Uses

The many unique features of ketamine pharmacology, especially its propensity to produce unwanted emergence reactions, have placed ketamine outside the realm of routine clinical use. Nevertheless, ketamine has an important niche in the practice of anesthesiology when its unique sympathomimetic activity and bronchodilating capability are indicated during induction of anesthesia. It is used for premedication, sedation, induction, and maintenance

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Induction and Maintenance of Anesthesia

Poor-risk patients (ASA class IV) with respiratory and cardiovascular system disorders (excluding ischemic heart disease) represent the majority of candidates for ketamine induction; such induction is particularly appropriate for patients with bronchospastic airway disease or those with hemodynamic compromise based on either hypovolemia or cardiomyopathy (not coronary artery disease). The bronchodilation and profound analgesia that allow the use of high oxygen concentrations make ketamine an excellent choice for induction in patients with reactive airway disease. Otherwise healthy trauma victims whose blood loss is extensive are also candidates for rapid-sequence induction with ketamine.^[544] Patients in septic shock may also benefit from ketamine.^[545] However, ketamine's intrinsic myocardial depressant effect may become manifested in this situation if trauma or sepsis has caused depletion of catecholamine stores before the patient's arrival in the operating room. The use of ketamine in these patients does not obviate the need for appropriate preoperative preparation, including restoration of blood volume. Other cardiac diseases that can be well managed with ketamine anesthesia are cardiac tamponade and restrictive pericarditis.^[546] The finding that ketamine preserves the heart rate and right atrial pressure through its sympathetic stimulating effects makes it an excellent anesthetic induction and maintenance drug in this setting. Ketamine is also often used in patients with congenital heart disease, especially those with a propensity for right-to-left shunting. In addition, the use of ketamine has been reported in a patient susceptible to malignant hyperthermia who had a large anterior mediastinal mass^[547] when spontaneous ventilation was required and inhaled anesthetics were contraindicated. ^{[547] [548] [549]}

Ketamine combined with diazepam or midazolam can be given by continuous infusion to produce satisfactory cardiac anesthesia in patients with valvular and ischemic heart disease. The combination of a benzodiazepine^[542] or a benzodiazepine plus sufentanil^[550] with ketamine attenuates or eliminates the unwanted tachycardia and hypertension, as well as postoperative psychological derangements. This technique produces minimal hemodynamic perturbations, profound analgesia, dependable amnesia, and an uneventful convalescence. No comparison of this technique with a continuous benzodiazepine-opioid technique has been made.

In patients with known depression, small doses of ketamine administered on induction significantly improved the postoperative depressive state when compared with matched controls.^[551]

Low-dose ketamine has been used as an analgesic after thoracic surgery^[552] ; its lack of respiratory depressant properties and its pain relief equivalent to that of meperidine make it a third choice when one wishes to avoid narcotics because of their respiratory depressant effects and has reason to also avoid nonsteroidal agents such as ketorolac. Additional analgesic use can be considered in asthmatic patients.^[553] Ketamine administered in small doses of 10 to 20 mg preoperatively decreases postoperative analgesic consumption. Patients given up to three doses of 0.25 mg/kg ketamine combined with 0.15 mg/kg morphine, or only 0.3 mg/kg morphine, had better visual analog pain scores, were more alert, and experienced less nausea and vomiting.^[554] Ketamine administered in a one-to-one combination with morphine and the use of an 8-minute lockout interval provided optimal postoperative analgesia for this combination. ^[555] Epidural/caudal administration of ketamine (0.5 to 1 mg/kg) is increasingly being reported. Although the efficacy of these doses of ketamine seems to be established, the safety of this technique has not yet received regulatory approval. The preservative in the racemic mixture is potentially neurotoxic, but studies to date indicate that preservative-free S-(+)-ketamine may be safe.^[556]

Sedation

Ketamine is particularly suited for the sedation of pediatric patients undergoing procedures away from the operating room. Pediatric patients have fewer adverse emergence reactions^[490] than adults do, and this feature makes the use of ketamine in pediatrics more versatile. Ketamine is used for sedation or general anesthesia, or for both, for the following pediatric procedures: cardiac catheterization, radiation therapy, radiologic studies, dressing changes,^[557] and dental work.^[464] Caution is advised in the use of ketamine for cardiac catheterization in pediatric patients with elevated pulmonary vascular resistance because such resistance can be increased by ketamine.^[558]

Ketamine is often used repeatedly in the same patient. Unfortunately, the literature does not provide information on how many times ketamine anesthesia can safely be administered to one individual, whether the frequency of administration is related to tolerance after multiple administrations, and whether frequent/long-term use can produce detrimental effects.

Generally, a subanesthetic dose (≤1.0 mg/kg IV) is used for dressing changes; this dose gives adequate operating conditions but a rapid return to normal function, including the resumption of eating, which is important in maintaining proper nutrition in burn patients.^{[285] [444]}

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In adults and children, ketamine can be used as a supplement or an adjunct to regional anesthesia to extend the usefulness of the primary (local anesthetic) form of anesthesia. In this setting, ketamine can be used before the application of painful blocks,^[559] but more commonly it is used for sedation or supplemental anesthesia during long or uncomfortable procedures. When used for supplementation of regional anesthesia, ketamine (0.5 mg/kg IV) combined with diazepam (0.15 mg/kg IV) is better accepted by patients and is not associated with more side effects than in unsedated patients.^[560] Ketamine in small doses can also be combined with nitrous oxide and propofol for supplementation of conduction or local anesthesia. These techniques of ketamine administration are used in outpatient and inpatient settings, and although patients are comfortable and cooperative, dreams and other unpleasant emergence reactions can occur.^[444] In outpatients, premedication with midazolam, concurrent propofol infusion, and intermittent ketamine (for analgesia) in doses less than 3 mg/kg are recommended.^[561]