Anaesthetic depth at induction. An evaluation using clinical eye signs and EEG polysomnography

We describe the stages of non-REM sleep induced by anaesthesia with sevoflurane 8% in oxygen and relate these stages to clinical eye positions. We explored John Snow's observation (1847) that 'when voluntary movement ceases, with the eyes fixed in an upward gaze during the gas induction of anaesthesia, the patient is protected against the risk of mental suffering' (awareness). Unpremedicated ASA 1 patients undergoing elective tonsillectomy were studied using EEG polysomnographic principles and clinical eye movement tracking. The results expressed as median and range were: latency to stage 1 sleep 4.5 min [2.5-7.5], stage 2 sleep 5 min [3.5-8.5], stage 3 sleep 5.5 min [4-12] and stage 4 sleep 6 min [4.5-15.5]. Eye position 5, the point of no further eye movement, was reached after 9 min [5.5-18.5]. This was significantly longer than the time taken to reach the stage 4 sleep EEG, p < 0.01, supporting Snow's observation and encouraging investigation into eye movement tracking technology as a potential monitor of anaesthetic depth.