Ketamine is the cyclohexamine most commonly used for sedation, analgesia, and anesthesia. The cyclohexamines have been categorized as sympathomimetic anesthetics that produce a state of anesthesia termed ''dissociative." The original preparation, phencyclidine hydrochloride (Sernylan®), was used in cats, nonhuman primates, and some wild species, but had a prolonged recovery. It has been withdrawn from the market because of its potential for human abuse. The cyclohexamines have analgesic properties in low doses and produce a state of general anesthesia at higher doses. The analgesic and anesthetic properties of ketamine in most rodents are poor, and it does not produce surgical anesthesia when used alone in the rat (Wilson and Wheatley, 1981), except in very high doses that produce severe respiratory depression (Lumb and Jones, 1984; Flecknell, 1987). Many investigators believe that it should be administered in combination with a sedative, such as diazepam or xylazine (Flecknell, 1987). Ketamine has been used successfully as a preanesthetic agent before the use of inhalational agents in rodents and rabbits (Wass et al., 1974; White et al., 1975).
It is desirable to administer atropine before ketamine in some species to minimize the excessive salivary and bronchial secretions associated with ketamine. Xylazine and diazepam are excellent adjuncts to the use of ketamine to produce surgical anesthesia in cats, dogs, guinea pigs, hamsters, horses, rabbits, sheep, pigs, and primates (Tables 5-4 and 5-5). The combination of xylazine or diazepam with ketamine permits smoother induction of anesthesia and deeper and longer-lasting anesthesia than ketamine alone (Wixson et al., 1987a,b,c).
Ketamine anesthesia is not accompanied by the classic, progressive CNS depression that is noted after use of barbiturates and inhalational agents; rather, it is a cataleptic anesthetic state (Winters et al., 1972; Chen, 1973). In many species, it produces an appearance similar to that of decerebration, including hypertonicity of the antigravity muscles and an apneustic respiratory pattern that is characterized by breathholding at the peak of inspiration followed by passive exhalation (Calderwood et al., 1971).
The characteristic clinical signs seen in animals after the administration of ketamine are head weaving, progressive ataxia, complete loss of coordination, collapse, and immobilization. There are usually increases in muscle tone, salivation, and bronchial secretions; muscle twitching; and, while the animal is under anesthesia, spontaneous movements unrelated to surgical stimuli. Ocular, oral, and swallowing reflexes are present, and the animal's eyes remain open with corneal and pupillary reflexes present. Nystagmus might occur, and lacrimal secretions persist. An increase in respiration might occur, but death from the high dose necessary for surgical anesthesia is eventually due to respiratory depression (Flecknell, 1987), especially in small rodents. Ketamine has been used for general anesthesia in