Ketamine is a cyclohexanone derivative whose pharma-cological actions are quite different from those of the other IV anesthetics. The state of unconsciousness it produces is trancelike (i.e., eyes may remain open until deep anesthesia is obtained) and cataleptic; it has fre-quently been characterized as dissociative (i.e., the pa-tient may appear awake and reactive but does not re-spond to sensory stimuli). The term dissociative anesthesia is used to describe these qualities of pro-found analgesia, amnesia, and superficial level of sleep.
Slow IV administration of ketamine does not cause gradual loss of airway reflexes, apnea, or general muscu-lar relaxation. The onset of the ketamine-induced “anes-thetic state” is accompanied by a gradual, mild increase in muscle tone (which greatly resembles catatonia), con-tinued maintenance of pharyngeal and laryngeal re-flexes, and opening of the eyes (usually accompanied by nystagmus). Although reflexes may be maintained, the airway still must be protected, since ketamine sensitizes laryngeal and pharyngeal muscles to mucous or foreign substances, and laryngospasm may occur.
Ketamine also can be contrasted to other intra-venous drugs in its ability to cause cardiovascular stim-ulation rather than depression. The observed increases in heart rate and blood pressure appear to be mediated through stimulation of the sympathetic nervous system. In a healthy, normovolemic, unpremedicated patient, the initial induction dose of ketamine maintains or stim-ulates cardiovascular function. In contrast, patients withpoor cardiac reserve, compromised autonomic control, or hypovolemia may undergo a precipitous fall in blood pressure after induction of anesthesia with ketamine. If selection of the patient and preoperative preparation are carefully done, however, ketamine may be an excel-lent drug for the induction of anesthesia in individuals who cannot tolerate compromise of their cardiovascular system.
The analgesia induced by ketamine also is a prop-erty that separates it from other IV anesthetic drugs. Analgesia is obtained without a deep level of anesthe-sia. When subdissociative doses of ketamine are given either IV or intramuscularly (IM), they provide ade-quate analgesia for postoperative pain relief as well as analgesia for brief operations on the skin, such as de-bridement of third-degree burns. Because it can be re-garded as a nearly complete anesthetic (hypnosis and analgesia), does not require anesthesia equipment, and is relatively protective of hemodynamics, ketamine also can be very useful outside of normal operating room conditions, such as may be found during painful radio-graphic procedures.
A most important advantage of ketamine over other anesthetic agents is its potential for administration by the IM route. This is particularly useful in anesthetizing children, since anesthesia can be induced relatively quickly in a child who resists an inhalation induction or the insertion of an IV line. Ketamine has a limited but useful role as an IM induction agent and in pediatrics.
The most serious disadvantage to the use of ketamine is its propensity to evoke excitatory and hallucinatory phenomena as the patient emerges from anesthesia. Patients in the recovery period may be agitated, scream and cry, hallucinate, or experience vivid dreams. These episodes may be controlled to some extent by main-taining a quiet reassuring atmosphere in which the pa-tient can awaken or if necessary by administering tran-quilizing doses of diazepam.
Other reported side effects include vomiting, saliva-tion, lacrimation, shivering, skin rash, and an interaction with thyroid preparations that may lead to hypertension and tachycardia. Ketamine also may raise intracranial pressure and elevate pulmonary vascular resistance, es-pecially in children with trauma or congenital heart dis-ease. Increases in intraocular pressure also may occur, and vigilance is required if ketamine is used in ocular surgery.