Liz is a licensed veterinary medical technologist. She acquired a B.S. in veterinary medical technology from Lincoln Memorial University.
When anesthetizing veterinary patients, particularly dogs, cats, and horses, acepromazine, ketamine, and propofol are three of the most commonly used injectable sedatives/anesthetics used by veterinary anesthetists. Whether being used as a pre-anesthetic or as an induction anesthetic, these are great anesthetic drug choices and, when used properly in conjunction with other appropriate drugs, they are very safe. That being said, not every drug is safe for every patient, and thorough exams and metabolic work-ups should be performed before inducing anesthesia in any patient; every anesthetic case is different, and must be treated as such. A drug protocol that is great for one young, healthy patient may be dangerous to another senior, unstable patient. The type of procedure being performed while under anesthesia is also important to consider when selecting anesthetic drugs. Plumb's Veterinary Drug Handbook is my personal go-to for all my anesthetic drug questions, and is a necessity to any veterinary or veterinary technology student who is or will take veterinary pharmacology.
Acepromazine, or “ace”, is a phenothiazine sedative/tranquilizer. This drug has a complex mechanism of action that is not fully understood. Major side effects may include depression of the reticular activating center of the brain and blockage of alpha-adrenergic, dopamine, and histamine receptors. Major adverse effects include hypotension (low blood pressure), penile prolapse in male horses and other large male animals, and decreased PCV (packed cell volume or hematocrit). As it is a phenothiazine, ace lowers the seizure threshold and therefore should not be used in epileptic animals or those who have a history of seizures. Ace is metabolized by the liver and crosses the placental barrier slowly, therefore fetuses are affected. Onset of action is about 15 minutes after intramuscular injection in dogs or intravenous injection in horses, and peak effect occurs within 30 to 60 minutes. The duration of action is 4 to 8 hours in small animals, but may be longer if higher doses are used or if the drug is given to old, sick, or debilitated patients, or those with liver disease. The duration of action in horses is shorter.
Patients injected with ace should be moved into a darker, quiet location free from stimulation, as the sedative effects can be overridden and ineffective if the patient becomes excited or overstimulated by the stimuli around it. It has been found that the manufacturer’s recommended dose for ace is actually higher than actually required for adequate pre-anesthesia, and the dose should therefore be reduced to minimize the risk of adverse effects. In my personal opinion and experiences, ace should only be used as a pre-anesthetic in conjunction with other appropriate anesthetic drugs; it should never be used alone to simply sedate an animal for a simple procedure or to calm a fractious animal, as some animals actually experience an excitatory effect from ace. The commonly accepted doses are about 0.05 to 0.1 mg/kg in small animals, with a maximum dose of 3 mg in dogs and 1 mg in cats, and 0.03 to 0.05 mg/kg in horses. Higher doses will increase hypotension, but not sedation. Hydration is important when using phenothiazine derivatives because they can cause vasodilation. It should also not be used when treating vomiting animals with abnormal gastrointestinal motility because it may promote ileus and worsen vomiting.
When using acepromazine, one should be aware that this drug has increased potency in geriatric animals, neonates, animals with liver or cardiac dysfunction, and generally debilitated patients. Responses to this drug are also species and breed-dependent; for example, doses should be reduced by 25% in collies and Australian shepherds to minimize the possibility of prolonged sedation. Giant breed dogs, greyhounds, and boxers have been found to be especially sensitive to this drug and may experience severe bradycardia (low heart rate) and hypotension. Terriers and cats are typically more resistant to ace's sedative effects. Severe hypotension and bradycardia associated with the use of ace is treated with IV fluid therapy and anticholinergics. Although ace has a relatively low toxicity, severe overdoses require immediate treatment. Hypotension resulting from overdose is actually worsened by epinephrine and should instead be treated with phenylephrine or norepinephrine.
