Bispectral index monitoring in the neurointensive care unit.
The Monitoring System
The BIS (Fig 1) is a noninvasive monitoring system that consists of a sensor and a monitor. The sensor adheres to the patient's forehead, and the monitor provides continuous electroencephalogram (EEG) readings. The calculations obtained from the EEG indicate the sedation level in the patient. The higher the amount of burst suppression (i.e., decreases of brain activity), the greater the degree of sedation. Originally, file BIS monitor was designed for use during general anesthesia to measure sedation but has since crossed over to the ICU.
[FIGURE 1 OMITTED]
The BIS monitor was created to numerically display two factors: the suppression ratio (SR) and BIS, which indicate the sedation state. The first number, SR, is a percentage of the last 63 seconds of EEG suppression and has a 15- to 30-second lag time. The SR ranges from 0 to 100 with the highest number corresponding to the complete suppression of EEG activity. The other number, BIS, is a biphasic analysis of EEG waveforms that relate to levels of sedation, anesthesia, loss of consciousness, and recall. The calculated BIS number ranges from 0 to 100 and is inversely proportional to the SR, with 0 indicating no EEG activity and 100 indicating an EEG level of a fully awake adult.
More than 1,000 research studies involving the B|S monitor have been done in the operative and ICU setting (Aspect Medical Systems, Inc., personal communication, 2003). Before the development of the BIS monitor, verifying that the anesthetized patient was not only paralyzed but also sedated was sometimes difficult. The BIS monitor provided a visible way to verify sedation through interpretation of the EEG waves.
The need to monitor sedation in the ICU lends itself to similar usefulness of the BIS monitor in continually sedated intensive care patients. Forty-seven studies have included patients in the ICU. Many of the studies have compared the BIS with other measures of sedation, including the Sedation Agitation Scale, Spectral Edge Frequency, Auditory Evoked Potential, Median Frequency, Ramsey Sedation Scale, and the Fast Fourier Transform Scale (Fletcher, Aneja, Heard, Kielma, & Heard, 2001; Riker, Fraser, Simmons, & Wilkins, 2001; Shah, Clack, & Chea, 1996; Venn, Cusack, Rhodes, Newman, & Grounds, 2000). All studies found the BIS system to be an effective method for measuring sedation. Riker et al. (2001) found the BIS monitor to be more effective in the trau ma population than in the cardiac or general population.
Neurological, neurosurgical, and trauma patients have been included in the populations studied in the ICU setting. Strong correlations have been shown between the level of sedation and the BIS display in the observation of neurological patients. Hana, Inchoisa, and Frost (1999) indicated a strong correlation between the Glasgow Outcome Scores (used to predict outcome after head injuries) and the average BIS value during the first week after injury. An average BIS value of less than 40 was associated with poor outcome.
Gilbert, Wagner, and Halukurike (2001) plotted multiple neurological measurements (i.e., APACHE III Neurologic Score, Glasgow Coma Score, Reaction Level Scale, and Modified Ramsey Sedation Scale) against various spectral parameters of the EEG, including BIS. The BIS correlated significantly with each of the neurologic scores and was more strongly correlated with neurologic status than any other EEG spectral parameter. Neurologic function was better associated with higher values of BIS. Riker, Wilkins, and Fraser (1999) documented that the BIS score correlated well with the SR in patients on a mean pentobarbital infusion of 3 mg/kg/hr and who attained a mean pentobarbital level of 31.7 mcg/ml. A BIS of 15 and an SR of 66 strongly correlated with an EEG burst suppression of 3-5/min.
Clarian Health Partners, Methodist Hospital (CHPMH) began using the BIS monitor in its ICUs several years ago. In the neurological population it is most often used for patients placed on a neuromuscular blockade drip such as cisatracurium besylate (Nimbex), which is accompanied by sedation such as propofol (Diprivan), and for patients placed in a pentobarbital-induced coma. In neurological patients, the Nimbex drip is most often used in the patient with uncontrolled intracranial pressure (ICP) or for ventilation management. Verification that the patient is adequately sedated while paralyzed is vitally important.
In the patient receiving a neuromuscular blockade and sedation, the optimal BIS level for sedation appears to be between 40 and 60. Because benzodiazepines are used at levels to sedate and do not depress EEG activity and slow brain metabolism as much as barbiturates used to induce coma, the EEG bursts are not as significantly depressed. As a result, the SR reading often is not affected and is not as significant in the sedated patient as it is in the pentobarbital patient.
