Alfaxalone

Overview

Alfaxalone (Alfaxan®) is a neuroactive steroid intravenous anesthetic widely used in veterinary medicine for the induction and maintenance of general anesthesia in dogs and cats. It produces rapid, dose-dependent unconsciousness and muscle relaxation but provides no analgesia.

Alfaxalone is FDA-approved for dogs and cats and used extra-label in many other species, including exotics and livestock. Its rapid onset and short duration allow precise titration, making it suitable for anesthetic induction and short procedures. When administered slowly at appropriate doses, it provides relatively stable cardiovascular effects, but dose-dependent respiratory depression or apnea may occur, especially with rapid IV administration. Recovery excitation, tremors, or hyperreactivity can be seen when alfaxalone is used without premedication.

Mechanism of Action (MOA): Alfaxalone exerts its anesthetic effects by modulating gamma-aminobutyric acid (GABA-A) receptors in the central nervous system. By enhancing chloride ion conductance across neuronal membranes, it produces CNS depression and loss of consciousness. Alfaxalone does not possess analgesic properties and must be combined with appropriate analgesic agents when pain is anticipated.

Indications

Alfaxalone is indicated for use as an injectable general anesthetic in veterinary patients, primarily for induction and maintenance of anesthesia in dogs and cats. Its rapid onset and ability to be titrated to effect make it suitable for a wide range of clinical situations.

• Induction of general anesthesia (dogs and cats): FDA-approved for IV induction of anesthesia, either as a sole agent for short procedures or prior to maintenance with inhalant anesthetics.
• Maintenance of anesthesia: May be used for maintenance of anesthesia via intermittent IV boluses or continuous rate infusion (CRI), particularly for short procedures or when inhalant anesthesia is not feasible.
• High-risk or compromised patients: Can be titrated IV to effect and used at reduced doses, making it a potential option in selected high-risk, geriatric, or critically ill patients, with appropriate monitoring.
• Laryngeal examination in dogs (extra-label): Used for light anesthesia during assessment of laryngeal motion, typically in premedicated dogs, although results may vary compared with other induction agents.
• Sedation or light anesthesia via IM administration (extra-label): Used in cats, small dogs, and small mammals when IV access is difficult. IM use provides dose-dependent sedation to light anesthesia but is limited by injection volume and variable recovery quality.
• Diagnostic and minor procedures: Utilized for short procedures such as imaging, echocardiography (in combination with other sedatives), wound management, and minimally invasive interventions.
• Use in multiple species (extra-label or indexed): Employed in horses, livestock, small mammals, birds, reptiles, amphibians, fish, and exotic species for induction or short-term anesthesia, particularly when rapid, injectable anesthesia is required.

Dosage (Reference)

Dog

Alfaxalone is FDA-approved in dogs for IV induction and maintenance of general anesthesia. Doses should always be titrated slowly to effect, especially in premedicated or high-risk patients.

Cat

In cats, alfaxalone produces rapid induction and smooth anesthesia when administered slowly IV. IM use is extra-label and mainly reserved for sedation in fractious or difficult-to-handle cats.

Warnings & Precautions

Alfaxalone is a general anesthetic agent that must be administered with appropriate monitoring and supportive equipment available. The primary concerns relate to dose-dependent respiratory and cardiovascular depression, particularly with rapid IV administration or overdose.

• Respiratory depression and apnea: Dose-dependent hypoventilation or apnea may occur, especially if alfaxalone is administered rapidly IV. Continuous monitoring, oxygen supplementation, and the ability to provide assisted or controlled ventilation are mandatory.
• Rate of administration: IV induction must be performed slowly (approximately over 60 seconds) and titrated to effect. Rapid injection significantly increases the risk of apnea and cardiorespiratory depression.
• No analgesic effect: Alfaxalone provides no analgesia. Adequate pain control must be provided using appropriate analgesic drugs.
• Premedication effects: The use of preanesthetic agents (opioids, benzodiazepines, alpha-2 agonists, phenothiazines) reduces the required alfaxalone dose but also influences cardiorespiratory responses. Doses must be adjusted accordingly.
• Hepatic dysfunction: Use cautiously in patients with significant liver disease, as reduced clearance may prolong anesthetic effects. Lower doses or longer dosing intervals may be required.
• Geriatric, debilitated, or critically ill patients: Increased sensitivity to cardiorespiratory depression may occur. Use the lowest effective dose and titrate carefully.
• Postanesthetic excitement: Excitement, muscle tremors, opisthotonos-like posturing, and hyperreactivity may occur during recovery, especially when alfaxalone is used as a sole agent. Specific signs reported in dogs and cats include disorientation, nervousness, violent movements, vocalization, paddling, trembling, and myoclonus. Premedication and a quiet recovery environment substantially reduce these effects.
• Recovery management: Animals should recover in a quiet, minimally stimulating environment. Excessive handling or stimulation during recovery may precipitate excitement or dysphoria.
• Intramuscular administration: IM use is extra-label in dogs and cats and is limited to animals with small body size due to the large injection volumes required.
• Young animals: Safety has not been fully established in kittens younger than 4 weeks or puppies younger than 10 weeks. Use with caution and appropriate monitoring.
• Concurrent IV anesthetics: Alfaxalone should not be used concurrently with other IV general anesthetic agents (eg, propofol).
• Facility requirements: Alfaxalone should only be used in facilities equipped for anesthetic monitoring and emergency airway management.
• Pregnancy & lactation: Safe use has not been formally established in pregnant or lactating animals; however, alfaxalone is routinely used for cesarean sections in dogs and cats. CRI of alfaxalone during c-section produced longer recovery times and lower Apgar scores but no difference in puppy survival vs isoflurane maintenance. Effects on fertility not evaluated. Use only when maternal benefits outweigh potential risks.

