Depth of anaesthesia

SUMMARY One hundred and fifty years after the first general anaesthetic in 1846 our knowledge about the mechanisms of general anaesthetics is still very sparse. The concept ‘depth of anaesthesia’ was introduced by John Snow (1847). He described ‘5° of narcotism’. Because one single agent had to provide all the components of general anaesthesia, the main problem for the anaesthetist was to avoid morbidity and mortality associated with excessively deep anaesthesia. The introduction of curare in 1942 allowed muscle relaxation required for surgery during a lighter level of anaesthesia, but also changed the emphasis from the problem of too deep anaesthesia and death, to too light anaesthesia and litigation. The problem of awareness during general anaesthesia with muscle relaxants provided the main impetus for monitoring depth of anaesthesia. During daily clinical practice the anaesthesiologist relies on clinical signs to evaluate anaesthetic depth, although several studies have shown a poor correlation between the 2 (Cullen et al . 1972; Evans and Davies 1984; Russell 1993). Different methods have been used in attempts to measure anaesthetic depth (Evans and Davies 1984; Stanski 1994), but none have been developed to a state where they can be used routinely in the operating theatre. This review will cover some of the parameters used to evaluate anaesthetic depth.