Chapter 10
Intravenous Anesthetic Induction Drugs
Time for takeoff!
Tanya Duke-Novakovski
Department of Small Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Canada
- Q. What are ideal characteristics of an injectable induction anesthetic drug?
- A. inexpensive;
- easy to obtain;
- no record-keeping required;
- can be obtained in multidose vials;
- not toxic to tissues;
- easily titratable;
- rapid metabolism and recovery;
- produces good quality of induction and recovery with no apnea;
- provides cardiovascular stability;
- has good shelf-life and does not become contaminated once the seal is broken.
- A. inexpensive;
- Q. Which drugs closely meet these expectations (apart from cost and record-keeping)?
- A. No drug meets all the requirements, but propofol, alfaxalone, and ketamine/benzodiazepine combinations are popular because they can meet most of the requirements.
- Q. Which drugs are considered suitable anesthetic induction agents for IV use in veterinary patients?
- A. Thiopental, propofol, alfaxalone, etomidate, and ketamine (generally with a benzodiazepine).
- Q. What are the suggested doses for the currently available induction drugs?
- A. See Table 10.1 for drugs and dosages.
Table 10.1 Suggested doses for IV induction drugs in healthy dogs and cats.
Induction drug
IV dose (no premedication)
IV dose (with premedication)
Comment
Thiopental
Cat and Dog:
20–25 mg/kg
Cat and Dog:
5–12 mg/kg
[drug] 2.5% concentration. Do not use in Sighthounds
Propofol
Cat: 6–8 mg/kg
Dog: 6–8 mg/kg
Cat: 2–6 mg/kg
Dog: 2–4 mg/kg
Heinz body formation in cats with repetitive daily use (>3 days)
Alfaxalone
Cat: 3–5 mg/kg
Dog: 2–3 mg/kg
Cat: 1–3 mg/kg
Dog: 0.5–2 mg/kg
Pre-anesthetic medication advisable
Ketamine with Diazepam or Midazolam Combination
Cat and Dog:
10 mg/kg of ketamine and
0.5 mg/kg of benzodiazepine
Cat and Dog:
2–5 mg/kg ketamine with 0.25 mg/kg of benzodiazepine
Pre-anesthetic medication advisable
Tiletamine/zolazepam combination
Cat: 4–5 mg/kg
Dog: 2–5 mg/kg
Cat: 1–4 mg/kg
Dog: 1–2 mg/kg
Lengthy recovery possible with high doses
Etomidate
Cat and Dog:
1–3 mg/kg
Cat and Dog: 0.5– 2 mg/kg
Propylene glycol formulation can cause tissue injury and phlebitis
Myoclonus without pre-anesthetic medication
Note: It should be noted that these drugs should be given “to effect” and tailored for individual physical health status. Heavy sedation with premedications that include an alpha-2 agonist + opioid will usually lead to induction dose requirements at the lowest end of the suggested dose range.
- Q. How do injectable anesthetic drugs work?
- A. All injectable anesthetic drugs (except ketamine) act mainly upon inhibitory gamma amino-butyric acid (GABA) receptors within the brain to hyperpolarize the neuron. These receptors normally respond to the indigenous ligand GABA and the anesthetic drug can attach to the receptor in other places and enhance the action of the naturally occurring GABA molecules. Other actions might affect cell-signaling mechanisms, such as those involving calcium and neurotransmitter release, to also cause a state of anesthesia. Ketamine and tiletamine have a different action and mainly produce a “dissociated” state by blocking NMDA receptors. Ketamine “dissociates” the part of the brain responsible for consciousness from the other areas of the brain so the animal is not aware of its surroundings. It is not considered to be true “anesthesia” but rather a cataleptic state. Anesthetic drugs may also have an effect on inhibitory receptors within the spinal cord to produce muscle relaxation or to reduce spinal reflexes.
- Q. Why should I use injectable anesthetic drugs for induction and not inhalant anesthetics?
- A. Injectable anesthetics require slightly more skill using the intravenous (IV) route because of the need for accurate IV administration, but when administered IV they have a more rapid action compared to inhalant anesthetics. A suitable depth of anesthesia to allow endotracheal intubation can be rapidly achieved and this will reduce the risk of pulmonary aspiration. The excitatory stages of anesthesia are not observed with appropriate dosing and relatively rapid administration, and therefore a suitable depth of anesthesia is achieved efficiently without any potential struggling from the animal. Induction of anesthesia using inhalational techniques in unsedated animals can cause struggling, release of catecholamines, and is potentially riskier to the animal. See Chapter 11 for more information on inhalant inductions.
- Q. What are the main anesthetic effects of thiopental?
- A. While thiopental is not currently available in the USA, it is available in other countries and may return to the USA at some time in the future. Thiopental is a barbiturate with a reliable onset within 20–30 s following IV administration and is available as a powder for reconstitution with sterile water. Its alkaline nature (pH 11–14) prevents bacterial contamination and it cannot be mixed with other drugs or fluids. Exposure of the solution to carbon dioxide may result in some cloudiness, but otherwise the solution is stable for at least six weeks with only a small decrease in potency with time. It is easily titratable and can produce a rapid recovery if it is only used for induction of anesthesia. If anesthesia is maintained using several injections of thiopental, accumulation can occur and a prolonged recovery will follow. Sighthounds (Afghans, Greyhounds) and other animals with a high muscle mass to fat ratio have clinically relevant prolongation of recovery even with one induction dose just to allow endotracheal intubation [1]. Greyhounds also lack some hepatic enzyme systems required for metabolism of barbiturates. See Chapter 41 for more breed details regarding anesthetic drugs.
Thiopental can cause a decrease in mean arterial blood pressure (MAP) because of vasodilation and the heart rate will increase to compensate (baroreceptor reflex arc). Therefore, good blood pressure is generally preserved in healthy animals. Thiopental can also cause induction apnea.
Thiopental can be used without pre-anesthetic medication, but higher doses are required and recovery from these higher doses can result in paddling and an excitable recovery. Thiopental should not be used in concentrations greater than 2.5% in cats and dogs as it can cause tissue irritation, especially if the drug is accidentally administered extravascular. Perivascular administration of thiopental can lead to significant tissue sloughing, especially when more concentrated solutions are used or it is injected into a small peripheral vein.
Stay updated, free articles. Join our Telegram channel
- A. Thiopental, propofol, alfaxalone, etomidate, and ketamine (generally with a benzodiazepine).
Full access? Get Clinical Tree
