Alfentanil is a synthetic opioid analgesic and a derivative of fentanyl. It is widely used for analgesia to supplement general anesthesia for various surgical procedures, or as a primary anesthetic agent in very high doses during cardiac surgery.

Compared to similar anesthetics, including fentanyl and sufentanil, alfentanil is the least potent, has the fastest onset of action, and the shortest duration of action. Because of alfentanil’s short duration of action, it is typically preferred for shorter procedures, or when rapid changes in the level of consciousness are necessary. Such procedures include gynecological procedures, short ambulatory procedures such as colonoscopies, and shorter surgeries such as tonsillectomies, appendectomies, and cholecystectomies, to list a few. Alfentanil has demonstrated to be effective and safe in both children and adults at appropriate dosing.[1][2][3][4][5][6][7][8]

Alfentanil is of the opioid-class of medications and thus acts by stimulation of the opioid receptors, of which there are three subtypes: mu (m), kappa (k), and delta (d). The receptor primarily involved in pain transmission is the m-opioid receptor. The opioid receptors are predominately present in the central nervous system, brain, and spinal column, and peripheral nervous system, but are also present on vascular, cardiac, lung, gut, and even peripheral blood mononuclear cells. The natural ligands for the opioid receptors are called the “endogenous opioid peptides” and include enkephalins, endorphins, and endomorphins.

Opioids have a variety of clinical effects but are predominately known and used for their profound pain-relieving effects. Their pain-relieving effects are from binding the m-opioid receptor, which is coupled to G1 proteins, resulting in the closure of N-type voltage-operated calcium channels and opening of calcium-dependent inwardly-rectifying potassium channels. The result is intracellular hyperkalemia and the decrease in the neurotransmission of pain signals resulting in increased tolerability of pain. Opioid receptor binding also decreases intracellular cyclic-adenosine-monophosphate (cAMP), which modulates the release of nociceptive (relating to the perception of pain) neurotransmitters (e.g., substance P).

There have not been acute changes to intracellular sodium noted to occur with opioid-receptor binding; however, intracellular sodium may hypothetically have involvement in opioid-receptor affinity to both endogenous and exogenous opioid-receptor binders. Researchers have observed that opioid-receptor binders seem to bind with a higher affinity in the presence of lower intracellular sodium concentrations and higher intracellular potassium concentrations. Of note, there was no observed change in the total number of binding sites of the opioid receptors during these observations. This phenomenon was observed specifically in cells known to have the m- and d-type opioid receptors.

Opioids are also known for their euphoric effects. Euphoria is a feeling or state of intense excitement and happiness. These effects usually are due to the binding of endorphins to opioid receptors in both the central and peripheral nervous systems. Endorphins are primarily synthesized and stored in the anterior pituitary gland from their precursor protein proopiomelanocortin (POMC). POMC is a large protein that is cleaved into smaller proteins such as beta-endorphin, alpha-melanocyte-stimulating hormone (MSH), adrenocorticotropin (ACTH), and others. Interestingly enough, studies have suggested that cells of the immune system are also capable of beta-endorphin synthesis because immune cells possess mRNA transcripts for POMC and T-lymphocytes, B-lymphocytes, monocytes, and macrophages have shown to contain endorphins during inflammation.

Normally, euphoric effects are primarily due to endorphin binding at opioid receptors in the peripheral nervous system. However, in opioid administration, it is directly due to opioid drugs binding the opioid receptors at both the pre- and postsynaptic junctional in the peripheral nervous system, and at the pre-synaptic junction in the central nervous system. The analgesia produced by endorphins in the peripheral nervous system mainly stems from a decreased synthesis of substance P. In the central nervous system, there is also a degree of analgesic effects from endorphins binding opioid receptors, but it is from inhibiting the release of GABA, resulting in excess production of dopamine. Upon opioid administration, the opioid drug takes on this role.

It bears mention that when administering opioid analgesic drugs to patients, there is a decrease in synthesis and secretion of endogenous endorphins and opioid peptides; this is due to negative feedback on the endogenous endorphin and opioid systems.  

Other effects of opioid medications include changes in mood, drowsiness, and mental clouding. However, the distinctive feature of the analgesia induced by opioids is the lack of consciousness. The patient still perceives pain but describes it as less intense. Thus, opioids do not decrease or treat the cause of a painful stimulus, but rather decrease its perception.[9][10][11][12][13][14][15]

Administration of alfentanil to adults is typically intravenous over three minutes or by continuous intravenous infusion, and administration to children is typically intravenous over three to five minutes, or by continuous intravenous infusion. However, other methods of administration of alfentanil have demonstrated efficacy, including epidural (injection into the epidural space), intrathecal (injection into the spinal canal or, into the subarachnoid space), transdermal (across the skin), and intranasal.[1]

With alfentanil being an opioid-class analgesic, it has adverse effects common to this group of medications, including respiratory depression, decreased gastrointestinal motility, sedation, nausea, vomiting, constipation, and intestinal bloating. An interesting note about alfentanil is that it causes less intense respiratory depression than that of fentanyl likely due to its lower total body clearance, a smaller volume of distribution, and shorter half-life than other commonly used similar opioid analgesics, such as fentanyl and sufentanil. Additional adverse effects noted to be more specific to alfentanil include hypotension, chest wall rigidity, bradycardia, and tachycardia.[10][4][7]

Alfentanil is contraindicated in patients who have a history of hypersensitivity to alfentanil (e.g., anaphylaxis) or any component of the formulation. It is also contraindicated in those who have shown hypersensitivity to other similar opioid-class medications.

There has been evidence that alfentanil can increase the risk of seizures in those with a history of seizures and can increase the duration of seizures. In such patients, electroencephalogram monitoring may be necessary if another analgesic is unable to be used, but a history of seizures is not necessarily a contraindication to alfentanil.

Outside of the United States, specifically in Canada, contraindications to alfentanil include suspected surgical abdomen, mild pain that is manageable with other pain medications, acute/severe bronchial asthma, chronic obstructive airway, acute respiratory depression, status asthmaticus, hypercapnia, cor pulmonale, acute alcoholism, convulsive disorders, delirium tremens, severe CNS depression, increased intracranial or cerebrospinal pressure and head injury, or using monoamine-oxidase inhibitors concurrently or within 14 days of use, as well as women or who are nursing or pregnant, and during labor/delivery.[16][17]