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Analgesic drugs

Analgesic drugs. MBChB: a crammer. This talk. Pain: classification Paracetamol NSAIDs Opiates Drugs for neurogenic pain. Drugs for gout. Pain. Somatic inflammation of epithelial surfaces, trauma, sepsis felt at site of pathology Visceral e.g. myocardial ischaemia, colic

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Analgesic drugs

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  1. Analgesic drugs MBChB: a crammer

  2. This talk • Pain: classification • Paracetamol • NSAIDs • Opiates • Drugs for neurogenic pain. • Drugs for gout

  3. Pain • Somatic • inflammation of epithelial surfaces, trauma, sepsis • felt at site of pathology • Visceral • e.g. myocardial ischaemia, colic • poor localisation, often ‘referred’ • Neurogenic • e.g. neuralgia • no response to analgesics

  4. Paracetamol • Mechanism of analgesic activity not fully understood: ?  prostaglandin synthesis in the CNS • Mechanism of antipyretic activity:  PG-E2 in the hypothalamus

  5. Paracetamol • Safe effective analgesic used OTC • analgesia • lowering elevated temperature • it has no anti-inflammatory effect

  6. Dangerous in overdose

  7. Dangerous in overdose saturated

  8. Dangerous in overdose The main problem is hepatotoxicity Takes around 24-36 h to become apparent saturated Antidote: n-acetyl cycteine But use in the first 24 h Damage correlates with paracetamol conc. Measure conc. No earlier than 4 h

  9. NSAIDs

  10. Phospholipase A2 Arachidonic acid COX-I COX-II Leukotrienes PGs with gastric protective effects PGs with inflammatory effects Membrane phospholipid

  11. steroids X Membrane phospholipid Phospholipase A2 Arachidonic acid COX-I COX-II Leukotrienes PGs with gastric protective effects PGs with inflammatory effects

  12. steroids X Membrane phospholipid Phospholipase A2 Older NSAIDs Arachidonic acid X X COX-I COX-II Leukotrienes PGs with gastric protective effects PGs with inflammatory effects

  13. steroids X Membrane phospholipid Phospholipase A2 Arachidonic acid COX-II inh X COX-I COX-II Leukotrienes PGs with gastric protective effects PGs with inflammatory effects

  14. Aspirin • Acetylsalicylic acid • Analgesic/antipyretic at low dose • Anti-inflammatory at high dose • (Anti-platelet activity at low dose) • Upper GI irritation and bleeding

  15. Aspirin Overdose • Partly eliminated unchanged in the urine. • Aspirin is a strong acid, and is lipid soluble at acid pH (remember Henderson Hasselbach). • ‘Alkaline diuresis’: iv bicarbonate to yield alkaline urine. • Hence water soluble aspirin and higher aspirin clearance. • Dialysis in the most serious cases.

  16. Other NSAIDs • (Ibuprofen: OTC as an analgesic) • Naproxen • Diclofenac • Useful in inflammatory arthritis • Useful adjunct to opiates in terminal care

  17. COX-II inhibitors • Anti-inflammatory, useful for Rh-D • Much more expensive than older NSAIDs • Reserve for selected patients with PUD or GORD. • Rofecoxib withdrawn because of cardiovascular risk

  18. NSAID adverse effects • GI • Salt and water retention • Renal impairment • Asthma may be precipitated

  19. Opiates

  20. Opiates • Endorphins are endogenous compounds released in the CNS in response to pain. Three receptor sub-types. •  - analgesia and euphoria •  - analgesia •  - dysphoria and hallucination • Opiate analgesics bind to endorphin receptors.

  21. Effects of the opiates • CNS: analgesia, euphoria, sedation (inc. resp depression), cough suppression, nausea. • GI tract: slow transit, constipation, sphincter of Oddi contraction. • CVS: vasodilatation leading to drop in BP and heart work.

  22. Clinical PK of the opiates • All are well absorbed from IM and SC injection. • All subject to first-pass metabolism. • All are terminated by liver metabolism.

  23. Examples • Codeine: relatively low potency • Pethidine: higher potency, short half-life. Less effect on sphincter of Oddi. • Morphine and diamorphine: most potent, longer half-life than pethidine.

  24. Uses • Severe somatic and visceral pain. • No benefit in patients with neurogenic pain. • Pulmonary oedema. • Cough suppression. • Diarrhoea.

  25. Adverse effects and contraindications • ADRs: • Nausea • Constipation • Addiction • Respiratory depression: type-2 resp failure. • Biliary colic. • Contraindications: • Severe respiratory problems • Hepatic impairment • Head injury (resp depression and CO2 retention

  26. Reversal • Naloxone. • Rapid eliination: • Faster than the opiates. • You may be suffering a false sense of security: the patient needs observation. • Dose may need to be repeated. • Infusion may be required.

  27. Neurogenic pain.

  28. Neurogenic pain • Examples: • Trigeminal neuralgia • Post-herpetic neuralgia • Thalamic pain following stroke • Painful peripheral neuropathy

  29. Drugs for neurogenic pain • Carbamazepine • Anti-epileptic • Remember enzyme induction • Gabapentin • Antiepileptic • Tricyclic antidepressants • Remember suicide risk

  30. Drugs for gout.

  31. Acute gout. • Paroxysmal arthritis. Any joint (other than the axial skeleton) but 1st MTPJ is the commonest. • Commonly precipitated by diuretics (esp thiazides). • May be very severe and resemble septic arthritis. • Joint aspirate: uric acid crystals.

  32. Treating acute gout. • Rest. • NSAID until arthritis settles. • Worst cases merit prednisolone. • If prophylactic drug, such as allopurinol, is to be used then NSAID ‘cover’.

  33. Allopurinol • Purine bases metabolised via xanthines to uric acid. • Xanthines are water soluble. • Uric acid is pretty insoluble. • Allopurinol inhibits xanthine oxidase. • Used to reduce frequency of paroxysms. • May precipitate acute gout when first started: NSAID ‘cover’.

  34. Allopurinol ADRs and interactions • GI upset • Rash • 6-mercaptopurine and azathioprine. • Warfarin.

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