonatal resp.
most common way fentanyl is administered
transfermal patch
what is fentanyl used for
- For profound pain relief in small animals
- Also commonly used as an analgesic and sedative prior to induction
do you need a reversal agent for etorphine? what is its potency compared to morphine
yes, its required for recovery
4000x more potent than morphine
what is butorphenol used for
Used for mild/moderate pain; ineffective for severe pain
what type of agonist is butorphenol
kappa agonist and mu antagonist
main advantage of buprenorphine
-Main advantage is for home (oral transmucosal) administration for animals with chronic pain (long
duration)
what is apomorphine used for
used to cause dogs to vomit
can naloxone sotp vomit reflex
no
what is an alternative to apomorphine
ropinirole
what is a common anti-diarrheal in vet med
loperamide (imodium)
what is the main opioid antagonist and how does it work
naloxone
bind and block mu opioid receptors through competitive inhibition
