Because of the risks of anaesthetic overdose and the variable anaesthetic requirements between individuals, anaesthetists have always looked for methods to judge anaesthetic depth.

In 1917, Arthur Guedel divided anaesthesia into four stages and better defined the important third, or surgical, stage of anaesthesia by dividing it into planes.

The stimulus of the pungent ether vapour was such that once a patient was breathing quietly (automatic breathing) on it, they were unlikely to react to the skin incision. The onset of Guedel’s stage three (surgical anaesthesia) had a ‘soft’ signpost but it was clinically indispensable as this was when surgery on the body surface could start.

Deepening the ether anaesthetic further increased muscle relaxation and allowed body cavity surgery. From this point on, further increase in ether dose would result in flaccid muscles and death.