14 - Opioids Flashcards
What is the difference between pain and nociception?
Nociceptive pain is pain due to actuall tissue damage (google)
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How do we feel pain in general terms?
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How can we modulate pain peripherally?
- Centrally: via periaqueductal grey
- Peripherally: via subsantia gelatinosa
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How can we modulate pain centrally?
Endogenous opioids like enkephalins released from the periaqueductal grey and inhibit the second order neurone
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What are the different types of endogenous opioid receptors and what are their actions when stimulated?
All are GPCRs and all decrease cAMP levels
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What is the WHO analgesic ladder?
- Go up the ladder for severe or worsening pain e.g start on morphine straight away for malignany or acute severe pain
- Move down the ladder with resolution of pain
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What are opiates used for?
- Mainly act on u-receptors to modulate pain
- Can also be indicated in cough, diarrhoea and palliation
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What are the different types of opioids?
- Can range from agonists to anatagonists
- Know the ones in bold plus tramadol, diamorphine and methadone
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What is the mechanism of action of morphine?
Strong u-agonist causing analgesia and euphoria
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- Absorbed really quickly IV but gut absorption is erratic and low oral bioavailability (40%)
- Distributed across all tissues as lipophillic, including fetus, but cannot get across BBB
- Eliminated renally so be careful with AKI and CKD
What are some of the side effects of morphine?
- Respiratory depression: medullary resp centre less responsive to CO2
- Emesis: as stimilates chemoreceptor trigger zone
- Constipation: due to decrease motility and increased sphincter tone
- Syncope: decreased b.p
- Miosis
- Histamine release causing itch, be careful in asthmatics
- Cannot use in pregnancy
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What is the mechanism of action of fentanyl?
Strong u receptor agonist (higher potenticy and affinity than morphine) causing analgesia and anaesthetic
- Given IV, epidural, intrathecal or nasal in children. High bioavailability of 80-100%
- Highly lipophillic and protein bound and can get into CNS
- Metabolised hepatically by CYP3A4
- Renally excreted but less so than morphine so better in AKI and CKD
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Why may fentanyl be used instead of morphine?
- Less histamine release, sedation and constipation so less side effects than morphine
- Has a higher affinity for the u-receptor so shifts morphine off
What are the side effects of fentanyl?
- Respiratory depression
- Constipation
- Vomiting
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What is the mechanism of action of codeine?
Moderate u-receptor agonist used for mild/moderate analgesia and cough depressant
- Given PO or SC
- Converted to morphine by CYP2D6
- Elimination by glucoronidation of morphine and renal excretion
- 1/10th potency of morphine
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What can affect the CYP2D6 enzyme and therefore affect how a patient reacts to codeine?
- Can be inhibited by Fluoxetine and other SSRIs so need lower dose of morphine with this
- Variable expression amongst people
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