Ketamine
Ketamine is a phencyclidine derivative chemically related to
cyclohexamine.
· Ketamine is a general anaesthetic
that is rapid acting, producing profound analgesia with normal
laryngeal-phar-yngeal reflexes, skeletal muscle tone, and cardiovascular and
respiratory stimulation. When given intravenously, it induces sedation
associated with immobility, amnesia, and analgesia (dissociative anaesthesia).
· Ketamine is popular as an
anaesthetic in cases of trauma and emergency surgical procedures.
·
It is also frequently used for short-term sedation during
clinical procedures; most frequently in the paediatric
population.
·
Onset of effects after IV
administration is 30 to 40 seconds, and after IM administration, 3 to 8
minutes. The duration of unconsciousness following the usual anaesthetic dose (vide infra) is about 40 to 60 minutes.
·
Ketamine undergoes first-pass
metabolism by the liver necessitating higher doses when taken by the oral or
rectal route. It undergoes N-demethylation by the cytochrome P450 system to
form norketamine. Norketamine is an active metabolite with an anaesthetic
potency one-third that of ketamine.
·
During the period of
unconsciousness, patients are usually noncommunicative though the eyes may be
open and they appear to be awake. Skeletal muscle spasm is often present.
·
Following usual anaesthetic doses,
side effects include significant transient increases in blood pressure and
heart rate, respiratory depression, airway obstruction, apnoea, muscular
hypertonus, psychomotor, psychomimetic, and acute dystonic reactions.
·
Ketamine elevates intracranial and
intraocular pressure.
·
Mydriasis and nystagmus may occur.
· Emergence phenomena are also
commonly reported char- acterised by vivid dreams, nightmares, hallucinations,
screaming, crying, disorientation, and delirium. They can be minimised by
concomitant administration of a benzodi-azepine. Nightmares following ketamine use sometimes
lastfor several days to weeks. Children and the elderly seem
less sensitive to these effects.
·
Ketamine must not be used for
anaesthetic purposes in alcoholics since exaggerated psychotomimetic effects
are commonly precipitated in such individuals during the recovery phase.
Overdose is associated with convulsions, hypertension,
tachy-cardia, and respiratory depression. The usual anaesthetic dose of
ketamine is 2 mg/kg IV or 5 to 10 mg/kg IM.
·
The minimal toxic or lethal dose is
not well established in the literature. Deaths have been reported in adults
following doses of 900 to 1000 mg.
· Treatment
of the adverse and toxic effects of ketamine involves mainly symptomatic and
supportive measures. Plasma ketamine levels are not clinically useful. Airway
control and ventilatory support are essential.
· Seizures and panic attacks must be
treated with benzodi-azepines; dystonic reactions respond to diphenhydramine (5
to 50 mg IV over 2 minutes) or benztropine.
· Droperidol intravenous
administration as a pre-anaesthetic agent in doses of 75 mcg/kg has been
reported to be effec-tive in reducing the incidence of psychotomimetic and
circulatory side effects of ketamine. However, based on cases of QT
prolongation and/or torsades de pointes in patients receiving droperidol at
doses at or below recom-mended dosing, it should be reserved for use in
patients who fail to show an acceptable response to other agents.
· Alpha and beta blocking agents,
benzodiazepines, and vera-pamil have been shown to block cardiovascular
stimulation. Pulmonary hypertension and pulmonary oedema may be reversed by
fentanyl.
· There is evidence to suggest that
4-aminopyridine reverses most of the effects of ketamine toxicity.
· Atropine or glycopyrrolate can help
in reducing tracheo-bronchial secretions.
·
Physostigmine has been suggested in the management of
phencyclidine overdose, but there is little evidence that indicates
physostigmine is of value in the treatment of ketamine overdose and it is NOT
recommended.
·
Ketamine is increasingly gaining popularity as a recre-
ational drug and is employed in “rave” parties for its disso- ciative
hallucinatory effects. It is usually taken intranasally or by inhalation, but
may also be injected (IM or IV). It is |most commonly taken in powdered form
and either mixed in a drink or snorted.
·
Slang names: K, Kay,
K-amine, Special K, Vitamin K, Ketaset, Green, Jet, Mauve, Purple, Special LA
coke, Super acid, Super C.
·
Ketamine has become popular in the teen and young adult
culture because of its unique combination of hypnotic, analgesic and amnesic
effects with limited respiratory depression. It can produce a
"dissociative" effect which is characterised by analgesia and amnesia
without causing a loss of consciousness. Users describe the effects of ketamine
as an “out of body” experience or “near death”, characterised by exhilarating
sensations of immobility and a disregard for death.
·
Other common effects include tachycardia, altered mental
status, anxiety, palpitations, slurred speech, hallucinations, nystagmus,
mydriasis, mild hypertension, and chest pain. Ketamine may produce
bronchodilation and increased salivary and tracheobronchial secretions. Confusion,
vomiting and memory loss are less common. Rarely seizures, polyneuropathy, or
respiratory arrest may occur. Epistaxis and anosmia (loss of sense of smell)
have been reported following chronic snorting of ketamine. Death is rare
following ketamine abuse or overdose.
· Ketamine crosses the placenta.
Severe adverse effects on the infant, including respiratory depression, have
been reported following ketamine use for obstetric analgesia.
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