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Introduction
  • Fentanyl is a potent synthetic opioid similar to morphine but produces analgesia to a greater extent. It is 50-100 times more potent than morphine. However, exhibits vastly different properties and pharmacokinetics. It is indicated for: preoperative analgesia, general anaesthesia, acute pain
Mechanism of Action
  • Fentanyl molecules target a subclass of opioid receptor systems in the body, which are localised in the brain within specialised neuroanatomical structures. It is a mu-selective opioid antagonist. It has the capabilities to activate other opioid system receptors such as the delta and potentially the kappa-receptors.  The activation of these receptors produces there analgesic affects.  The dopamine neurotransmitter is increased in the reward areas of the brain, which elicits the stereotypical exhilaration and relaxation effects, and is associated with the addiction to the drug
Adverse effects
  • Side effects of Fentanyl can include: confusion, respiratory depression, drowsiness, nausea, visual disturbance, dyskinesia, hallucinations
Precautions
  • Use caution when using other narcotic medication
  • Ensure the patient is monitored for signs of respiratory or CNS depression 
Drug Interactions
  • Fentanyl can interact with MAOI anti-depressants with the effects of both medications potentiated.
Overdose
  • Overdose side effects include: respiratory depression, somnolence, coma, skeletal muscle flaccidity, bradycardia, hypotension and death.
  • There are a wide range of opioid data not a lot independent of Fentanyl alone. Fentanyl is 50-100 times more potent than morphine and is dependent on the person ingesting it
Pharmacology

Pharmacokinetics

Absorption
  • Fentanyl is absorbed rapidly due to its lipophilicity and has an almost immediate onset.
  • Bioavailability (amount that enters the venous system) of each route of administration:
    • Sublingual tablets: 54% bioavailable
    • Transmucosal lozenges: 50% bioavailable
    • Buccal tablets: 65% bioavailable
    • Sublingual spray: 76% bioavailable
    • Nasal spray: 64% bioavailable
Distribution
  • The IV volume of distribution is 4L/kg. The oral volume is 25.4L/kg. In hepatically impaired patients, the IV distribution ranges from 0.8-8L/kg.
  • Fentanyl quickly crosses the blood brain barrier and produces analgesia in one to two minutes
  • Fentanyl is a strong agonist of opioid receptors with 80-100 times the potency of morphine.
Metabolism
  • Fentanyl is metabolised to a number of inactive metabolites. It is 99% N-dealkylated to norfentanyl by cytochrome P450. It can also be amide hydrolysed to despropionylfentanyl or alkyl hydroxylated to hydroxfentanyl which is N-dealkylated to hydroxynorfentanyl
Elimination
  • Within 72 hours, 75% of a dose of fentanyl is excreted in the urine with <7% unchanged, and 9% is excreted in the faeces with <1% unchanged

Pharmacodynamics

Intended activity
  • Fentanyl is a synthetic opioid with high lipophilicity which allows it to cross the blood-cerebrospinal fluid barrier more rapidly. It is oxidised by hepatic microsomal cytochrome P450 into norfentanyl, an inactive metabolite that is excreted through the renal system 
Therapeutic window
  • Fentanyl has an elimination half-life of 4 hours with a shorter duration of action of 30-40 minutes
Duration of action
  • Fentanyl crosses the blood brain barrier and produces analgesia within one to two minutes. Serum levels decline rapidly from peak concentrations because of extensive tissue uptake. The brain concentration of fentanyl falls in conjunction with the serum level. Duration of action is 30-40 minutes. In high doses can be several hours due to the release of the bound drug from tissue stores

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