Fentanyl
**Fentanyl** is a highly potent, synthetic phenylpiperidine-derivative **Class-II opiate analgesic** used extensively in veterinary medicine for the management of moderate to severe pain. **Key Pharmacological Features:** * **Potency:** Approximately 75 to 100 times more potent than morphine. * **Lipophilicity:** Fentanyl is highly lipophilic, which allows it to rapidly cross the blood-brain barrier (resulting in a very fast onset of action when given IV) and makes it an excellent candidate for **transdermal delivery**. * **Duration:** When administered as a single IV bolus, its duration of effect is extremely short (15-30 minutes) due to rapid redistribution from the brain to other tissues. Therefore, it is most commonly administered via **Constant Rate Infusion (CRI)** or **transdermal patch** for sustained analgesia. **Clinical Applications:** * Adjunctive control of postoperative pain. * Management of severe, chronic, dull, or non-specific widespread pain (e.g., cancer, pancreatitis, aortic thromboembolism). * Perioperative use to significantly reduce the Minimum Alveolar Concentration (MAC) of inhalant anesthetics, providing cardiovascular stability in compromised patients. > **Clinical Pearl:** While transdermal patches are highly convenient, absorption is notoriously variable among individual animals. A "rescue" injectable analgesic should always be available when relying on transdermal fentanyl.
Mecanismo: Fentanyl is a highly selective **full agonist at the mu-opioid receptor (MOR)**. **Mechanism Pathway:** 1. **Receptor Binding:** Fentanyl binds to presynaptic and postsynaptic mu-receptors primarily located in the central nervous system (limbic system, spinal cord, thalamus, midbrain) and peripheral tissues. 2. **G-Protein Activation:** Binding activates inhibitory G-proteins (Gi/Go). 3. **Cellular Effects:** * Inhibits **adenylate cyclase**, decreasing intracellular cAMP. * Promotes the opening of **inward-rectifying potassium (K+) channels**, leading to neuronal hyperpolarization. * Inhibits the opening of **voltage-gated calcium (Ca2+) channels** on presynaptic nerve terminals. 4. **Analgesia:** This cascade profoundly inhibits the release of excitatory, nociceptive neurotransmitters (such as **Substance P** and **glutamate**) and decreases the excitability of postsynaptic neurons, effectively blocking pain transmission.
Dosificación por especie
- Perioperative pain · 2-3 micrograms/kg IV plus 2-3 micrograms/kg/hour IV infusion · IV/CRI · Continuous · Perioperative
- Pain management · 1-3 micrograms/kg IV, followed by a CRI at 1-4 micrograms/kg/hr · IV/CRI · Continuous · Loading dose followed by CRI
- Surgical analgesia · CRI at 10-30 micrograms/kg/hr · CRI · Continuous · Intraoperative
- Analgesia · 2-4 micrograms/kg/hour · CRI · Continuous
- Induction · 0.001-0.002 mg/kg IV · IV · Single dose
- MAC reduction during general anesthesia · 10-20 micrograms/kg/hr CRI · CRI · Continuous · Intraoperative
- Severe to excruciating pain in the emergent patient · Fentanyl at 1050 micrograms/kg, administered IV titrated to effect; use the effective dose as an hourly CRI. · IV/CRI · Titrated to effect · Note: '1050' appears exactly as written in source text, likely a typo for 10-50.
- Epidural for pain control · 0.004 mg/kg (4 micrograms/kg), diluted 0.2 mL with preservative-free saline or local anesthetic. · Epidural · Single dose · 1/2 hour · Onset in less than 10 minutes
- Maintenance analgesia/heavy sedation in critically ill hypotensive animals · fentanyl at 0.5-0.8 micrograms/kg/minute (equivalent to 30-50 micrograms/kg/hr). · CRI · Continuous · Used in addition to a local line block. May combine with midazolam.
- Transdermal Analgesia · 25 mcg/hr or 12.5 mcg/hr · Transdermal · q120h · Up to 5 days · Apply 6-24 hours prior to need. 12 mg patches are available for very small cats.
Vías de administración
Contraindicaciones
- Known hypersensitivity to fentanyl or patch adhesive
- Conditions where additional respiratory or CNS depression would be deleterious
- No specific contraindications available in the monograph (use with caution in patients with respiratory compromise)
Efectos adversos
- Dose-related respiratory depression
- CNS depression (sedation)
- Circulatory depression (bradycardia)
- Dysphoria or agitation (especially in cats)
- Urine retention
- Constipation
- Contact dermatitis/rash at patch site
- Altered thermoregulation (hypothermia or hyperthermia)
- Respiratory depression
- Severe bradycardia
- Asystole (with rapid IV injection)
- Reduction in heart rate
Interacciones farmacológicas
- Azole Antifungals (ketoconazole, itraconazole, fluconazole) · May inhibit fentanyl metabolism, increasing risk of toxicity
- CNS Depressants (other) · Additive CNS and respiratory depressant effects
- Diuretics · Opiates may decrease diuretic efficacy in congestive heart failure patients
- Macrolide Antibiotics (erythromycin, clarithromycin) · May inhibit fentanyl metabolism
- Monoamine Oxidase Inhibitors (amitraz, selegiline) · Severe and unpredictable opiate potentiation; generally not recommended within 14 days of MAOI use
- Skeletal Muscle Relaxants · Fentanyl may enhance neuromuscular blockade
- Nitrous Oxide · High fentanyl doses combined with nitrous oxide may cause cardiovascular depression
- Phenobarbital, Phenytoin · May increase the metabolism of fentanyl, reducing its efficacy
- Rifampin · May increase the metabolism of fentanyl
- Tricyclic Antidepressants (clomipramine, amitriptyline) · Fentanyl may exacerbate the effects of tricyclic antidepressants
- Warfarin · Opiates may potentiate anticoagulant activity
- Other anaesthetic drugs · Reduces the dose requirement for other anaesthetic agents · major
Monitorización
- Analgesic efficacy (pain scoring)
- Heart rate (monitor for bradycardia)
- Respiratory rate and depth
- Body temperature (monitor for fever which increases patch absorption)
- Neurologic status (sedation, dysphoria)
- Heart rate and rhythm (monitor for bradycardia)
- Blood pressure
- Adequacy of analgesia
- Body temperature (avoid external heat on patches)
Sobredosis
**Clinical Signs of Overdose:** Profound respiratory and/or CNS depression, cardiovascular collapse, bradycardia, hypothermia, tremors, neck rigidity, and seizures. Newborns are particularly susceptible. **Treatment:** * **Naloxone** is the specific reversal agent of choice for treating respiratory depression. * Because naloxone's duration of action is often shorter than fentanyl's, **repeated doses or a CRI of naloxone** may be required. * Patients must be closely monitored for re-narcotization. * Mechanical respiratory support should be instituted in cases of severe respiratory depression.
La referencia de fármacos de VetSheet está destinada a veterinarios colegiados como apoyo a la decisión clínica, no sustituye el juicio profesional ni la ficha técnica vigente del fabricante.