Utility of bispectral index (BIS) monitoring during general anesthesia

Abstract

The possibility to objectively measure effects from anesthetics on the level of consciousness has since long been desired. General anesthetics cause changes in brain electrical activity, seen as changes in the electroencephalogram (EEG). Devices utilizing processed EEG for pseudo-quantification of anesthetic depth have been available for more than ten years. These monitors display the anesthetic, or rather, hypnotic depth as an index number. In this thesis, some aspects of the use and utility of one such device, the Bispectral index scale (BIS) has been evaluated.

The effects from experience and education on the use and utility of BIS monitoring were analyzed within a group of nurse anesthetists. Available BIS monitoring was found to have minimal effect on anesthetic gas delivery and BIS levels, and this did not change with growing experience and repeated education. However, the user approval, expressed as subjectively rated utility of the monitoring, increased markedly with mounting experience.

The BIS has been thoroughly investigated concerning potential short term benefits, achieved by titrating of anesthesia by the use of the monitor. Marginal reduction in wake up time, shortening of time spent in the postoperative ward, and a reduced incidence of postoperative nausea and vomiting have been shown, but only when BIS values were close to the upper limit of the recommended range.

Regarding long term effects from deep anesthesia, time spent at deep hypnotic level as displayed by BIS < 45, has been associated with increased postoperative mortality. It has been speculated that deep anesthesia could cause or worsen malignant disease, possibly by negatively affecting the immune system. In this thesis, no such relation was found between postoperative mortality within two years after surgery and time spent with BIS< 45. Neither was any relation found between the development of new malignant disease within five years after surgery and time spent at deep hypnotic level.

Underdosing of anesthetics may result in unintended wakefulness, awareness, which besides being a very unpleasant experience, can have long term psychological consequences for the individual. We compared the incidence of awareness in a BIS monitored cohort of patients to that in a historical control group with no such monitoring. Compared to standard praxis, we found a reduction in the incidence of awareness by 77% when BIS monitoring was used.

In summary, the possibility to avoid unnecessarily deep anesthesia by the use of BIS was not adhered to despite mounting experience and educational efforts. Increasing experience was, however, associated with increased subjectively rated utility of the BIS technology. Since anesthesia exposure in terms of time with low BIS was not found to affect long term mortality or the occurrence of malignant disease, the main benefit of BIS monitoring thus far seems to be the possibility to reduce the risk of underdosing of anesthetics, that is, awareness.