fentanyl

A short-acting parenteral opiate analgesic.

The pharmacokinetics of fentanyl can be described by a three compartment model, with a distribution time of 1.7 minutes, redistribution of 13 minutes and a terminal elimination half-life of 219 minutes. The volume of distribution for fentanyl is 4 L/kg.

The onset of action is almost immediate when the drug is given intravenously; however, the maximal analgesic and respiratory depressant effect may not be noted for several minutes. The usual duration of action of the analgesic effect is 30 to 60 minutes after a single intravenous dose of up to 100 microgram. Following intramuscular administration, the onset of action is from seven to eight minutes and the duration of action is one to two hours.

Unlike morphine, assays in humans show no clinically significant histamine release at usual doses.

The duration and degree of respiratory depression is dose related.

• Adjust dosing according to risk of respiratory depression (more likely in the very young, the elderly, use of concomitant resp. depressants such as midazolam, other opioids, neuroleptics or ethanol, and in those with severely impaired respiratory function such as COPD).
• Patients on chronic opioid therapy or with a history of opioid abuse may require higher doses to achieve an adequate therapeutic effect.
• Adequate facilities should be available for postoperative monitoring and ventilation.
• Resuscitative equipment, oxygen and an opioid antagonist should be readily available to manage apnoea.
  • appropriate surveillance should be maintained because the duration of respiratory depression of doses of fentanyl employed during anaesthesia is usually longer than the duration of opioid antagonist action.
• Fentanyl should only be used by experienced doctors and in patients who are under constant supervision.

Risk of muscle rigidity, especially chest wall rigidity and impaired ability to breathe

• This effect is related to the dose and speed of injection and may be reduced by slow intravenous injection.
• If this effect occurs, it may be managed by the use of assisted or controlled respiration and, if necessary, by administration of a neuromuscular blocking agent compatible with the patient's condition.
• High risk in patients with myasthenia gravis and the need for reversal with muscle relaxants thus contraindicates the use of fentanyl in these patients.

Risk of bradycardia and potentially asystole (rare):

• This is more likely in patients who already have bradycardia, thus avoid fentanyl in these patients.
• Bradycardia ma be treated with atropine.

Usual dosages:

• IV induction bolus: 1-4mcg/kg (max. 200mcg)
• IV titrated analgesia boluses: 0.5-1mcg/kg
• Infusion: 2-4mcg/kg/hr
• Intranasal: 1.5mcg/kg, repeat ONCE if needed
  • Intranasal use of fentanyl for analgesia in children:
    • see Sunshine Hospital policy August 2008

There is now also a buccally-absorbed Fentanyl lozenge available to assist with breakthrough pain in patients with chronic pain managed by opiates.

for more information see MIMS Online

see also opiates and opioids