Etomidate

Overview

Etomidate (Amidate®) is a nonbarbiturate injectable anesthetic agent used intravenously for induction of general anesthesia in dogs and cats. It is valued for producing rapid unconsciousness with minimal cardiovascular depression, making it a useful option in hemodynamically unstable or critically ill patients.

In veterinary practice, etomidate is most commonly considered as an alternative induction agent to propofol or alfaxalone, particularly in patients with preexisting cardiac dysfunction, shock risk, or limited cardiovascular reserve. Hypnosis is short — about 3 to 5 minutes, and dose-dependent — and the drug is used for induction only, not maintenance, because prolonged administration suppresses cortisol and aldosterone production.

Mechanism of Action (MOA): Etomidate enhances gamma-aminobutyric acid (GABA) receptor activity within the central nervous system, increasing chloride ion conductance and causing neuronal hyperpolarization. This results in hypnosis, CNS depression, and rapid anesthetic induction.

Etomidate provides no analgesia, so appropriate analgesic medications are required for painful procedures. It also suppresses adrenal cortisol production for roughly 2 to 6 hours in dogs and up to 5.5 hours in cats after a single induction dose.

Indications

Etomidate is primarily used in dogs and cats as an intravenous induction agent when rapid anesthetic induction is required with minimal cardiovascular compromise. It is especially valuable in patients where maintenance of blood pressure and cardiac function is a priority.

• Induction of general anesthesia: Used to produce rapid unconsciousness prior to endotracheal intubation and transition to inhalant or injectable maintenance anesthesia.
• Patients with cardiac disease: May be selected for dogs and cats with pre-existing cardiac dysfunction because it produces relatively mild haemodynamic changes.
• Critically ill or unstable patients: Useful in emergency or high-risk patients where hypotension from other induction agents is a concern.
• Shock or poor perfusion risk: May be selected when preservation of arterial pressure is clinically important during induction.
• Neurologic cases requiring induction: Used at the standard induction dose given below when reduction of cerebral blood flow, cerebral oxygen consumption or intracranial pressure is desirable.
• Alternative to propofol or alfaxalone: Used when those agents are unavailable, less suitable, or expected to cause excessive cardiovascular depression.

Dosage (Reference)

Dog

Etomidate is used intravenously as an induction agent in dogs. Dose requirements vary with patient status, level of premedication, concurrent drugs, and overall anesthetic risk. Give slowly intravenously over approximately 30 to 60 seconds rather than as a rapid bolus; the required dose is a range that varies with the patient rather than a single fixed figure.

Cat

In cats, etomidate is used as an IV induction agent with dosing principles similar to dogs. Midazolam co-induction markedly reduced myoclonus in cats: after etomidate, myoclonus occurred in 9 of 12 saline-treated cats and none of 12 midazolam-treated cats.

Warnings & Precautions

Etomidate is a potent intravenous induction anesthetic that should be administered only in settings where airway control, oxygen supplementation, assisted ventilation, and cardiovascular monitoring are immediately available. Although cardiovascular effects are usually mild, careful patient selection and monitoring remain essential.

