THERAPY – KETAMINE AND OTHER NMDA ANTAGONISTS
Ketamine is an anaesthetic drug with a few notable features which have resulted in its introduction to pain clinic practice. The rationale for its use is supported by animal studies which allow tentative conclusions to be drawn about a mechanism of action. However, the extrapolation of animal data, based on electrophysiological and behavioural studies, to the human experience of pain is always difficult. The evidence to support the use of ketamine in chronic pain is necessarily limited. Ketamine is valuable for its excellent analgesic properties at subanaesthetic doses, and at anaesthetic doses for its freedom from the effects of cardiovascular and pharyngeal reflex depression which characterize other anaesthetic agents. As such it has a unique place for providing analgesia and anaesthesia for environments in which other agents would be difficult to use: for example at the site of a major accident or on the battlefield. The drug is limited by major side effects, however: notably cardiovascular stimulation, increased cerebral blood flow, and psychological disturbance. The role of the NMDA system and ‘windup’ in the production of a chronic pain syndrome has not been clearly elucidated. It has been shown however that subanaesthetic doses of i.v. ketamine of 0.5 mg kg-1 bolus followed by 0.25 mg kg-1 h-1 reduce hyperalgesia and are an adjuvant in postoperative analgesia.