Fentanyl is a short-acting, full agonist opioid with a potency of approximately 100X that of morphine. Administered intravenously, it has a rapid onset of action with a relatively short duration of action (i.e. about 30 minutes). Due to its short duration of action, it is used most routinely by constant rate infusion (CRI) following a loading dose. Intravenous administration is not associated with histamine release.
Side effects are consistent with other full agonist opioids—dose dependent respiratory and cardiovascular depression. At clinical doses, these drugs exhibit few negative cardiovascular effects, with bradycardia being the most common side effect. This bradycardia is highly responsive to anticholinergic (e.g. atropine) treatment.
Fentanyl is often used in combination with midazolam, a benzodiazepine, to anesthetically induce patients whose cardiovascular or hemodynamic parameters are deemed to be unstable in a method termed “neuroleptic analgesic induction.” Fentanyl is also manufactured as a transdermal patch which has found a limited place in veterinary medicine. While no longer commercially available, a concentrated solution of fentanyl (50 mg/mL) for topical, transdermal application was labeled for use in dogs. This formulation was approved for and demonstrated an analgesic duration of 4 days after one application.
Sufentanil, remifentanil and alfentanil are all derivatives of fentanyl and all have been used in veterinary applications. They are typically more potent than fentanyl, with equal or shorter durations. Remifentanil is unique in that it is primarily metabolized by nonspecific plasma esterase which provides more predictable clearance in patients with liver or renal insufficiencies.
All of these short acting opioids agonists can be clinically applied to provide analgesia for severe pain and significantly reduce the requirement for gas inhalants.