Slow IV administration of ketamine does not cause gradual loss of airway reflexes, apnea, or general muscular relaxation. The onset of the ketamine-induced "anesthetic state" is accompanied by a gradual, mild increase in muscle tone (which greatly resembles catatonia), continued maintenance of pharyngeal and laryngeal reflexes, 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 intravenous drugs in its ability to cause cardiovascular stimulation 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 stimulates cardiovascular function. In contrast, patients with poor 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 excellent drug for the induction of anesthesia in individuals who cannot tolerate compromise of their cardiovascular system.
The analgesia induced by ketamine also is a property that separates it from other IV anesthetic drugs. Analgesia is obtained without a deep level of anesthesia. When subdissociative doses of ketamine are given either IV or intramuscularly (IM), they provide adequate 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 regarded 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 radiographic 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.
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