Guedel’s revolution was to include the pupillary changes that occurred with increasing doses of ether. In an age without monitors, pupil changes were easily observable physiological monitors of depth of anaesthesia. Guedel’s original eye signs included the activity of the motor muscles of the eye (deviant gaze) and pupillary dilation. The eyelid, corneal and eyelash reflexes were added later.
A 1972 study of Minimum Alveolar Concentration (MAC) confirmed that the pupillary changes of ether anaesthesia correlate with its alveolar concentration, and a 2004 paper showed good correlation between Guedel’s signs and depth of anaesthesia, as measured from the Bispectral Index (BIS).
For more information refer to:
Cullen, DJ, Eger EI, Stevens WC et al. Clinical signs of anesthesia. Anesthesiology. 1972;36:21-36.