Analgesics

Opiods
1. Morphine
  1. Morphine analogue
    1. Naloxon (Antagonist)
  2. Synthetic Derivatives
    1. Phenylpiperdine series
      1. Pethidine
      2. Fentanyl
    2. Methadone series
      1. Methadone
    3. Semisynthetic Thebaine derivatives
      1. Etorphine
        Immobilin
        Very potent
      2. Buprenorphine
        Large animals
        Partial Agonist
  3. Metabolism
    1. First pass metabolism w/ PO (???)
    2. Conjugated in Liver
    3. Morphine - 6- glucuronide => Active analgesic
  4. Absorption
    1. Oral (Variable)
      1. IV
        NOT Pethidine
        IM
        S/C
        Intrathecal
  5. Distribution
    1. Morphine analogues t 1/2: 3 - 6 hrs
  6. Elimination
    1. Morphine Glucuronides excreted in urine
      1. Glucuronides hydrolysed in gut, but Morphine reabsorbed
        Enterohepatic circulation
  7. Side effects
    1. Sedation
    2. Respiratory Depression
    3. Negative Chronotrophy
      1. Except Pethidine
    4. Emesis (Morphine)
    5. Dysphoria
    6. Histamine release
      1. Pethidine
        Why given IM vs. IV
    7. Decreased GI motility
2. Opiod Receptors
  1. GPCRs
  2. INHIBIT Adenylate cyclase
    1. Decrease cAMP
    2. Promote opening of K+ channels
      1. Inhibit opening of voltage gated Ca2+ channels
        Blocks/reduces transmission to higher brain centers
        Prevents perception of pain
  3. u- receptors (MOP)
    1. CNS (Cerebral cortex, basal ganglia, spinal cord, PAG)
    2. Analgesia (Resp. depression, Decreases gut motility, CVS depression)
  4. k-receptors (KOP)
    1. Spinal Analgesia
  5. Delta - receptors (DOP)
    1. Supraspinal & Spinal Analgesia
    2. Mood regulation ?
  6. NOP receptor
    1. Supraspinal & Spinal
  7. Omega receptors
    1. Not selctive Opiod receptors
      1. Dysphoria
3. CNS
  1. Analgesia
    1. Acute & Chronic pain
  2. Sedation
    1. Dysphoria
  3. Emesis
  4. Myosis
    1. u & k receptors (Oculomotor nucleus)
  5. Reduced GI motility
    1. Codeine & Loperamide
4. Resp.
  1. Respiratory Center (Medulla Oblongata): Decreased sensitivity to Pco2
  2. Cough suppression
5. GI
6. Histamine release
  1. Bronchoconstriction & Hypotension
NSAIDs
1. Peri-operative analgesia
Local Anaesthetics
1. Absorption
  1. Available as solutions for:
    1. Injection
    2. Sprays
    3. Creams
    4. Gels
    5. Formulated as hydrochlroide salt
  2. Usually administered around nerves requiring block
  3. Absorption depends on pKa
    1. Infected tissue has a lower pH than normal, therefore local anaesthetic absorption is impaired
  4. Local infiltration
  5. Splash blocks
  6. Specific nerve blocks
  7. IVRA
  8. Extradural
  9. Topical
  10. EMLA (Eutectic mixture of Local Anaesthetic)
    1. Mixture of Anaesthetics w/ a single chemical composition
      1. Ex. Lidocaine & Prilocaine
  11. Cause Vasodilation, thus Vasoconstrictors added to reduce systemic absorption
    1. Ex. + Adrenalin
    2. Cocaine causes Vasoconstriction
2. Distribution
  1. Higher PPB, longer duration
    1. Ex. Lidocaine: PPB = 65%
    2. Ex. Bupivicaine: PPB = 95%
    3. Bupivicaine > Ropivicaine>Lidocaine > Prilocaine
    4. Albumin & alpha 1-Acid Glycoprotein
    5. Free drug crosses the placenta & can become trapped if the Foetus is acidotic
3. Metabolism and Excretion
  1. Esters
    1. Broken down by Plasma Cholinesterases (Inactivates)
    2. Excreted by kidneys
      1. PABA metabolite
  2. Amides
    1. Metabolized by Hepatic Amidases
    2. Excreted by kidneys
4. Side Effects
  1. CNS
    1. Tremors
    2. Convulsions
    3. Respiratory Depression
  2. CVS
    1. Reduced myocardial contractility
    2. Vasodilation
      1. Low BP
    3. Bupivicaine especially Cardiotoxic
  3. Reduces tissue repair
  4. Inadvertent IV administration
  5. Tissue Irritation
  6. Allergic Rxns
NMDA Antagonists
1. NMDA (N-methyl-D-aspartate) Receptor
  1. Found in CNS
  2. Receptors for Excitatory Amino Acids (EAA)
    1. Binding of EAA modulates pain transmission => Enhancing pain perception
      1. Glutamate
      2. Aspartate
2. Block EAA receptors
  1. Reduces pain modulation & CNS activity
3. Low doses: Analgesia
4. High doses: Dissociative Anaesthesia
5. Ketamine
  1. Dogs, Cats, Horses
  2. Cyclohexane derivative
  3. Uses
    1. Peri-operative & Intra-operative Analgesia
    2. Chemical Restraint
    3. Dissociative anaesthesia
    4. Chronic pain
      1. Inhibits central sensitization
  4. Absorption
    1. Bolus
    2. CRI
Alpha 2 Agonists
1. Intra-operative Analgesics
  1. Medetomidine
    1. Dexmedetomidine
  2. Cats, Dogs, Horses
2. Reversed
Unlicensed Products
1. Tramadol
  1. Atypical synthetic Opiod
    1. 1/10th potency of Morphine
  2. Binds MOP receptors
    1. But also INHIBITS re-uptake of Seratonin and NA in the CNS
      1. Tx of Chronic pain
  3. No Tissue damage
2. Gabapentin
  1. Stabilizes electrical nerve activity
    1. Decreases nerve firing
  2. Analgesic (Neuropathic pain)
Nitrous Oxide (N2O)
1. Colorless, Non-flammable gas
2. Anaesthetic sparing
3. Little cardiac effects

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