To compare Bispectral index and entropy during maintenance of anesthesia

Several monitors are currently proposed to evaluate the depth of hypnosis. Among them, Bispectral Index (BIS) is a well known and widely used parameter derived from the patient's electroencephalograph (EEG). Spectral entropy relies on the extent of disorder in both EEG and electromyography (EMG) signals and returns 2 values: State Entropy (SE) and Response Entropy. SE is computed over the frequency range of 0.8-32 Hz and reflects the level of hypnosis while Response Entropy is computed over the frequency range of 0.8-47 Hz and reflects also EMG activity. BIS and SE are dimensionless numbers scaled from 100 to 0 for BIS and from 91 to 0 for SE.

The BIS and Entropy manufacturers claim that adequate level of anesthesia is defined by a BIS or SE value between 40 and 60. But some observations have shown that the monitors provided different information. These observations led us to carry out this prospective observational study designed to study the comparability between BIS and SE during the maintenance period of propofol-sufentanil anesthesia.

Inclusion Criteria:

• age > 18 years,
• American Society of Anesthesiologists physical status 1-3,
• surgery under general anesthesia using propofol, sufentanil, and a relaxant agent,
• surgery lasting more than one hour,
• tracheal extubation planned at end of surgery

Exclusion Criteria:

• pregnant women,
• allergy to propofol,
• neurological or muscular disorder,
• treatment with opioids or any psychoactive medication,
• cardiac surgery,
• thyroid disorder,
• emergency surgery,
• prone or lateral decubitus position (difficulty to have a correct probe position)