Ketamine is a class III controlled substance and is classified as a dissociative general anesthetic and NMDA- receptor antagonist. It acts very rapidly, has significant analgesic (pain-reducing) activity, and has a relative lack of cardiopulmonary depression, making it great for cardiac patients. It is typically used in combination with acepromazine and xylazine (both as pre-anesthetics), and/or diazepam to provide muscle relaxation and deep anesthesia. Personally, I always keep diazepam on hand regardless when using ketamine in the event that the ketamine induces a seizure. Ketamine is given intramuscularly or intravenously, and is actually only approved for use in cats and primates; it is actually used extra-label in other animal species. Animals keep their eyes open when given ketamine, so ocular lubricants are a necessity to prevent eye injuries and corneal ulcerations. Spastic muscle jerking, hypertension, hyperthermia, seizures, and increased salivation may be seen with this drug. Due to its low pH, pain at the injection site is frequently noted. Apneustic breathing (holding of the breath for extended periods of time) is often seen, and anesthetic depth in animals given ketamine may be difficult to assess because of the associated altered palpebral (blinking) reflexes.
Ketamine is generally contraindicated in cases of severe blood loss, hyperthermia, increased ocular pressure, head trauma, and procedures involving the larynx, pharynx, or trachea. Peak action of ketamine is about 1 to 2 hours after intravenous injection, and about 10 minutes after intramuscular injection. The total duration of effect is about 20 to 30 minutes. A higher dose increases sedation, but does not increase anesthetic effect. As a dissociative, it is redistributed and metabolized by the liver, and may also be excreted unchanged in the urine and should be used cautiously in patients with liver or kidney disease.
|Drug||Onset of Action||Duration of Action||Recovery|
15 minutes; peaks in 30 to 60 minutes
4 to 8 hours
Varies upon other drugs used
10 minutes after IM injection, 1 to 2 hours after IV injection
20 to 30 minutes
Varies upon other drugs used
30 to 60 seconds
5 to 10 minutes
20 to 30 minutes
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Propofol is an ultra-short acting nonbarbituate injectable anesthetic with a very wide margin of safety. It is the most commonly used injectable agent for induction in small animals. Propofol is my personal favorite anesthetic inducer due to its rapid onset of action, wide margin of safety, and smooth, rapid recovery. Propofol’s chemical structure is unique to other anesthetic agents. Although it has a milky appearance, it is currently the only exception to the rule that white liquids should never be given intravenously. Propofol can also be used to treat status epilepticus (seizures) because it tends to cause less cardiovascular depression and produce smoother recoveries than pentobarbital. Repeated use of propofol in cats may produce Heinz body anemia because cats cannot metabolize it well due to their relatively low circulatory concentrations of the enzyme glucoronyl transferase.
Although its mode of action is not fully understood, propofol seems to affect GABA receptors in a similar was as barbituates. It has a rapid onset and short duration of action because it is highly fat-soluble; it is rapidly taken up by vessel-rich tissues but is very quickly redistributed to muscle and fat, where it is rapidly metabolized and eliminated. This results in a smoother, more rapid recovery with minimal residual sedative effects, even after repeated injections. Propofol is only truly contraindicated in patients that have a hypersensitivity to any component of the drug.
The onset of action of propofol is very rapid, only about 30 to 60 seconds. It has an extremely short duration of action of about 5 to 10 minutes, with complete recovery in as little as 20 to 30 minutes. Side effects may include CNS depression, bradycardia, hypotension, apnea, muscle twitching, and muscle relaxation. This drug should be given slowly intravenously, over 1 to 2 minutes until the desired anesthetic depth is reached. I often keep my leftover propofol on hand during the surgery or procedure in the event that my patient becomes too light, or if I need to adjust anesthetic depth quickly. Intramuscular injection of propofol may cause some sedation and ataxia, but it will not induce anesthesia because the drug is too quickly metabolized.
Personal experience as a veterinary anesthetist.
Lerche, P., Thomas, J. (2011). Anesthesia and Analgesia for Veterinary Technicians. (4th ed.) St. Louis, MO. Mosby-Elseveir.
Plumb, D. (2011). Plumb’s Veterinary Drug Handbook. (7th ed.) Ames, IO. Wiley-Blackwell.
Romich, J. (2010). Fundamentals of Pharmacology for Veterinary Technicians. (2nd ed.) Clifton, NY. Delamar-Cengage.
© 2018 Liz Hardin
Liz Hardin (author) from Tennessee on October 09, 2018:
Thanks for reading!
Linda Crampton from British Columbia, Canada on September 14, 2018:
The information in your article is very interesting. Thanks for sharing so many details about the drugs.