Alfentanil
Alfentanil is a **potent, short-acting synthetic opioid** belonging to the phenylpiperidine class, structurally related to fentanyl. Key characteristics include: * **Rapid Onset & Short Duration:** Highly lipophilic, allowing rapid penetration of the blood-brain barrier, making it ideal for short procedures or as a continuous rate infusion (CRI). * **Anesthetic Adjunct:** Primarily used to provide profound analgesia and reduce the required dose of induction or maintenance anesthetic agents (e.g., propofol, isoflurane). * **Species Specificity:** While veterinary experience is somewhat limited compared to fentanyl, it is noted to be particularly useful in cats as an adjunctive therapy during anesthesia. * **Regulatory Status:** It is a **Class-II controlled substance** due to its high potential for abuse. *Clinical Pearl:* Alfentanil is less lipid-soluble than fentanyl but more so than morphine. Its rapid redistribution and clearance make it highly titratable in a critical care or surgical setting.
Mechanism: Alfentanil acts primarily as a potent and highly selective agonist at **mu (μ) opioid receptors** in the central nervous system. * **Mechanism:** Binds to pre- and post-synaptic **μ-receptors** → inhibits adenylate cyclase → decreases intracellular cAMP → promotes opening of inward-rectifying potassium channels → hyperpolarization of the neuron. * **Neurotransmitter Inhibition:** Concurrently inhibits voltage-gated calcium channels → decreases the release of excitatory nociceptive neurotransmitters (such as **substance P** and **glutamate**) → profound analgesia and sedation. * **Systemic Effects:** Causes dose-dependent respiratory depression by decreasing the responsiveness of the brainstem respiratory centers to carbon dioxide.
Dosing by species
- As a premed · 5 micrograms/kg alfentanil with 0.3-0.6 mg of atropine · IV · 30 seconds before injecting propofol · Can reduce the dose of propofol needed to induce anesthesia to 2 mg/kg, but apnea may still occur.
- As a constant rate infusion for pain · Loading dose of 0.5-1 micrograms/kg, then a CRI of 0.5-1 micrograms/kg per minute. · IV · CRI
- As an analgesic supplement to anesthesia · 2-5 micrograms/kg · IV · q20 minutes
- For intra-operative analgesia in patients with intracranial disease · 0.2 micrograms/kg/minute · IV · CRI
- Intraoperative analgesia · 0.001-0.005 mg/kg · IV · single bolus · Administer slowly to avoid severe bradycardia
- Intraoperative analgesia · 0.001-0.0025 mg/kg/min · IV · continuous rate infusion · Best method of administration due to short duration of action
- Intraoperative analgesia · 0.001 mg/kg · IV · single bolus · Administer slowly to avoid severe bradycardia
- Intraoperative analgesia · 0.001 mg/kg/min · IV · continuous rate infusion · Best method of administration due to short duration of action
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Patients hypersensitive to opioids
Adverse effects
- Dose-related respiratory depression
- Bradycardia
- CNS depression
- Dose-related skeletal muscle rigidity
- Asystole (rare)
- Hypercarbia (rare)
- Hypersensitivity reactions (rare)
Drug interactions
- CYP3A4 Inhibitors (erythromycin, cimetidine, ketoconazole, itraconazole, fluconazole, diltiazem) · May increase the half-life and decrease the clearance of alfentanil leading to prolonged effect and an increased risk of respiratory depression
- Beta-blockers · May produce bradycardia or hypotension if used concurrently with alfentanil
- Other anesthetic agents · May produce bradycardia or hypotension; respiratory or CNS depression may be exacerbated
- Inhaled anaesthetics · Reduces the dose requirements of concurrently administered anaesthetics by at least 50% · major
- Monoamine oxidase inhibitors (MAOIs) · Risk of serotonin toxicity · major
Monitoring
- Anesthetic and/or analgesic efficacy
- Cardiac rate and rhythm
- Respiratory rate and depth
- Pulse oximetry or other methods to measure blood oxygenation
Overdose
### Signs of Toxicity * **Severe IV Overdose:** Circulatory collapse, pulmonary edema, seizures, cardiac arrest, and death. * **Less Severe Overdose:** CNS and respiratory depression, coma, hypotension, muscle flaccidity, and miosis. ### Treatment * **Supportive Care:** Provide cardiovascular and respiratory support (e.g., mechanical ventilation, IV fluids) as necessary. * **Antidote:** Administration of an opiate antagonist such as **naloxone**. Although alfentanil has a relatively rapid half-life, multiple doses of naloxone may be necessary. > **Measurement Precaution:** Because of the drug's extreme potency, the use of a tuberculin syringe to measure dosages less than 1 mL, along with a dosage calculation and measurement double-check system, is strongly recommended.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.