• No analgesia: Etomidate provides hypnosis only and does not supply pain control. Appropriate analgesics are required for painful procedures.
• Adrenal suppression: Etomidate can temporarily inhibit cortisol production after administration. Use cautiously in critically ill patients, septic patients, or those with impaired adrenal reserve.
• Induction use only: Use for induction only. The drug should not be given for any other purpose, and the formulation is not intended for administration by prolonged infusion because of the hazards of sustained cortisol and aldosterone suppression.
• Respiratory depression: Apnea, hypoventilation, or transient respiratory compromise may occur after induction. Be prepared to intubate and ventilate immediately.
• Myoclonus / excitatory movements: Involuntary muscle movements, paddling, or poor induction quality may occur, especially without adequate premedication.
• Nausea and regurgitation: Vomiting, gagging, retching, or regurgitation may occur; protect the airway in at-risk patients.
• Hepatic or renal dysfunction: Elimination half-life may be significantly increased in patients with impaired hepatic or renal function, and the propylene glycol carrier in the injection may itself be problematic where there is liver dysfunction.
• Ocular procedures: Etomidate alone usually lowers intraocular pressure, but increases have been reported with some combined protocols; use caution during ocular procedures and monitor intraocular pressure.
• Cats and hemolysis risk: The propylene glycol carrier may cause haemolysis, especially in cats; haemoglobinuria and intravascular haemolysis occurred in two dogs receiving etomidate infusion.
• IV administration only: Administer through a secure intravenous catheter whenever possible. Slow titration to effect is preferred over rapid bolus dosing.
• Recovery quality: Compared with propofol in dogs, etomidate produced higher systolic and mean arterial pressures but longer and poorer recoveries.
• Pregnancy and lactation: Safety has not been established during pregnancy or nursing. Embryocidal effects and maternal toxicity have both been reported in animal reproduction studies, and some etomidate is excreted into milk; use only when maternal benefit outweighs risk to offspring.
• Hypersensitivity: Etomidate is contraindicated in patients with known hypersensitivity to the drug.
• High-alert medication: Etomidate is a high-alert medication requiring special safeguards against dosing error, and it must not be confused with etidronate. Use redundant dose and volume checks and special alert labels.
• Laboratory testing: Etomidate-induced cortisol inhibition may invalidate ACTH stimulation and glucose tolerance tests for 2 to 6 hours in dogs and up to 5.5 hours in cats.
• Malignant hyperthermia: Etomidate does not induce malignant hyperthermia, but in susceptible patients it can speed the onset of an episode triggered by another agent.

Drug Interactions

Most clinically relevant interactions with etomidate involve additive central nervous system depression, respiratory compromise, or altered anesthetic depth. Dose adjustments and close anesthetic monitoring are recommended whenever etomidate is combined with other sedatives or anesthetic agents.

• Opioids (e.g., methadone, fentanyl, hydromorphone): Additive sedation and respiratory depression may occur. Opioid premedication may reduce the seizure risk reported in humans, but additive central nervous system and respiratory depression may occur.
• Benzodiazepines (e.g., midazolam, diazepam): Commonly combined to reduce myoclonus and lower induction dose requirements, but CNS depression may be enhanced.
• General anesthetics (e.g., propofol, alfaxalone, inhalants): Combined anesthetic effects may deepen anesthesia and increase cardiopulmonary depression.
• Barbiturates: Additive hypnotic and respiratory depressant effects may occur when used concurrently.
• Ketamine: Low-dose use in premedication protocols may improve induction quality and reduce etomidate dose needs.
• Verapamil: Has been associated with potentiation of anesthetic and respiratory depressant effects.
• Other CNS depressants: Drugs that depress central nervous system or respiratory function may produce additive pharmacological effects with etomidate.
• Corticosteroids: May be intentionally administered in selected critically ill patients to offset temporary cortisol suppression associated with etomidate.

Side Effects & Overdose

Side Effects

Adverse effects of etomidate are usually associated with induction and the immediate peri-anesthetic period. Many reactions can be reduced by appropriate premedication, slow IV administration, and close monitoring.

• Pain on injection: Pain at the intravenous injection site and phlebitis may occur; transient venous pain was reported in about 20% of human patients. Injecting into a running intravenous line may reduce pain.
• Myoclonus / muscle tremors: Involuntary muscle movements, paddling or rigidity may occur during induction. Transient skeletal muscle movements were reported in about 32% of human patients, while midazolam prevented post-etomidate myoclonus in one study of healthy cats.
• Vomiting / retching / regurgitation: Gastrointestinal reactions may occur around induction or recovery.
• Hypersalivation: Excess salivation may be seen in some patients.
• Apnea or hypoventilation: Temporary respiratory depression may occur after dosing and requires prompt airway support if significant.
• Tachypnea or altered breathing pattern: Short-term respiratory irregularities may be observed.
• Transient blood pressure changes: A brief decrease in arterial blood pressure may occur despite generally mild cardiovascular effects; both hypertension and hypotension have been reported in humans.
• Arrhythmias: Cardiac rhythm disturbances have been reported in humans receiving etomidate.
• Pigmenturia / hemolysis: May occur due to the propylene glycol formulation, with cats considered more susceptible.
• Poor recovery quality: Some patients may have rougher or longer recoveries compared with propofol.
• Eye movements: A recognised effect during induction; miosis has also been reported when etomidate is combined with midazolam.
• Effects reported in humans: Additional effects reported in humans include laryngospasm, hiccups, hyperventilation, lactic acidosis and seizures; opioid premedication may reduce the reported seizure risk.

