Analgesia Flashcards

1
Q

Opioids - Mechanism of Action

A
  • Bind and stimulate μ receptors. With increasing doses, starts to stimulate κ and δ receptors
  • Pre-synaptic - close calcium channels, decreasing intracellular calcium in presynaptic terminals of afferent fibres, inhibiting NT release
  • Post-synaptic - opens potassium channels, increasing intracellular calcium causing hyper-polarisation (inhibitory post-synaptic potential)
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2
Q

Opioids - Central Effects

A
  • Analgesia
  • Sedation, but no amnesia
  • Cough reflex suppression (e.g. codeine and dextromethorphan)
  • Euphoria
  • Respiratory depression (important to note in asthmatics), increases carbon dioxide, causing vasodilation. Pethidine does so less than morphine (so used in labour)
  • Raised intracranial pressure (secondary to vasodilation)
  • Miosis/ pinpoint pupils (pre-existing miosis must be diagnosed before drug administration, important consideration with suspected head injury, does not occur with pethidine)
  • Nausea and vomiting (counteracted by cyclizine/ H1 receptor antagonist)
  • Truncal muscle rigidity at supraspinal level - particularly fentanyl (counteracted by a neuromuscularblocker)
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3
Q

Opioids - Peripheral Effects

A
  • Constipation (worsened with atropine and metoclopramide)
  • CVS - peripheral arterial and venous dilation and decreased TPR, resulting in hypotension (mediated by histamine release)
  • Histamine release - pruritis/ rash, bronchospasm (therefore opiates CI in poorly controlled asthmatics or COPD patients)
  • Genitourinal tract - increased sphincter tone, and urine retention
  • Neuroendocrine - increased ADH, prolactin and somatotrophin release
  • Biliary tract - biliary colic (use pethidine rather as it has an antimuscarinic effect)
  • Uterus - morphine can prolong labour and decrease contraction rate (use pethidine as it has an antimuscarinic effect)
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4
Q

Opioids - Contraindications

A
  • Head injury
  • Shock
  • Liver dysfunction
  • Heroin addicts
  • Asthma
  • COPD
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5
Q

Pethidine

A
  • Antimuscarinic properties (counteracts miosis and causes tachycardia)
  • Drug of choice in asthmatics
  • Metabolised to norpethidine, which is neurotoxic (resulting in seizures)
  • Naloxone reversal is less effective
  • Half lives - pethidine 2.5-4 hours; norpethidine 8-21 hours
  • Serotonin syndrome with monoamine oxidase inhibitors
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6
Q

Fentanyl

A
  • Rapid onset of action
  • Short duration of action (half life 1-1.6 hours)
  • Effective for continuous infusion (especially the analogue remifentanil)
  • Potent analgesic that does not depress the CVS, therefore used as post-surgical analgaesia in cardiac patients
  • Duration of reparatory depression is less than that of morphine
  • Less histamine release, therefore less side effects
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7
Q

Tramadol

A
  • Partial μ receptor antagonist and serotonin & noradrenaline reuptake inhibitor
  • Causes nausea and vomiting
  • High oral bioavailability
  • At analgesic doses, there is no respiratory depression or CVS adverse effects
  • May cause seizures (especially if there is head trauma)
  • CNS depressent effects
  • No histamine release
  • Drug interactions with serotnin and noradrenaline drugs
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8
Q

NSAIDS - Contraindications

A

Absolute

  • GIT bleeding or ulceraton on history
  • Hypersensitivity
  • Severe liver dysfunction
  • Cardiac failure
  • Dehydration, hypotension, hypovolemia
  • Hyperkalemia
  • Renal impairment
  • Aspirin-induced asthma

Relative

  • Bleeding or coagulation disorders
  • Surgery with high risk of hemorrhage
  • Surgery where absence of bleeding is important
  • Drugs (ACE-I, anticoagulants, methotrexate, cyclosporin, gentamycin)
  • Pregnancy and lactation
  • Age > 65
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9
Q

Ketamine

A
  • MOA: antagonist at NMDA glutamate receptors
  • No respiratory depression
  • Psychiatric effects
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10
Q

Morphine Dose

A
  • Infusion: 0.07-0.5mg/kg/hour
  • Intermittent dose: 0.01-0.5mg/kg 1-2 hourly (10-30mg usually…)
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11
Q

Fentanyl Dosage

A

100-200mcg IV

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