Dissociative anaesthetic

Dissociative anaesthetic

·         Dissociative anaesthetics are drugs that produce a state of CNS depression that is characterized by a feeling of dissociation from surroundings, unconsciousness and analgesia but with little or no muscle relaxation.

·         Skeletal muscle tone is usually maintained and may be increased during anaesthesia.

·         Oral, ocular and swallowing reflex remain intact.

·         Eyelids remain open, nystagmus present and mydriasis occurs during dissociative anaesthesia.

·         Analgesia is selective, visceral pain is not abolished.

·         Eg:  ketamine, phencyclidine and tiletamine.

Ketamine

·         Ketamine is a prototype drug and is widely used.

·         It is least potent and short acting.

·         Versatile agent because it can be administered by intravenous or intramuscular route without tissue irritation.

·         They act by antagonizing NMDA receptors i.e., glutamate gated ion channels.

·         Increases heart rate and cardiac output.

·         Does not cause respiratory depression at normal dose but higher dose cause respiratory depression.

·         Does not have significant effect on the liver and kidney at normal dose.

·         Ketamine stimulates salivation and lacrimation which may have become copious. Hence an anticholinergic agent is usually recommended to minimize salivation.

·         Produces selective analgesia and poor muscle relaxation.

·         Important adverse effects associated with ketamine include increased salivation, muscle twitching, mild tonic convulsions, emesis, vocalization and unpredictable and prolonged recovery.

·         It is contraindicated in patients with prone to seizure activity.

·         It should not be recommended in head injuries as it increases the CSF pressure.

·         Its use in patients with hypertension, heart failure could be harmful.

·         Ketamine is associated with open eyelids, hence artificial tears or eye ointment should be used to prevent corneal drying and injury.

·         In veterinary medicine it is widely used in cats for restraining or as sole anaesthetic agent for diagnosis and minor surgical procedures of brief duration that do not require smooth muscle relaxation.

·         For general anaesthesia it is often used in combination with sedative Preanaesthetic agent (Xylazine) and anticholinergic agent (Atropine).

Phencyclidine

Because of high adverse effects and drug abuse in humans, no longer freely available and used in veterinary medicine.

Tiletamine

Pharmacological effects are similar to ketamine.

It is used in veterinary anaesthesia mostly in combination with Zolazepam, a benzodiazepam.