Dissociative anaesthesia is a state of profound analgesia and anterograde amnesia with minimal hypnosis during which the eyes may remain open (see ketamine, p. 353). It is particularly useful where modern equipment is lacking or where access to the patient is limited, e.g. at major accidents or on battlefields.
Sedation and amnesia without analgesia are provided by midazolam i.v. or, less commonly nowadays, diazepam. These drugs can be used alone for procedures causing mild discomfort, e.g. endoscopy, and with a local anaesthetic where more pain is expected, e.g., removal of impacted wisdom teeth. Benzodiazepines produce anterograde, but not retrograde, amnesia. By definition, the sedated patient remains responsive and cooperative. (For a general account of benzodiazepines and the competitive antagonist flumazenil, see Ch. 19.)
Benzodiazepines can cause respiratory depression and apnoea especially in the elderly and in patients with respiratory insufficiency. The combination of an opioid and a benzodiazepine is particularly dangerous. Benzodiazepines depress laryngeal reflexes and place the patient at risk of inhalation of oral secretions or dental debris.
Entonox, a 50:50 mixture of nitrous oxide and oxygen, is breathed by the patient using a demand valve. It is particularly useful in the prehospital environment and for brief procedures, such as splinting limbs.
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