Ketamine is a phencyclidine derivative that produces dissociation of the cortex from the limbic system, producing reliable sedation and analgesia while preserving upper airway muscle tone and respiratory drive.
Ketamine is a non-competitive antagonist at the phencyclidine site of the N-methyl-d-aspartate (NMDA) receptor for glutamate. However, its effects are also mediated by interactions with many other receptors. It is a short-acting anesthetic that has been widely used by emergency physicians and can be given intravenously, intramuscularly, orally, and even nasally. Multiple ketamine anesthetics may be safe.
Ketamine was introduced as early as the 1960s and is not generally used today as a general anesthetic, because of adverse psychological reactions, including delirium, disturbed dreaming, motor adverse reactions, and emergence reactions in about 12% of patients. However, subanesthetic low-dose ketamine has been used for acute pain therapy, day-case surgery, and chronic pain management. Ketamine for sale is available in chiral (S + and R −) forms as well as the standard racemic form. S-ketamine has twice the analgesic potency of racemic ketamine and four times that of R-ketamine. Thus, low dose S-ketamine may avoid adverse reactions while providing high-quality analgesia.
Ketamine relaxes smooth muscles in the airways and may therefore be a useful induction agent in children with asthma. If endotracheal intubation is required, lidocaine 1–2 mg/kg intravenously before intubation has been recommended, although the use of a laryngeal mask airway may be more appropriate. When used in combination with midazolam by infusion, ketamine provides analgesia and prevents and relieves bronchospasm.
The pharmacology of ketamine, including adverse reactions, has been reviewed, as has the use of S-ketamine in the intensive care unit.
The addition of ketamine to bupivacaine for spinal anesthesia has been studied in 60 patients undergoing spinal anesthesia for the insertion of intracavitary brachytherapy implants
for cervical carcinoma. They were randomly assigned to
receive either bupivacaine 10 mg or bupivacaine 7.5 mg plus
ketamine 25 mg. Motor recovery was significantly quicker
in the ketamine group. Blood pressure was significantly
lower in the bupivacaine group 5 minutes after administration, and perioperative intravenous fluid requirements were
significantly higher. Patients given ketamine reported more
sedation and dizziness, both intraoperatively and postoperatively. There were no nightmares or dissociative features.
Overall satisfaction was better with bupivacaine. The study
was abandoned after 30 patients, because of the high rate of
adverse with ketamine. Although ketamine had local
anesthetic-sparing properties, adverse reactions made it
unsuitable for intrathecal administration.
When added to standard doses of morphine and a nonsteroidal analgesic, S-ketamine 0.5 mg/kg had no additional benefit in a randomized, double-blind study in 30
patients undergoing anterior cruciate ligament repair.
The effects of midazolam on ketamine-induced emergence reactions have been investigated in 100 children
undergoing adenotonsillectomy. All were given intramuscular ketamine 7 mg/kg and atropine 0.015 mg/kg and
half were given midazolam 0.1 mg/kg. There were no
severe adverse events in either group. In the immediate
postoperative period, mild reactions were similar, but
moderate reactions were less common in those who
received midazolam (3.6% versus 25%). On day 1, mild
reactions were frequent in both groups. On day 2, 71% of
the children who received ketamine without midazolam
had mild reactions, whereas 75% of those who received
the midazolam group had no emergence reactions. The incidence of immediate postoperative nausea and vomiting
(at 0–4 hours) was significantly reduced by midazolam, but
this was reversed in the period up to 24 hours, when those
who received midazolam had a higher incidence of vomiting. This may have been attributable to the relative half-lives of the drugs.