Surgical anaesthesia is classically divided into four stages: analgesia, delirium, surgical anaesthesia (subdivided into four planes), and medullary paralysis (overdose). This gradual procession of stages was described when ether was given to un-premedicated patients, a slow unpleasant process. Ether is obsolete and the speed of induction with modern inhalational agents or intravenous anaesthesia drugs makes a detailed description of these separate stages superfluous.
Balanced surgical anaesthesia (hypnosis with analgesia and muscular relaxation) with a single drug requires high doses that will cause adverse effects such as slow and unpleasant recovery, and depression of cardiovascular and respiratory function. In modern practice, different drugs are used to attain each objective so that adverse effects are minimised.
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