A New Monitor of Patient Sedation in the ICU?
November 1, 1999 4 minutes read
A New Monitor of Patient Sedation in the ICU?
Abstract & Commentary
This paper reports a clinical evaluation of the bispectral index (BIS), in comparison with the revised sedation-agitation scale (SAS), to assess the level of sedation in a series of mechanically ventilated adult patients in a multidisciplinary ICU. The BIS is an electronically generated value based on the electroencephalogram (EEG) that attempts to quantitate a patient’s level of sedation. Processing the EEG signal with Fourier transformation produces a power-frequency spectrum from which a number of derived indices can be obtained and that has been used in the operating room to assess sedation level during anesthesia. The BIS is an attempt to generate a single value indicating sedation depth despite the biphasic effects many medications used for sedation have on the EEG signal. It consists of a whole number between 0 and 100, with awake patients scoring in the 90s, conscious sedation dropping the value into the 70s and 80s, and general anesthesia further depressing the BIS into the 40-60 range.
Simmons and associates had previously devised the SAS, a seven-point subjective but standardized scale for assessing the level of sedation in an ICU patient. In the present study they simultaneously determined SAS and BIS data on 63 patients during mechanical ventilation. BIS values were assigned to baseline, stimulated, and average conditions for each patient by an investigator unaware of the SAS scores.
In the patients evaluated, sedation varied from very deep (SAS score 1, BIS score 43) to mild agitation (SAS score 5, BIS score 100). More heavily sedated patients by clinical criteria also had lower BIS scores, whereas the opposite trend was observed in more lightly sedated patients. Average BIS score correlated statistically with average SAS score (r2 = 0.21; P < 0.001). Simmons et al conclude that both the SAS and the BIS work well to describe the depth of sedation for ventilated ICU patients. (Simmons LE, et al. Crit Care Med 1999;27:1499-1504.)
COMMENT BY DAVID J. PIERSON, MD, FACP, FCCP
Although the degrees of analgesia, amnesia, and immobilization have been standardized in the operating room, such is not the case in the ICU, despite the need for prolonged sedation and sometimes also muscle relaxation in critically ill, ventilated patients. The BIS is an attempt to bring a greater degree of objectivity and standardization in patient sedation to the bedside in the ICU.
In an editorial accompanying this article, Shapiro (Shapiro BA. Crit Care Med 1999;27:1663-1664) lists five features of the BIS that clinicians should understand:
1) Bispectral analysis of the EEG provides information about interactions between cortical and subcortical areas that change with increasing amounts of hypnotic drugs.
2) The BIS is an empirical, statistically derived measurement that was generated by analysis of a large database of EEGs from subjects receiving hypnotic agents and applying multivariate statistical methods to derive apparently optimum combinations of these features.
3) The BIS measures the state of the brain, not the concentration of any drug.
4) Interpretation of the BIS is predicated on the assumption that sedation is intended to produce a state of sleep that includes a lack of awareness and a lack of recall (amnesia), in contrast to analgesia, which is intended to produce a state of reduced pain perception.
5) In general, a BIS score of 100 reflects the awake state, 80 reflects some sedation, 60 reflects a moderate hypnotic level, and 40 reflects a deep hypnotic level.
I first encountered the BIS monitor when I came back on service in the medical ICU a couple of months ago, after being away from the unit for only a few weeks. During the presentation of the first patient on rounds, the managing intern included a BIS score of 70 as part of the morning’s neurological findings. Although the intern hesitated when I interrupted to ask what that was, the patient’s nurse quickly interjected that it was a new index of how sedated the patient was, which we were now monitoring on all ventilated patients in the unit. That last part turned out to be an exaggeration, but this vignette illustrates how quickly a previously unknown new technology can be put into routine clinical use in the ICU.
It is easy to see the potential advantages of something like the BIS in managing critically ill patients during mechanical ventilation. However, if the BIS monitor follows its predecessors, the pulse oximeter and the in-line capnometer, from the operating room to the ICU and becomes part of the "standard of care," will it produce clinically helpful, reliable data in this setting, under what are likely to be quite different conditions of use? Will the numbers have the same meaning as they do in the operating room? Will the use of this device permit less use of some other expensive monitor or therapy, thus justifying the added cost? Will it make patients more comfortable and easier to manage, decrease complications, reduce the duration of mechanical ventilation, or shorten average length of stay? Chances are, this latest in a long line of rationally intended, vigorously promoted electronic devices will see a lot of clinical use on ICU patients before data from properly designed clinical trials begin to answer these questions.
Monitoring the level of sedation of mechanically ventilated ICU patients using the bispectral index (BIS) has been shown to:
a. reduce ICU length of stay.
b. reduce the utilization of neuromuscular blocking agents.
c. decrease the incidence of ventilator-associated pneumonia.
d. All of the above
e. None of the above
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