Chapter 5
Pre-Anesthetic Sedative Drugs
Let’s make the patient more relaxed and comfortable!
Lesley J. Smith1 and Jo Murrell2
1Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
2School of Veterinary Sciences, University of Bristol, UK
- Q. What is the purpose of pre-anesthetic sedation?
- A. Pre-anesthetic sedation has many potential benefits. Among them are reduced patient stress and anxiety, and easier patient handling resulting in less stress to personnel. Because patients are easier to handle there is reduced risk of injury to veterinary staff or to the patient from excessive or aggressive restraint. Perhaps most importantly, pre-anesthetic sedation, depending on the drugs chosen, significantly reduces the required dose of induction agent and of anesthetic inhalant. Reducing doses of induction and inhalant drugs results in less overall cardiovascular and respiratory depression, which are the main players with respect to anesthetic morbidity and mortality. Finally, pre-anesthetic sedation will smooth the recovery process because there is less risk of emergence delirium or dysphoria on recovery.
- Q. What drugs are used in small animal medicine for pre-anesthetic sedation or restraint?
- A. Most commonly the following drugs are used for sedation/restraint in the pre-anesthetic period.
- The phenothiazine tranquilizer: acepromazine.
- The benzodiazepine anxiolytics: midazolam and diazepam.
- Telazol: A proprietary combination of the dissociative tiletamine and the benzodiazepine zolazepam.
- The dissociative: ketamine.
- The alpha-2 agonists: medetomidine, dexmedetomidine.
Chapter 6 will cover opioids and NSAIDs, which are generally used for their analgesic effects. Opioids can provide sedation in dogs and cats, but not reliably so, and NSAIDs provide no sedation.
- Q. What should I know about acepromazine in dogs and cats?
- A. Acepromazine is a relatively long-acting sedative that is metabolized by the liver. Depending on the dose used, this drug may have residual sedative effects and prolong the time to extubation [1]. Acepromazine is a relatively reliable sedative drug, but aggressive or fractious animals will override its effects, particularly if it is used alone. The long duration of acepromazine may be an advantage in excitable or anxious patients because it can help to smooth recovery. However, animals that are scheduled to go home the same day, geriatric patients, any patient with compromised hepatic function, and any patient with depressed mentation will exhibit undesirably prolonged sedation from acepromazine.
The most important negative side effect of acepromazine is a decrease in blood pressure secondary to vascular smooth muscle relaxation and vasodilation [2]. This decrease in blood pressure is dose-dependent, so low doses of acepromazine (see Table 5.1) will cause only modest decreases in blood pressure in healthy patients. It is also important to keep in mind that hypotension secondary to acepromazine will be additive when inhalants are administered, particularly at higher vaporizer settings.
Table 5.1 Suggested doses for pre-anesthetic sedative drugs.
Dose ranges for pre-anesthetic sedation
IV
IM
acepromazine
0.005–0.02 mg/kg
0.01–0.1 mg/kg
midazolam
0.05–0.1 mg/kg
0.1–0.2 mg/kg
diazapam
0.05–0.1 mg/kg
not useful
ketamine
2–5 mg/kg
5–10 mg/kg
telazol
2–4 mg/kg
medetomidine
2–10 µg/kg
4–20 µg/kg
dexmedetomidine
1–5 µg/kg
2–10 µg/kg
atipamezole
(for reversal of alpha-2 agonist sedation)
not recommended
equal volume to administered dose of the alpha-2 agonist (dog); ½ volume of administered dose of alpha-2 agonist (cat)
- Q. So when should I think about using acepromazine for sedation and when should I avoid it?
- A. Acepromazine is an excellent sedative choice, particularly in combination with opioids (see Chapters 6 and 8) in young, healthy non-fractious dogs. In this population, sedation is generally reliable, blood pressure decreases are modest, and the long-ish duration of acepromazine helps in the recovery phase. In cats, acepromazine combined with an opioid does not always provide adequate sedation, so often a dissociative is added to the combination (see below on Dissociatives).
Acepromazine should probably be avoided, or used at the very lowest end of the dose range, in geriatric patients or those with significant systemic disease (ASA 3–5). It should be avoided in any patient with dehydration or blood loss from hemorrhage because negative effects on blood pressure can be anticipated. It should perhaps be avoided in patients with coagulopathies, because surgical blood loss will contribute to hypotension. Historically, acepromazine was reported to “lower the seizure threshold,” but there is no convincing evidence that acepromazine administration is associated with an increase in seizure frequency, even in dogs with a seizure history [3].
- Q. When is it useful to consider the benzodiazepine drugs, midazolam or diazepam? How do they work?
- A. Benzodiazapine drugs work by enhancing Cl– conductance at the GABA receptor, producing mild sedation and reduced anxiety in animals. The sedative effects are minimal in young, healthy dogs and cats. Often, these patients will become excitable and more difficult to restrain, particularly if the benzodiazepine is used alone. Most often, in order to achieve some degree of sedation, the benzodiazapines are combined with opioids in dogs and cats (see Chapters 6 and 8). The duration of effect of the benzodiazapines is short; in dogs an IM dose of midazolam results in sub-therapeutic plasma concentrations within an hour, with similar results via the IV route [4].
Benzodiazapines, combined with an opioid, are good choices for pre-anesthetic sedation in patients with significant systemic disease (ASA 3–5) and in geriatric patients who cannot tolerate low blood pressure during anesthesia because of reduced organ “reserve.” Even combined with opioids, the benzodiazapines will not provide reliable sedation in fractious patients.
The benzodiazapines are extremely safe, however, as they cause virtually no change in cardiovascular or respiratory function in dogs and cats. There is some debate about their use in patients with hepatic failure (see Chapter 34
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- The phenothiazine tranquilizer: acepromazine.
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