Opioids and Pain Medicine COPY Flashcards
What is an opioid agonist-antagonist?
- high mu affinity, little mu efficacy - Provide analgesia but with less risk of respiratory sedation and euphoria as compared to pure opioid agonists. - Have a ceiling effect –> escalating a dose beyond a certain level yields only side effect - May reverse respiratory depression and pruritis associated with pure agonists.
Name 4 opioid agonists-antagonists.
Pentazocine, butorphanol, buprenorphine, nalbuphine
What receptors do opioid agonists-antagonists act on?
- mu and kappa receptor partial agonists
Describe the mechanism of action of naloxone.
- A pure mu, kappa, and delta opioid receptor antagonist. - poor oral bioavailability - Reverses the effects of agonist drugs.
Describe the duration of naloxone.
- Peak effect occurs within 1-2 minutes of IV administration. - Duration of action is within 30-60 minutes.
Name several side effects associated with naloxone administration.
- Acute opioid withdrawal (e.g. nausea/vomiting) - Severe hypertension - Ventricular dysrhythmias - Pulmonary edema - NO seizure
Name typical side effects of opioids.
- Respiratory depression - Nausea/vomitting - Pruritis - Cough suppression - Urinary retention - Biliary tract spasm
Name allergic side effects of opioids
- histamine release causing hives, bronchospasm, and hypotension.
Name side effects associated with IV administration of opioids (think during induction).
- Abdominal and chest wall rigidity.
Name cardiovascular effects of opioids.
- dose-dependent bradycardia caused by vagal nucleus stimulation. - Minimal inotropic effect. - Meperidine has physical structure similar to atropine and may induce tachycardia.
Which opioids are associated with histamine release?
Morphine, meperidine, and codeine all associated with cutaneous reactions and hypotension. Morphine’s reactions appear to be dose-dependent.
Describe the mechanism of opioid-induced nausea.
- bind directly to opioid receptors in the area postrema of the medulla. - also sensitize the vestibular system. - incidence of N/V similar for all opioids regardless of route of administration.
What is methylnaltrexone?
A peripheral opioid receptor antagonist derived from naltrexone. Does not cross BBB and used for palliative use in opioid-induced constipation.
Describe the typical ventilatory pattern and response to CO2 in presence of opioids.
- Slow respiratory rate and increase tidal volumes - impair hypoxic ventilatory drive - Shift alveolar ventilatory response-to-CO2 curve to the right.
Describe analgesic onset, peak effect, and duration of IV fentanyl.
- Analgesic onset: 1-3 minutes - Peak effect: 3-5 minutes - Duration: 30-60 minutes
Describe analgesic onset, peak effect, and duration of IV hydromorphone.
- Analgesic onset: 5 minutes - Peak effect: 15-30 minutes - Duration: 4-5 hours
Describe analgesic onset, peak effect, and duration of IV morphine.
- Analgesic onset: 5-15 minutes - Peak effect: 30-90 minutes - Duration: 4-5 hours
Explain how fentanyl can have shorter duration of action but longer elimination half-life compared with morphine.
Elimination half-lives correspond with duration of action in a single-compartment pharmacokinetic model. Lipophilic opioids such as fentanyl are better represented by a multicompartment model since redistribution plays a much larger role than elimination in determining their duration of action.
Explain the concept of context-sensitive half-time and its relevance to opioids.
- Context-sensitive half-time is the time required for a 50% reduction in the steady-state drug plasma concentration once a constant infusion is terminated. - This time is determined by both elimination and redistribution, and it varies considerably as a function of infusion duration for commonly used opioids.
Explain why morphine may cause prolonged ventilatory depression in patients with renal failure.
- Morphine is primarily conjugated in the liver as morphine-3-glucuronide (50% to 75%) and morphine-6-glucuronide (10%), of which 90% is renally excreted. - Morphine-3-glucuronide is inactive, but morphine-6-glucuronide is approximately 100 times more potent than morphine as a m-receptor agonist.
