Chapter 8
Time to Premedicate
How do I decide from all these choices?
Lesley J. Smith
Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
- Q. What am I trying to achieve by my choice of premedication?
- A. The main purposes of premedication are to produce a calm and tractable patient, to smooth the induction process, to reduce the required doses of injectable and inhalant anesthetics for better cardiovascular and respiratory stability, and to provide analgesia when needed. The available premedication drugs can produce sedation, anxiolysis, analgesia, and chemical restraint. Which drug or combination of drugs you choose, and the doses of each, will influence where your patient is on the spectrum of premedication results. Temperament, species, breed, health status, and underlying disease will also influence how that patient responds to different premedications.
- Q. What is the difference between sedation and anxiolysis?
- A. Sedation is characterized by CNS depression and a patient that is generally less aware, or less wary, of its surroundings, but that can still be aroused by painful stimuli or loud/sudden disturbances [1]. Anxiolysis, also referred to by some clinicians as tranquilization, refers to a patient that is rendered more calm and relaxed as a result of the drug, but that is still aware of and responsive to its surroundings. Acepromazine is a classic drug that provides anxiolysis and, at higher doses, sedation. Benzodiazepines provide anxiolysis but little sedation in most patients. Alpha-2 agonists are generally reliable sedatives.
- Q. What is chemical restraint?
- A. Chemical restraint refers most specifically to the dissociative drugs ketamine and tiletamine that produce a cataleptic state in which the patient is unaware of its surroundings, unresponsive to stimuli, but with dilated pupils and some reflexes intact. Animals treated with dissociatives often retain a swallow, will salivate profusely, and will have rigid muscle tone if no other drug is given concurrently. Higher doses of alpha-2 agonists can also produce a state of relative chemical restraint.
- Q. What is analgesia?
- A. Analgesia is reduced or absent pain perception [1]. None of the available premedication drugs when administered at clinically relevant doses can reduce pain to non-existent, but some do reduce pain perception. These are most notably the opioids and alpha-2 agonists, with the possible addition of the dissociatives. The NSAIDs also provide analgesia, but do not produce any sedation or anxiolysis.
- Q. Should I give my premedications IV or IM? What about subcutaneous routes?
- A. The route of administration of your premedications depends on your patient and your technical staff and/or their technical skills. Dogs and cats that are difficult to restrain due to temperament are best premedicated by the IM route, so that IV catheter placement prior to anesthesia is easier to achieve and less stressful for the patient. Less experienced personnel may also benefit from having a sedated patient prior to attempts at IV catheter placement. If IV access is already established, or if it is easy to place a catheter in a tractable, calm patient, then premedications can also be given IV. Generally, doses are reduced when the IV route is chosen. Subcutaneous administration of premedications is also acceptable, but absorption may be slower and less predictable, resulting in lower plasma concentrations of drug and a smaller effect. Subcutaneous administration may be desirable, however, when larger volumes of drug must be given or in animals that are thin with little muscling.
- Q. OK, so I have all these choices, where do I start?
- A. A good place to start is to evaluate your patient and decide what you want to achieve with your premedication. Do you want heavy sedation? Just a calmer animal? One that is completely immobilized? How much analgesia do you want to add depending on how painful the procedure is anticipated to be? Remember too that all of the premedication drugs have dose-dependent effects – and this means dose-dependent desirable effects (e.g., sedation or analgesia) as well as dose-dependent negative side effects. It might be useful to look at the available sedative and opioid drugs on a spectrum – see Table 8.1.
Table 8.1 Spectrum of premedication effects for a healthy dog or cat.
Minimal sedation but some anxiolysis
Moderate sedation/anxiolysis
Profound sedation
midazolam or diazepam
Low–middle dose acepromazine
high dose acepromazine
low dose medetomidine or dexmedetomidine
middle-high dose medetomidine or dexmedetomidine
Mild analgesia/sedation
Moderate analgesia/sedation
Profound analgesia/sedation
butorphanol
low dose buprenorphine
high dose buprenorphine
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- Q. What is chemical restraint?