Relationship Between Bispectral Index (BIS) and Spectral Entropy (SpEn) During Desflurane Anaesthesia

During the last decade, an increasing number of monitor systems were developed designed to assess the depth of anesthesia.

Awareness is a rare occurrence of explicit recalls during anaesthesia with an incidence of 0,18% and the use of cerebral monitoring prevents about 50% of these events.

Different stages of awareness should be defined and implicit memorization is subconscious awareness without explicit recall and possible implicit recall with an incidence greater than awareness.

To assess implicit recalls, a large number of test can be used, for example the "word stem completation test" and the "free recall association test". These result easy to use and reliable in detect implicit memory.

Bispectral Index (BIS) and Spectral Entropy (SpEn) has been proposed, in recent years, as electroencephalographic monitors of anaesthesia depth.

The bispectral index is a monitor of anaesthetic depth approved by the Food and Drug Administration in the USA. The BIS monitor reports a dimensionless number between 0 (complete EEG inactivity) and 100 (awake state); BIS values from 40 to 60 are thought to be suitable for general anaesthesia. Spectral Entropy acquires the electroencephalograph (EEG) and frontal electromyography (FEMG) signals which are processed into three parameters: State Entropy (SE), Response Entropy (RE) and Burst Suppression Ratio (BSR).

SE is computed over the EEG dominant part of the spectrum (0.8-32 Hz), and therefore primarily should reflect the cortical state of the patient and results useful to assess hypnotic effects of drugs .

RE is computed over a frequency range from 0.8 to47 Hz. It includes both the EEG-dominant and EMG-dominant part of the spectrum . The SE and RE values below 60 correlate with a low probability of consciousness. Also Bispectral Index provides BRS, an evaluation of burst suppression in which cerebral activity results suppressed.

BIS and Spectral Entropy are intended to decrease the risk of intraoperative awareness and to enable the anesthetist to reduce risk of the under- and overdosing of anesthetic agents and to ensure faster and more predictable wake-up and extubation.

Previous studies result contrasting in the correlation between these devices with lack in evaluation of implicit memory.

The aim of this study was to estimate the relationship between BIS and Spectral Entropy values and any explicit or implicit memory or dreams recall during desflurane anaesthesia.

Inclusion criteria

• American society of anesthesiologist physical status I-II
• Italian native speakers

Exclusion criteria

• Psychiatric or neurologic diseases
• Acoustic impairment
• Obesity (Body Mass Index > 30)
• Substance dependence.