For years, anesthesiologists relied on a variety of methods to gauge how patients were reacting to anesthesia, such as measuring heart rate, pulse oximetry and end-tidal CO2. Then researchers discovered that anesthetized patients exhibited changes in the electrical signals emitted by the nerve cells in the brain. As patients lost consciousness and entered a hypnotic state, their EEG waves changed from low-amplitude, high-frequency signals to large-amplitude, low-frequency signals.

Aspect Medical Systems was the first company to capitalize on this finding by developing the Bispectral Index (BIS) monitor, which analyzed the complex EEG data and provided what the company claimed was the first true measure of the depth of anesthesia. Now, two other monitoring devices