General anesthesia may be induced by the use of inhalational agents or, more commonly, by the use of injectable drugs. The latter are usually administered by the intravenous route, but some will also work when given intramuscularly, e.g. ketamine. Other routes may occasionally be used (e.g. rectal, intraperitoneal, transmucosal) but, in common with all other extravenous routes, they prevent titration of the induction agent to a suitable endpoint. That is to say, with intravenous induction techniques, the drug can be given slowly to effect and administration stopped when the patient is at a suitable depth of anesthesia for endotracheal intubation: drugs given by other routes are administered based solely on body weight, implying that some animals will receive a relative overdose while others will be underdosed, as not all patients of a particular weight will require the same dose of anesthetic.
Induction agents are often chosen on the basis of anticipated recovery time, effects on the cardiovascular system and so on, but in many cases several agents may be suitable for a particular case, and personal preference and expense play a role.
A number of derivatives of barbituric acid have previously been used for induction and/or maintenance of general anesthesia (methohexital, pentobarbital) in veterinary patients. However, the only barbiturate currently in use as an anesthetic induction agent is thiopental, although newer induction agents have largely superseded it.
Doses of injectable anesthetic agents
Depending on the agents chosen, premedication may dramatically alter the dose of injectable anesthetic agent required (see Table Suggested drug doses for patients following ‘standard premedication’ with acepromazine ± opioid). For example, acepromazine will reduce the dose of thiopental for induction of anesthesia by about 30-50%, while medetomidine will decrease it by anything up to 90%. In addition, the state of the patient’s health will also have a major bearing on the quantity of induction agent required. It has been said that: ‘The dose of induction agent required is as much as the patient needs, and no more’, i.e. anesthetic agents should be given to effect. Thus, it is not possible to be completely prescriptive about induction doses, and the information provided in Table Suggested drug doses for patients following ‘standard premedication’ with acepromazine ± opioid should be considered only as a guideline.
Table Suggested drug doses for patients following ‘standard premedication’ with acepromazine ± opioid. Much lower doses will generally be required following medetomidine.
|Thiopental||7-10 mg/kg intravenous||7-10 mg/kg intravenous|
|Alfaxalone||2mg/kg intravenous||2-5 mg/kg intravenous|
|Propofol||3-4 mg/kg intravenous||4-6 mg/kg intravenous|
|Etomidate||0.5-2 mg/kg intravenous||0.5-2 mg/kg intravenous||No veterinary licence|
|Ketamine||0.2 mg/kg diazepam + 5 mg/kg ketamine intravenous||0.2 mg/kg midazolam + 5-10 mg/kg||Diazepam-ketamine combination useful for examination of laryngeal function. The two drugs can be administered in the same syringe, but should be mixed just before administration
Midazolam-ketamine can be mixed in the same syringe, and provide deep sedation in cats
Medetomidine-ketamine can be mixed in the same syringe, with potential for reversal with atipamezole (ketamine dose is so low in this combination that once medetomidine is reversed, ketamine exerts little effect). This combination is usually used without prior premedication
|ketamine intramuscular; 80 μg/ kg medetomidine + 2.5-7.5 mg/kg ketamine intramuscular|