Fentanyl

Mechanism of Action

Fentanyl is similar to other opioid drugs. Fentanyl molecules target a subclass of opioid receptor systems in the body, many of which are localized in the brain within specialized neuroanatomical structures, particularly involving the control of emotions, pain, and speaking to the point of its infamous addictive properties, reward. Biochemically, it is a Mu-selective opioid agonist. However, it has the capability to activate other opioid system receptors such as the delta and potentially the kappa-receptors. Consequently, the activation of these receptors, particularly the Mu-receptors, produces analgesia. Also, the neurotransmitter dopamine (Da) is increased in the reward areas of the brain, which elicits the stereotypical exhilaration and relaxation effects, and is typically associated with the addiction to the drug.[6]

Fentanyl is hepatically metabolized via the CYP450 enzyme system, specifically CYP3A4. The drug has a half-life of 3 to 7 hours. Excretion is 75% in the urine and 9% in feces.

Administration

Fentanyl is typically administered intravenously (IV), intramuscularly (IM), transdermally (TD) as skin patches, intranasally (IN) in the form of a volatile nasal spray, and intrathecally (IT). It is also available as a buccal soluble thin film, which can dissolve in the mouth, similar to the sublingual tablets. However, in contrast to other opiates, it is less common to find forms of synthetic drugs such as oral tablets or powders. A new medication currently being used to address the gastrointestinal (GI) side effects without counteracting its primary analgesic aims acts selectively on the GI mu-opioid receptors and can aid in alleviating constipation while still providing pain relief, a dilemma previously treated off-label in some hospitals by suspending a vial of naloxone in ice water and taking the mix enterally. The transdermal patch is for the long-term management of pain. This article focuses on injectable fentanyl; more detailed coverage of the transdermal formulation is available in the StatPearls specific article on transdermal fentanyl.

Fentanyl comes in an injectable form of 50 mcg/mL. Dosing is as follows by indication:

• Preoperative analgesia
  • 50 to 100 mcg IV/IM for a single dose 30 to 60 minutes before surgery; consider lower dosing in patients 65 and older.
• Anesthesia adjunct
  • 2 to 50 mcg/kg/dose IV for a single dose
    • 2 to 20 mcg for low dosing; 20 to 50 mcg for higher dosing. Low dosing is favored in patients 65 and older.
• Regional anesthesia adjunct
  • 50 to 100 mcg IV/IM for a single; consider lower dosing in patients 65 and older.
• General anesthesia
  • 20 to 50 mcg/kg/dose IV for a single dose; consider lower dosing in patients 65 and older.
  • For high-risk patients undergoing complicated surgical procedures, use with oxygen and a muscle relaxant; doses up to 150 mcg/kg/dose may be necessary.
• Post-operative pain control
  • 50 to 100 mcg IV/IM every 1 to 2 hours as needed; alternately 0.5 to 1.5 mcg/kg/hour IV as needed. Consider lower dosing in patients 65 and older.
  • PCA (patient-controlled analgesia): 10 to 20 mcg IV every 6 to 20 minutes as needed; start at the lowest effective dose for the shortest effective duration - refer to institutional protocols.
• Moderate to severe acute pain (off-label)
  • 1 to 2 mcg/kg/dose intranasally each hour as needed; the maximum dose is 100 mcg. Use the lowest effective dose for the shortest effective duration.

Adverse Effects

Fentanyl's side effects are similar to those of heroin, which produce euphoria, confusion, respiratory depression (which, if extensive and untreated, may lead to arrest), drowsiness, nausea, visual disturbances, dyskinesia, hallucinations, delirium, a subset of the latter known as "narcotic delirium," analgesia, constipation, narcotic ileus, muscle rigidity, constipation, addiction, loss of consciousness, hypotension, coma, and even death.[7] Alcohol and other drugs (i.e., cocaine, heroin) can synergistically exacerbate fentanyl's side effects, creating multi-layered clinical scenarios that can be complex to manage. These substances, taken together, generate undesirable conditions that complicate the patient's prognosis.[8][9]

Contraindications

The use of fentanyl is contraindicated in patients in the following situations:

• After operative interventions in the biliary tract, these may slow hepatic elimination of the drug.
• With respiratory depression or obstructive airway diseases (i.e., asthma, COPD, obstructive sleep apnea, obesity hyperventilation, also know as, Pickwickian syndrome)
• With liver failure
• With known intolerance to fentanyl or other morphine-like drugs, including codeine, or any components in the formulation
• With known hypersensitivity (i.e., anaphylaxis) or any common drug delivery excipients (i.e., sodium chloride, sodium hydroxide). 