Which opioids may be associated with seizure activity in patients with renal failure?
- Hydromorphone and meperidine - Hydromorphone-3-glucoronide and normeperidine can accumulate in renal failure and promote myoclonus and seizures.
What is remifentanyl and how is it different from other opioids?
- An ultrashort-acting opioid with 5-10 min duration and context-sensitive half-time of 3 minutes. - Metabolized by nonspecific plasma esterases. - Has been used as an IV bolus to facilitate intubation but with significant incidence of bradycardia + chest-wall rigidity. - Induces hyperalgesia and acute opioid tolerance –> do NOT use in patients with chronic pain.
Describe metabolism of codeine.
- Undergoes demethylation to morphine.
What are concerns with methadone dosing?
- Has a long and variable half-life. - Repeated dosing may lead to excessive plasma levels, especially on days 2-4.
What is the mechanism of action of methadone?
- Acts as mu-opioid receptor agonist - Acts as NMDA receptor antagonist. - Prolongs QT interval and may increase risk for torsades de pointes –> get baseline EKG and follow-up at 30 days.
What is tramadol?
- a codeine analog and atypical opioid - a reuptake inhibitor of norepinephrine and serotonin - has partial mu agonist activity and central GABA, catecholamine, and seratonergic activity - A moderately effective analgesic with lower incidence of respiratory depression, constipation, and dependence compared with other opioids - CONTRAINDICATED in patients with pre-existing seizure disorder (rarely may produce seizures) - can cause life-threatening serotonin syndrome with the use of other serotonin-norepinephrine uptake inhibitors, SSRIs, TCAs, or MAOis, so use cautiously in patients taking these medications
What are unique characteristics of meperidine? What cardiovascular effects may be seen?
- Has some weak local anesthetic properties, particularly when administered neuraxially. - Has atropine-like structure and may induce tachycardia (not bradycardia like other opioids) - can be used to suppress postoperative shivering.
In what class of patients is meperidine contraindicated?
- In patients receiving MAO inhibitors, because may lead to serotonin toxicity, hyperthermia, and death.
Describe the site and mechanism of action of neuraxial opioids.
- Neuraxial opioids bind to receptors in the dorsal horn of the spinal cord. - Activation of mu-receptors reduce visceral and somatic pain via GABA-mediated descending pain pathways. - Activation of k-receptors appears to reduce visceral pain via inhibition of substance P.
Discuss the effect of lipid solubility on neuraxial opioid action.
Lipophilic opioids: - diffuse across spinal membranes more rapidly than hydrophilic opioids. - have rapid onset of analgesia. - diffuse across vascular membranes more readily –> result in increased serum concentrations and a shorter duration of action. Hydrophilic opioids: - achieve greater cephalo-caudal spread when administered into the epidural or subarachnoid space. - attain broader analgesic coverage than lipophilic opioids but may result in delayed respiratory depression following cephalad spread to the brainstem.
Are opioid receptors exclusively in the central nervous system?
No. Peripheral opioid receptors exist on primary afferent neurons, but they are functionally inactive under normal conditions. Tissue inflammation may induce opioid receptor upregulation and signaling efficiency.
Describe the advantages of combining local anesthetics and opioids in neuraxial analgesia.
Epidural LAs side effects: motor blockade and systemic hypotension. Epidural opioid side effects: pruritus and nausea. Combined, opioids and local anesthetics function in a synergistic manner to provide analgesia with attenuated side effects.
True or False: Neuropathic pain is usually less responsive to opioids than pain originating from nociceptors.
True
True or False: Chronic pain is best treated by using multiple therapeutic modalities, including PT, psychologic support, pharmacologic management, and rational use of more invasive procedures such as nerve blocks and implantable technologies.
True
True or False: Doses of morphine differ by a factor of 10 between intravenous, epidural, and intrathecal routes.
True