The change in Bispectral Index with stimulation indicates depth of sedation in intensive care

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Introduction

Bispectral Index (BIS) is an EEG parameter which gives a continuous reading between 0-100 depending on depth of anaesthesia. It has been used in ICU as a monitor of sedation [1]. Clinical assessment of sedation requires the patient to be stimulated, which may cause elevation of the BIS. We proposed that this change in BIS in response to stimulation is important when assessing depth of sedation. The aim of this study was to assess the feasibility of BIS as a monitor of sedation in ICU and to quantify within different sedation groups the changes in BIS score before and after stimulation.

Method

ICU patients being ventilated and sedated were studied. BIS was recorded on an Aspect A2000 monitor. A nurse blinded to the BIS reading assessed sedation using the observer's assessment of alertness/sedation score (OAAS 1-5: 1=no response, 5=fully awake). Sedation was assessed hourly, average unstimulated BIS (BISu) measured before clinical assessment of sedation was compared to average stimulated BIS (BISs). Spearman's correlation coefficient was used to assess association between BIS and OAAS, Mann-Whitney U was used to compare differences between BIS in OAAS groups 4–5 (lightly sedated), OAAS groups 1-3 (deeply sedated) and in the change of BIS (ΔBIS) with stimulation in all OAAS groups.

Results

A total of 139 observations in 30 patients were recorded. There was a significant difference (P<0.05) between BIS score in the lightly sedated group and deeply sedated group. Correlation coefficient BISu/OASS r=0.53, BISs/OASS r=0.73. ΔBIS was larger in OAAS 3-4 than in OAAS 1,2 and 5.

Conclusion

BIS can distinguish between lightly and deeply sedated patients. BISs correlated more closely with OAAS than BISu. On stimulation ΔBIS was greatest in OAAS 3-4. BIS can be used in ICU patients but readings should be interpreted over time and in response to stimulation.

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