Ketamine
Ketamine is an N-methyl-D-aspartate antagonist (NMDA) that induces dissociative anaesthesia due to dissociation between the cortical and limbic systems. In addition to other pharmacological effects, it interacts with opioid receptors. It produces good analgesic effects at subanaesthetic doses.
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Use for acute or chronic pain
When used in subanaesthetic doses, ketamine can assist in controlling acute or chronic pain, particularly severe pain that is not well controlled by other agents, and particularly if there is evidence of central sensitisation. Ketamine should only be used in consultation with a specialist in pain medicine, anaesthesia or palliative care. When used for pain, it can be given by the subcutaneous route, either as intermittent doses or as a continuous infusion, often combined with an opioid analgesic. In persistent pain, it can be effective when given in short bursts of 3 to 5 days, then ceased and recommenced if pain returns or increases. Ketamine has also been used as a nasal spray.
Use for procedural sedation
Ketamine may be administered intramuscularly, intravenously or by continuous intravenous infusion. When administered intramuscularly, its onset of action is almost as rapid as when administered intravenously. The duration of its effect is partly dependent on the total dose administered. Amnesia may persist for one to two hours.
Precautions
Ketamine should be avoided in the following situations:
conditions where elevations in blood pressure and heart rate would be deleterious, such as uncontrolled hypertension, acute stroke, acute coronary syndromes, intracranial haemorrhage, stenotic valve disease, tachyarrhythmias and hyperthyroidism
raised intraocular pressure, including penetrating eye injuries
history of hallucinations
procedures involving the posterior pharynx, due to the risk of laryngospasm.
Adverse effects
Common adverse effects include hypersalivation and increased muscle tone resulting in random or purposeful movements that may resemble seizures. Emergence reactions, involving vivid dreams, confusion, hallucinations and irrational behaviour may occur during recovery and for up to 24 hours. In rare cases, these effects have recurred days or weeks after ketamine administration. Emergence phenomena occur less frequently in children (aged 15 years or less), in the elderly and with intramuscular administration. They can be minimised by monitoring the patient in a quiet, low-stimulus environment during recovery. Benzodiazepines, such as midazolam, have been used to prevent and treat such reactions. Ketamine may be useful in asthmatic patients due to its direct relaxant effects on bronchial smooth muscle and absence of respiratory depression. It is the induction agent of choice in status asthmaticus.
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