Current sedation assessment methods are subject to variability depending upon the observer (Habibi and Coursin 1996).  RSS is not sensitive or specific enough to determine agitation and over-sedation (Habibi and Coursin 1996).  Furthermore, evaluation using RSS is hard to duplicate in a busy ICU since it can be time-consuming and can agitate the patient (Habibi and Coursin 1996).  On the contrary, BIS is an objective, non-invasive method of continuously monitoring the patients sedation level.

Finally, RSS cannot be used to evaluate sedation levels in intubated patients (Polland 1993).  Many patients in the ICU require mechanical ventilation and/or muscle relaxants.  Patients who are on ventilators and who receive muscle relaxants cannot communicate effectively. RSS is an inadequate method of determining sedation level in such patients since the patients cannot respond (Polland 1993; Spencer et al. 1994).  BIS would be a good alternative for intubated patients and/or patients who are receiving muscle relaxants.  An EEG would provide non-invasive, continuous monitoring of sedation levels without requiring the patient to respond verbally.  Apparently, using BIS analysis could be the vehicle of providing objective sedation assessment in both intubated and non-intubated patients. Using BIS analysis, doctors and nurses may be able to achieve the ideal level of sedation that "provides a degree of sleepiness from which the patient is easily aroused and remains cooperative and responsive to commands" (Habibi and Coursin 1996).

The usefulness of other EEG parameters in determining sedation has also been investigated (Lui et al. 1996).  Our results also showed that SEF, MPF, Absolute alpha, Absolute beta, and EMG are statistically different between responders and non-responders.   However, these parameters are not as specific as BIS; similar to RSS, they may only be able to differentiate between responders and non-responders (Lui et al. 1996).   Also, the other power spectrum variables did not correlate with RSS as well as BIS did (Table 4).  According to a previous study, these simple EEG analyses are not effective; little correlation was found between MPF, SEF, and sedation score in critically ill patients (Spencer et al. 1994). Similarily, another study concluded that BIS provided the best correlation with the sedation scaling system used, and that only BIS consistently increased as anesthetic wore off (Spencer et al. 1994). Other EEG parameters do not correlate with RSS as well as BIS and hence, are not as effective in measuring sedation (Polland 1993; Spencer et al. 1994).

Other objective measures of sedation are also being investigated.  One study found that cardiac beat-to-beat variability (RR variability) correlates well with RSS in ICU patients (Haberthur et al. 1996).  It is an objective, continuous method that can be used to monitor intu
bated patients (Haberthur et al. 1996). More research needs to be done on alternative methods of assessing sedation in an objective manner. Presently, BIS can be used effectively to determine sedation in ICU patients. Other clinical uses of BIS are also being studied.  For example, one study concluded that BIS can be used to measure effects anesthetics have on EEG wave patterns (Billard et al., 1997). 

 

BIS is definitely useful in a clinical setting and may become the new method of monitoring depth of anesthesia in the ICU and possibly even in the operating room.

Acknowledgments

The author would like to thank Patricia Embree and all other staff at ASPECT Medical Systems for the support and for providing all the mechanical equipment necessary to complete this study.

Works Cited

Aspect Medical Systems. Technology Overview: Bispectral Analysis. Massachusetts, 1992.

Billard V., P. L. Gambus, N. Chamoun, D. Stansky, and S. L. Shafer. "A comparison of spectral edge, delta power and bispectral index as EEG measures of alfentanil, propofol, and midazolam drug effect." Clinical Pharmacology & Therapeitics 61 (1997): 45-58.

Glass P. S., M. Bloom, L. Kearse, C. Rosow, P. Sebel, and P. Manberg. "Bispectral analysis measures sedation effects of propofol, midazolam, isoflurane, and alfentanil in healthy volunteers." Anesth 86 (1997): 836-47.

Haberthur C., F. Lehmann, and R. Ritz. "Assessment of depth of midazolam sedation using objective parameters." Intensive Care Medicine 22 (1996): 1385-90.

Habibi S., and D. B. Coursin. "Assessment of sedation, analgesia, and neuromuscular blockade in the perioperative period." InH Anesthesiology Clinics 34 (1996): 215-41.

Lui J., H. Singh, and P. White. "Electroencephalogram Bispectral analysis predicts the depth of Midazolam-induced sedation." Anesth 84 (1996): 64-9.

Lui J., H. Singh, G. Wu, and P. F. White. "Electroencephalographic Bispectral Index correlates with intraoperative recall and depth of propofol induced sedation." Anesth Analg 84 (1997): 185-9.

Polland B. J. "Neuromuscular blocking agents in intensive care." Intensive Care Med 19 (1993): S36-S39.

Spencer E. M., J. L. Green, and S. M. Willatts. "Continuous monitoring of depth of sedation by EEG spectral analysis in patients requiring mechanical ventilation." British Journal of Anesthesia 73 (1994): 649-54.

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