Pentobarbital coma is used as the last line of treatment for uncontrolled ICP after all other measures have failed. The use of the pentobarbital does not come without risks, such as ileus, cardiac suppression, and hypotension. Because of the side effect of hypotension, vasopressors such as norepinephrine (Levophed) are often required, leading to other risks such as peripheral vascular constriction and organ failure.
Maintaining a therapeutic pentobarbital level is important for optimizing the decrease in brain metabolism and avoiding toxic levels that increase the chance of lifethreatening side effects. There is a poor correlation among pentobarbital serum concentration, therapeutic benefit, and systemic complications; thus, the EEG pattern of burst suppression is a more reliable tool of monitoring coma in a patient treated with pentobarbital for elevated ICP (Shah et al., 1996). In addition, the BIS monitor also may be utilized on any patient who is continually sedated and in which the nurse believes there is a potential benefit.
At CHPMH, checking pentobarbital levels requires sending samples that are only processed once a day to a laboratory at another location. The levels often are not returned for 8 hours or more and are only indicative of the pentobarbital level at the time it was obtained. Because of the boluses and changes in the concentration administered, the level may have changed greatly in that time. The BIS gives minute-to-minute read-outs indicating only a 63-second delay.
Maintaining the BIS level at 20 and the SR at 70-80 for these pentobarbital patients appears to indicate a therapeutic level of pentobarbital. A drop in BIS to 10 or below and a rise in SR to 90-100 is usually indicative of toxic levels. A BIS level of greater than 40 and an SR below 70 is evidence of subtherapeutic levels. If ICP is controlled at a subtherapeutic level, the level may not be adjusted in order to attempt to avoid untoward effects.
When the patient is prepared for the BIS monitor, the skin should be clean and dry before the probes are placed. The first electrode is placed in line with the nose, the second in line with the inner aspect of the eyebrow, the fourth in line with the outer aspect of the eyebrow, and the third electrode on the temple area over bone but close to the eye. While the probe is being placed, pressure must be placed over all four electrodes to ensure adequate adherence. For this reason, the probe should not be placed on a side where a bone flap has been removed or on areas with significant facial fractures.
The probes should be changed once a day, at which time the skin should be monitored for any breakdown. As of this writing, the ICU staff at CHPMH has not observed any skin breakdown caused by the BIS system.
There are important things to remember when using the BIS monitor. First, the probe must be checked for initial adherence and monitored until removal. The BIS monitor gives a continual measurement of the Signal Quality Index (SQI), which indicates adherence. For the BIS readings to be accurate, there should be at least 5 bars out of 10 filled on the SQI. Another measurement is the electromyography reading (EMG), which measures muscle movement. The EMG can help determine an accurate reading. If the patient has a BIS of 10 and an SR of 90, the patient should be at a drug-induced coma level and there should not be any muscle movement on EMG. Similarly, this would be true if a neuromuscular blockade is in use.
If the drip is at an adequate level, no muscle movement should be noted. If the patient is sedated with a BIS of 40-60 without also receiving a neuromuscular blockade, some muscle movement or EMG may be expected.
Also, it is important to remember that the BIS is not an accurate measurement of toxic levels of a neuromuscular blockade and train of four should still be used to monitor levels. The BIS does not monitor adequate analgesia and should not be used in that way.
Although the BIS monitor is relatively new to the intensive care setting, it can be a very beneficial tool in monitoring neurological patients. Having minute-to-minute indicators and reassurance of adequate sedation in the neuromuscular-blockaded patient enables neuroscience nurses to provide better care. More timely and current measurements of pentobarbital serum levels can help reduce and prevent life-threatening complications in this patient population. Much research has been done on its use, and further research is needed into the monitor's many uses.
Questions or comments about this article may be directed to: Chris Hilbish, BSN RN CNRN CCRN, by phone at 317/962-3826 or by e-mail at email@example.com. She is an outcome specialist neuroscience at Clarian Health Partners, Methodist Hospital, Indianapolis, IN.
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|Title Annotation:||Clinical Corner|
|Publication:||Journal of Neuroscience Nursing|
|Date:||Dec 1, 2003|
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