PO Pain Flashcards
What are the 3 majors sites of action of opioids?
- Brain (supraspinal)- opioids work prea nd post synaptically to activate descending inhibitory pathways
- Spinal cord (spinal)- directly on the dorsal horn of the SC
- Periphery- peripheral terminals of nociceptive neurons
What are opioids are used for in anesthesia?
- Attenuate the SNS response to noxious stimuli
- Adjunct to inhaled agents during anesthesia
- Sole anesthetic (fentanyl/sufent/morphine- cardiac anesthesia only high doses, critically ill pateints
- perioperative and post op control of pain
What are opioid characteristics that set them apart from other analgesics?
- Moderate to severe pain
- no max dose or ceiling effect
- tolerance can develop with chronic use
- tolerance associated with physical dependence but not necessarily with psychological dependence
- cross-tolerance- tolerant to one opioid, will be tolerant to others
- produce analgesia wihtout loss of
- touch
- proprioception
- consciousness (in smaller doses)
What are some opioid classifications by class?
- Naturally occuring
- morphine and codein
- Semisynthetic: analogs of morphine
- heroin and dihydromorphone
- Synthetic
- exogenous 4 groups
What are opioid classifications by action at receptor?
- Agonist- full activation
- Partial agonist
- Mixed agonist/antagonis- less activation
- Antagonist- prevents activation
What is mechanism of action of opioid?
Activate sterospeicfic G protein coupled receptors
- post synaptic- directly decrease neurotransmission
- increased K conductance (hyperpolarization)
- Ca channel inactivation (decreased NT release)
- Modulation of phosphoinositide- signaling cascade and phospholipase C
- Inhibition of adneylate cyclase (decreased cAMP)
- pre synaptic- inhibits release of excitatory NT
- decrease acetylcholine, dopamine, norepinephrine, substance P release
What are the main opioid receptors?
- Mu, kappa, delta
- theory- synthetic opioids mimic action of endogenous opioids by binding to opioid receptors
- activate endogenous pain modulating systems
- variable affinity and efficacy at different receptors types among the different drugs in this class
What are the Mu receptos?
- Subtypes mu-1 and mu-2 (all opioids interact with these 2 receptors)
- Mu-3 receptors thought to be involved in immune process
- All endogenous and exogenous agonists act on mu receptors
- Mu receptors in brain, periphery and spinal cord
What does Mu-1 cause?
- Supraspinal (most), spinal and peripheral analgesia
- euphoria
- miosis
- bradycardia
- urinary retention
- hypothermia
- all endogenous and synthetic opioid agonists work on these receptors
What does Mu-2 cause?
- Hypoventilation
- Physical depedence
- spinal analgesia (also some supraspinal)
- constipation
- all endogenous and exogenous agonists act on these receptors
What does Kappa receptor cause?
- Suprasinal, spinal and peripheral analgesia
- dysphoria
- sedation
- miosis
- diuresis
- dynorphins act on these receptors
- opioid agonist-antagonists often have principle actions at the kappa receptor
What does delta receptor cause?
- Peripheral, supraspinal and spinal analgesia
- Hypoventilation
- Constipation
- Urinary retention
- Physical dependence (on chart)
- Enkephalins work on these receptors
What are some genetic influencings on cyp 450 system and influence on opioids?
- CYP2D6 has 5 common mutations that can alter metabolism of
- codeine, oxycodone, hydrocodone and methadone
- can have unpredicatable pharmacokinetics and 1/2 lives
- Fentanyl metabolism least likely to be impaced by genetic variability in surgical population= has predictable pharmacokinetics (most immune to variances in metabolism
- Rate of metabolism may influence side effect rate
- studies indicate ultra rapid metabolizers at increased r/f PONV
- Believe someone if they say morphine makes them really nausous and avoid morphone analogs (fentanyl ok)
- studies indicate ultra rapid metabolizers at increased r/f PONV
What are some systemic effects of opioids?
- Many systemic effects similar among opioids
- althought diff opioids are active at diff receptors to diff degress
- variable s/e and efficacy profiles
- makes sense if you consider the variance in chemical structures
- morphine as prototype
What are some CNS effects of opioids?
- Analgesia
- Euphoria
- Drowsiness/sleep
- miosis (pupillary constriction)
- nausea- chemoreceptor trigger zone
- does NOT produce amnesia
Pulmonary effects of opioids?
- Decreased RR and increased TV
- at higher doses, decreased RR and TV, leading to apnea
- decreased response to CO2/hypoxia
What are some GI effects of opioids?
