Guedel’s original stages were a chart of clinical signs used to estimate depth of anaesthesia. The chart specifically described anaesthesia in un-premedicated patients using diethyl ether only. Thus, anaesthetic depth varied directly with the dose of inhaled agent.

For inexperienced anaesthetic assistants, Guedel’s wall chart displayed a series of signposts to describe the route in the continuum from the awake state through anaesthesia and finally respiratory paralysis, cardiovascular collapse and death.

These signposts used only basic clinical observation of the patient and therefore no special equipment was required.