Overdose

Etomidate overdose is expected to cause excessive anesthetic and cardiorespiratory effects. Severity depends on total dose, speed of administration, concurrent sedatives, and patient health status.

• Profound CNS depression: Acute overdose is expected to produce excessive hypnosis and central nervous system depression.
• Marked respiratory depression: Excessive respiratory depression, including hypoventilation or apnoea, may require mechanical ventilation.
• Cardiovascular compromise: Excessive pharmacological effects may include decreased arterial blood pressure; monitor cardiac rate, rhythm and blood pressure.
• Prolonged recovery: Delayed return to consciousness may occur, especially in compromised patients.
• Enhanced adrenal suppression: Prolonged administration can suppress endogenous cortisol and aldosterone production and should be avoided.
• Management: Treatment is supportive until the effects of etomidate diminish. Provide supportive treatment, including mechanical ventilation when required, and monitor respiratory and cardiovascular status until the drug’s effects diminish.
• Hospitalization: Continuous monitoring is recommended for symptomatic patients until normal respiration and mentation return.

Key Notes

Practical clinical points that may help optimize the safe and effective use of etomidate in dogs and cats:

• Best niche drug: Etomidate is most valuable when cardiovascular stability is more important than recovery smoothness or cost.
• Titrate to endpoint: Give etomidate intravenously over approximately 30 to 60 seconds; the required dose is a range rather than a single fixed figure.
• Use a balanced protocol: Etomidate provides no analgesia; provide appropriate analgesia for painful procedures. Suitable premedication can reduce vomiting, myoclonus and the required induction dose.
• Secure venous access first: Having a reliable IV catheter before induction improves safety because onset is rapid and airway control may be needed immediately.
• Consider procedure length: Because etomidate is an induction drug, a clear maintenance anesthesia plan should already be prepared before administration.
• Monitor high-risk patients aggressively: Frail, geriatric, septic, or trauma patients may appear cardiovascularly stable initially but still require intensive monitoring.
• Cost planning: Large dogs may require enough volume to make etomidate less economical than alternative induction agents.
• Team communication: Alert anesthesia staff in advance when etomidate is selected so airway equipment, monitoring, and follow-up drugs are ready.
• Case selection matters: In healthy routine patients, other induction agents may be simpler or more cost-effective, while etomidate excels in selected compromised cases.
• Therapeutic index: Etomidate has an unusually wide safety margin: a reported therapeutic index of 26, against 3 for propofol and 5 for thiopental.
• Pharmacokinetics: Onset is within about one minute and hypnosis lasts roughly 3 to 5 minutes, dose-dependently. Etomidate is about 75% protein bound, rapidly metabolised in the liver to inactive metabolites and excreted mainly in urine, with a reported elimination half-life of 1.25 to 5 hours. Recovery appears as fast as with thiopental but slower than with propofol. These figures come from human data — specific pharmacokinetic data in domestic animals has not been established.
• Formulation and storage: Etomidate is supplied as a human-labelled 2 mg/mL injection; no veterinary-labelled product is available. The solution contains 35% propylene glycol and is hyperosmolar at 4640 mosm/L. Store at room temperature and protect from light.
• Controlled-substance status: Etomidate is not a controlled substance.
• Monitoring: Monitor three things after induction: depth of anaesthesia and CNS effects (seizures, paddling, myoclonus); respiratory depression (rate, pulse oximetry, end-tidal CO2); and cardiovascular status (rate, rhythm, blood pressure).