Drug Interactions

Alfaxalone may be used safely with many commonly administered anesthetic and premedication agents; however, additive or synergistic central nervous system and cardiorespiratory depression can occur. Dose adjustments and enhanced monitoring are required when combining alfaxalone with other sedatives or anesthetic drugs.

• Opioids: Produce additive CNS and respiratory depression. Premedication with opioids significantly reduces the required alfaxalone induction dose.
• Benzodiazepines: Enhance sedation and muscle relaxation and reduce alfaxalone dose requirements. However, recovery excitement may be increased in some patients.
• Alpha-2 adrenergic agonists: Markedly reduce induction dose requirements but may increase the risk of bradycardia, hypotension, and respiratory depression.
• Phenothiazines: Additive sedative effects occur and alfaxalone dose requirements are reduced. Monitor closely for hypotension.
• Other IV general anesthetics: Concurrent use is not recommended. The UK label specifically advises against combining alfaxalone with other IV anesthetic agents.
• Inhalant anesthetics: Alfaxalone reduces inhalant anesthetic requirements during maintenance. Failure to adjust vaporizer settings may result in excessive anesthetic depth.
• Central nervous system depressants: Any drug with CNS depressant properties may potentiate the sedative and respiratory effects of alfaxalone.

Side Effects & Overdose

Side Effects

Alfaxalone is considered a relatively safe injectable anesthetic; however, adverse effects are primarily related to dose, rate of administration, and concurrent use of other sedative or anesthetic drugs. Respiratory depression and apnea are the most clinically significant concerns.

• Respiratory depression and apnea: The most common and clinically important adverse effect. Occurs more frequently with rapid IV administration or higher doses.
• Hypoxemia: May occur secondary to hypoventilation or apnea; supplemental oxygen is strongly recommended.
• Cardiovascular effects: Hypotension may occur due to myocardial depression and vasodilation, especially when combined with inhalant anesthetics. Transient hypertension and tachycardia have also been reported.
• Cardiac arrhythmias: Usually associated with hypoxemia or hypercapnia rather than a direct drug effect.
• Neuromuscular effects: Muscle tremors, myoclonus, paddling, opisthotonos-like postures, and hyperreactivity may be seen, particularly during recovery.
• Excitement during recovery: Vocalization, disorientation, and violent movements may occur; recovery quality improves with appropriate premedication and a quiet recovery environment.
• Hypothermia: Common during and after anesthesia; active temperature monitoring and warming are recommended.
• Ocular effects: Transient increases in intraocular pressure have been reported in dogs, followed by a decrease. Tear production may also be reduced.
• Anaphylaxis: Rare; a single case has been reported in a dog.

Overdose

Overdose of alfaxalone primarily results in severe cardiorespiratory depression. The severity of clinical signs depends on dose magnitude, speed of administration, and concurrent sedative or anesthetic drugs.

• Severe hypoventilation or apnea: The most likely and serious consequence of overdose.
• Hypotension: May be profound and prolonged due to combined myocardial depression and vasodilation.
• Hypoxemia: Common following prolonged apnea or inadequate ventilation.
• Cardiac arrhythmias: May occur secondary to hypoxia and acid-base disturbances.
• Management: Supportive care is essential, including airway protection, oxygen supplementation, assisted or controlled ventilation, and cardiovascular monitoring.
• Extended monitoring: May be required in cases of marked overdose or prolonged apnea, particularly in cats.
• Poison consultation: Consultation with a 24-hour veterinary poison consultation center is recommended for suspected overdose cases.
• Overdose dose thresholds: Hypoventilation, apnea, and hypotension are the most likely consequences of overdoses up to 5 times the induction dose (~20 mg/kg IV in dogs and ~50 mg/kg IV in cats). Marked cardiorespiratory depression and prolonged apnea/hypoxia have been documented in cats receiving 50 mg/kg and in a cat that received an alfaxalone CRI at 10 times the recommended rate due to a calculation error.

Key Notes

Practical clinical points to optimize the safe and effective use of alfaxalone in veterinary anesthesia:

• No analgesic effect: Alfaxalone provides anesthesia and sedation only and must always be combined with appropriate analgesic agents when painful procedures are anticipated.
• Rapid, titratable induction: The fast onset of action allows IV administration to effect, making alfaxalone useful for controlled inductions and in high-risk patients when carefully titrated.
• Premedication improves quality: Use of appropriate premedication reduces induction dose requirements and improves overall anesthetic stability and recovery quality.
• Recovery environment matters: A quiet, low-stimulation recovery area significantly reduces excitation, especially in cats.
• IM use is volume-limited: Intramuscular administration is most practical in small patients due to the relatively large injection volumes required.
• Suitable for sighthounds: Alfaxalone is considered a useful induction agent in sighthounds, where other anesthetics may have prolonged effects.
• Short anesthetic duration: When used alone, anesthesia duration is brief; plan maintenance strategy (inhalant or IV supplementation) in advance.
• Multispecies versatility: The drug’s reliable absorption and predictable effects support extra-label use in a wide range of species when IV access is difficult.
• Controlled drug handling: As a Schedule IV controlled substance, alfaxalone requires secure storage and strict inventory control.