Bispectral Index Monitoring

The bispectral index is a processed electroencephalogram-based parameter used in critically ill adults for assessment of level of consciousness and response to sedative, hypnotic, and anesthetic agents.1–4,10,13,16 The bispectral index also may indicate an arousal response to painful stimulation.² Information derived from bispectral index monitoring may be used to guide sedative, hypnotic, and analgesic therapy.^2,4,10,16

PREREQUISITE NURSING KNOWLEDGE

• Understanding of cerebral physiology is needed.

• Sedative, hypnotic, anesthetic, and analgesic agents produce clinical effects as a result of binding, in a dose-related manner, with specific receptors in the brain modulating cerebral physiology.24,34

• Understanding of the interrelationship between the electrical activity of the brain and cerebral metabolism is necessary.

• Electroencephalogram (EEG) tracings are obtained and recorded through the application of scalp electrodes and detect electrical activity in the brain.³³

• Examination of EEG waveforms provides a complement to central nervous system (CNS) evaluation obtained through clinical neurologic assessment.

• On its basic level, EEG activity requires multiple energy-using steps, which need to occur in succession. These steps include electrical impulse discharge at the thalamus and impulse conduction to the cerebral cortex with associated presynaptic release of neurotransmitters.

• Any clinical state or therapy that affects cerebral metabolism may also affect the EEG.12,18,22,30

• See Table 86-1 for terminology associated with bispectral index (BIS) technology.

Table 86-1

Bispectral Index Monitoring Terminology

Bispectral Index (BIS)	Processed EEG that assesses level of consciousness and response to sedative, hypnotic, and analgesic therapy.
Digital Signal Converter (DSC)	Amplifies, filters, and digitizes the patient’s EEG signals.
Electroencephalogram (EEG)	Measures electrical activity of the brain.
Electromyelograph (EMG)	Measures the presence of muscle activity or detects high frequency artifact from patient care devices.
Signal Quality Index (SQI)	A measure of the signal quality for the EEG channel source and is calculated based on impedance to electronic signal, electrode contact artifact and other variables.
Suppression Ratio (SR)	Percentage of EEG suppression (isoelectric EEG) over the past 63 seconds of collected data.

• The close relationship between BIS and EEG activity should be understood.25,33,35,36

• When BIS monitoring is initiated, a sensor is placed across the patient’s forehead per manufacturer recommendations to detect one channel of EEG activity (Fig. 86-1).

Figure 86-1 BIS sensor in place illustrating anatomic landmarks for optimal sensor placement. BIS Extend sensor in place. Sensor may be placed on right or left side.

• EEG activity is then subjected to multiple processing steps.

The EEG signal is filtered and digitized within the amplifier head box (digital signal converter [DSC])2–4 (Fig. 86-4, A) or BISx^5–7 near the patient’s head (Fig. 86-2, A and 86-3, A).

Figure 86-2 BIS VIEW monitoring system, including location of menu and power control keys, BISx, BIS index/trend, EMG, EEG, and sensor status displays. (Courtesy Aspect Medical Systems, Norwood, MA.)

Figure 86-3 BIS VISTA monitoring system illustrating display, BISx, and location of menu and power control keys. (Courtesy Aspect Medical Systems, Norwood, MA.)

Figure 86-4 Equipment and accessories for use of bispectral index monitoring system: BIS monitor, patient cable, patient interface cable, digital signal converter, and available sensors (Extend sensor, Quatro sensor, and pediatric sensors shown). BIS Extend sensor automatically defaults to a 30-second smoothing rate, which may be preferable when patients are lightly sedated and minimal changes in drug delivery or patient stimulation exist, conditions that may apply to an intensive care setting. The BIS Quatro sensor automatically defaults to a 15-second smoothing rate and may be preferable when rapid changes in hypnotic level are anticipated or when maximal sensitivity in assessment of arousal response is desired, which may be helpful in an operating room setting. (Courtesy Aspect Medical Systems, Norwood, MA.)

Artifacts (low-frequency and high-frequency) are eliminated.

Multiple processing steps are applied for calculation of a specific EEG state (frequency and amplitude) associated with the level of sedation, arousal, or anesthesia.

The level of EEG suppression and near suppression is determined.

The EEG features are combined to form the BIS, a single value that correlates with the level of consciousness and the specific EEG state.3,10,33,36

The BIS value is a single number based on the previous 10 to 30 seconds of EEG data (depending on the smoothing rate setting on the monitoring system) and is updated frequently; thus, changes in BIS value may lag behind clinical changes.⁵^,⁶

The BIS monitor provides a single channel of an EEG tracing from the right or left frontal-temporal montage electrode placement (BIS A2000™, BIS VISTA™, BIS VIEW™; Fig. 86-4B).^3–6

• The BIS monitor may also provide bilateral EEG data acquisition from right and left frontal-temporal electrode placement (BIS VISTA bilateral monitoring system).⁷

• Understanding of factors that affect cerebral metabolism and EEG activity is needed.

Sedation (dose-related): Related to the modulation of the EEG state and level of consciousness from medication administration.1–3,34

Analgesic agents (dose-related): Related to attenuation of the arousal response or sedation as a side effect of opioid analgesia in higher doses.1,2,16,34

Anesthetic agents (dose-related).25,36,37

Cerebral injury or hypoperfusion (hemodynamic stability, global neurologic injury, severe hypoxemia): Related to direct alterations in cerebral metabolic stability.9,18,26,29

• Potential indications for BIS monitoring include:

Use of neuromuscular blockade: BIS monitoring may help in identification of patients at risk for awareness, recall, and pain during paralysis.2,23,39,40

Use of BIS values to guide sedation and analgesia.²

Titration of sedation or analgesia in patients receiving controlled ventilation.5,7,15,23

Avoidance of extremes of undersedation and oversedation.1,37

Titration of medications for medication-induced coma.10,32,35

Procedural sedation.¹⁶

Determination of the dosage of sedation or analgesia during end-of-life care.12,20

• Table 86-2 provides BIS values and correlation with clinical endpoints and level of sedation.

Table 86-2

BIS Values, Corresponding Level of Sedation, and EEG State 3,13,17,20

BIS Value	Corresponding Level of Sedation	Descriptors
100	Awake state; patient able to respond appropriately to verbal stimulation	Baseline state before sedation Anxiolysis
80	Patient able to respond to loud verbal, limited tactile stimulation, such as mild prodding/shaking	High-frequency EEG activity (Beta augmentation) Moderate sedation
60	Low probability of explicit recall; patient unresponsive to verbal stimulation	Low-frequency EEG activity Deep sedation
40	Patient unresponsive to verbal stimulation, less responsive to physical stimulation	Deep hypnotic state Drug-induced coma; burst-suppression EEG pattern
20	Minimal responsiveness
0	No responsiveness mediated by brain function; spinal reflexes may be present	Isoelectric or completely suppressed EEG

Note: Levels of sedation and responsiveness, and corresponding BIS value and EEG state, occur on a continuum.

(Adapted from Arbour R: Continuous nervous system monitoring: EEG, the bispectral index and neuromuscular transmission, AACN Clin Iss 14(2):192, 2003.)