- Decreased gastric emptying
- direct stimulation of chemoreceptor trigger zone on the floor of 4th ventricle
- partial dopamine agonist?
- balanced by depression of the medullary vomiting center
What causes pruritis by opioids?
- Cause unknown
- occurs primarily on face, particularly nose
- “fenatnyl nose itch”
- histamine release most probably cause with some i.e. MSO4 (also demerol)
What is morphine?
- Naturally occuring opioid
- Severe acute pain almost always IM or IV admin
- PO used for chronic pain and cancer pain
- slow release formulations available- delayed onset 3-5 hours (not used preop/intraop much)
- considerable first pass effect
- 1/2 life 3-4 hours converted to active metabolite (morphine 6 glucuronide)
LAST SEMESTER STUFF (JUST IN CASE):
- Poor lipid solubility, PB= 35% (dr e syas this is high…) hightly ionized
- histamine release
- IM peak effect 45 min; IV 15-30 MIN
- DOA 4 hours
-
BRADYCARDIA VIA DIRECT STIMULATION OF VAGUS NERVE
- Inhibition of SA node as well
- metabolized by liver
- active metabolite M6G > potency than MSO4
- Kidneys play role in extrahepatic metabolism
- renal failure will have prolonged effects d/t M6G- AVOID!!
What is codeine?
- Mild pain relief
- PO
- e1/2 t= 3 hours
-
prodrug: 10% is metabolized by cyp2D6 to its active form
- remaining drug is demethylated to inactive metabolite
- active form= morphine
- 10% caucasians, 30% asians lack 2D6– no analgesic effect
- antitussive effect remains even without conversion
- better for cough (lower dose) than pain relief
What is hydrocodone?
- Vicodan
- PO
- Always combinesd with either ASA, ibuprofen, antihistamine, acetaminophen
- analgesic and antitussive
- used for chronic pain
- high abuse potential
What is oxycodone?
- PO
- AKA oxycontin, percocet, percodan
- available in sustained release preparation (oxycontin)
- mod to severe pain; useful for chronic pain, postop pain
- also in combo with ASA or tynelol
- no active metabolites- safer in patients with renal dysfunction
- high abuse potential
What is methadone?
- PO, IV, Sub q
- synthetic
- long plasma half life 8-59 hours or 13-100 hours (sources vary with range)
- opioid addiciton treatment (maintenance) dosed QD
- No active metabolites- safer in patients with renal dysfunciton
- chronic pain syndrome treatment: doses BID or TID q 4-8 hours
- neuropathic pain
- at risk for severe respriatory depression secondary to prolonged and unpredictable e1/2t
Tolerance to opioids?
- Tolerance is common with chronic use of opioids (after 2-3 weeks of use)
- pt first notices a reduction in adverse effects
- shorter duration of analgesia
- followed by decrease in effectiveness of each dose
- tolerance to most adverse effects include
- respiratory and CNS depression
- can be surmounted by increasing the dose
- cross tolerance exists amojng all full agonists but is not complte
- when switching to another opioid, start with half or less of the customary equianalgesic dose
- switching opioid tolerant patients to methadone may improve pain relief
- tolerance ot sedative and emetic effects develop rapidly but not constipation
- a stimulant laxative with or without a stool softener should be started early in tx
- senna/docusate
What is dependence?
- physical dependence causes abstinence symptoms upon sudden d/c
- clinically significant dependence develops only after several weeks of chronic tx
- addiction involves psychological dependence and biologic and social factors
- cancer pain and acute pain patients rarely experience euphoria an deven more rarely develop psychologicla dependence or addiction
Dosage of opioids?
- Dose vary widely from one pt to another
- no minimum or max dose except limitation by the dose of acetaminophen or aspirin
- dose required to maintain optiumum pain relief with tolerable side effects must be used
- after intiial titration with short-acting opioid in first 12-24 hours, dose determination by around the clock dosing is recommneded
- a sustained release forumlation should be used in chronic pain
- immediate release doses that are 10-15% of the total daily dose should be used for breakthrough pain
- PCA given IV, SQ, or other routes are now widely used when the oral route are not feasible
Equianalgesic doses for control of of chronic pain?
If pain is well controlled and switching opioids, calculate equivalent dose and reduce by 25% for dose of new drug
What is equianalgesic dose of meperidine (PO and SQ/IV?)
PO= 150 mg
SQ/IV= 50 mg