The drug fentanyl should not be used concomitant with certain medications such as CYP3A4 inhibitors like macrolide antibiotics or azole-antifungal agents, and protease inhibitors may increase plasma concentrations of fentanyl, extending the opioid drug action and exacerbating the opioid-induced respiratory depression (OIRD). On the other hand, the cessation of a CYP3A4 inducer medication (i.e., carbamazepine, phenytoin) in patients treated with fentanyl citrate injections may potentially increase fentanyl plasma concentrations prolonging the opioid adverse reactions. If the patient has used a monoamine oxidase inhibitor in the previous 14 days, fentanyl is contraindicated.

Monitoring

Baseline monitoring parameters include creatine, then ECG and vital signs should be monitored continuously.[10]

When using fentanyl citrate injections along with medications such as CYP3A4 inhibitors or stopping the CYP3A4 inducers in patients treated with fentanyl injections, the patients should receive frequent and close monitoring. Also, consider lowering the dose of fentanyl. Similarly, when using fentanyl citrate injection in conjunction with CYP3A4 inducer medications or stopping the CYP3A4 inhibitors, an undesirable reduction in fentanyl plasma concentrations may occur, thereby decreasing its systemic efficacy. The use of fentanyl citrate injection with CYP3A4 inducers or stopping the CYP3A4 inhibitor will require close and repetitive monitoring of the patient. Also, consider augmenting the dose of fentanyl as needed.

Toxicity

Typically, a fentanyl overdose manifests as an extrapolation of its pharmacological side effects. Although the clinical scenarios may vary among patients, one of the main concerns is the level of expression of OIRD. In such cases, oxygen administration and respiratory assistance are a priority. An opioid drug antagonist such as naloxone can help to correct OIRD symptoms. Commonly, repeated doses are necessary since, over time, respiratory depression may overcome the effects of the antagonist drug, particularly with opiates such as fentanyl, in which increased receptor affinity may be present as compared to other opiates. Thus, the therapeutic approach should continue until a normal respiratory rate coincides with appropriate oxygen saturation levels.

Naloxone/naltrexone are robust opioid antagonists that function by blocking µ-receptors. The FDA has approved these antidote medications for emergency treatment of patients with known or suspected opioid overdose experiencing respiratory and/or central nervous system (CNS) depression. Common routes of naloxone administration include IV, IM, IN, or subcutaneous (SC). Researchers have recently posed a caveat regarding the intranasal route for naloxone administration. Since the incidence of obstructive nasal pathology is relatively high in patients who experience serious OIRD, there is documentation of relevant instances of treatment failures when using the IN formulations.

Also, an IV neuromuscular blocker may be employed in cases of severe muscular rigidity to assist the controlled respiration treatments.

Enhancing Healthcare Team Outcomes

Fentanyl is a widely used narcotic in the hospital environment. However, with recent concerns about opiate overdose and toxicity, all interprofessional healthcare team members who prescribe, administer, or dispense this medication should be familiar with the drug's adverse effects and how to reverse them. This includes all clinicians (including mid-level practitioners), nurses, and pharmacists. When prescribing fentanyl, the dose and duration should be the lowest and shortest effective, and the patient is continually assessed for compliance and/or diversion in outpatient settings. Healthcare workers should be aware of new prescribing laws governing opioids, and unlike in the past, empirical prescribing of narcotics can lead to legal troubles, including loss of prescription privilege.[11][12] With interprofessional coordination of activities and open communication, fentanyl therapy can be successful and adverse events avoided or managed if they arise. [Level 5]