- Alfentanil is a synthetic phenylpiperidine derivative
- It is a colourless solution containing 500μg/ml or 5mg/ml alfentanil
- As an IV bolus for short-acting analgesia e.g.:
- At induction, where its rapid onset/offset makes it particular suited to obstetric GA
- During procedures performed under procedural sedation for periods of stimulation e.g. endoscopy
- As a continuous infusion e.g.:
- As a target-controlled infusion for surgical anaesthesia using Maitre or Scott models
- As a simple infusion on intensive care to facilitate ETT tolerance
- In palliative care settings, to provide analgesia to those with renal failure in whom morphine or oxycodone is unsuitable
- A pure MOP receptor agonist
- Potency 5x that of morphine, but 0.25x that of fentanyl
- pKa 6.5, therefore 89% unionised at pH 7.4
- Relative lipid solubility vs. morphine: 90x
- Although it has a lower lipid solubility than fentanyl, its onset of action is quicker than fentanyl's due to:
- Higher unionised fraction at physiological pH
- Lower potency = more drug required = higher concentration gradient
- 90-92% plasma protein bound
- Volume of distribution 0.6/kg
- Context-sensitive half-time after:
- 1hr - 30mins
- 2hrs - 40mins
- 8hrs - 60mins
- N-demethylated in the liver by CYP3A4 to noralfentanil
- As midazolam is also metabolised by CYP3A4, concurrent use increases the elimination half-life of both
- Metabolites are conjugated and excreted in the urine
- Clearance 6ml/min/kg
- Elimination half life is 100mins (much shorter than fentanyl (190mins) despite the lower clearance, as it has a lower volume of distribution)