86: Bispectral Index Monitoring

PROCEDURE 86


Bispectral Index Monitoring




PURPOSE: 


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.14,10,13,16 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.



• 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).



• EEG activity is then subjected to multiple processing steps.



image The EEG signal is filtered and digitized within the amplifier head box (digital signal converter [DSC])24 (Fig. 86-4, A) or BISx57 near the patient’s head (Fig. 86-2, A and 86-3, A).





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


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


image The level of EEG suppression and near suppression is determined.


image 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


image 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


image 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).36


• 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.



• Potential indications for BIS monitoring include:



• 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 State3,13,17,20































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.)


• See Fig. 86-1 for placement of the BIS sensor.


• Knowledge of factors that affect the BIS value is necessary.



image Sedation: Decrease in BIS value.11,14,15,21


image Analgesia: Decrease in BIS value from attenuation of cerebral arousal or sedation occurring as a side effect of high-dose opioid analgesia.1,2


image Neuromuscular blocking agents: Decrease in BIS value related to attenuation of high-frequency muscle activity across the patient’s forehead.8,17,23,40


image Painful (noxious) stimulation: If analgesia is inadequate, arousal response may be produced within the cerebral cortex.1,2,19


image Sleep: BIS range is lower (20 to 70) during deep sleep, and BIS range is higher (75 to 92) during rapid eye movement (REM) sleep.21


image Hypothermia: Decrease in BIS value.3,22,27


image Cerebral ischemia: Decrease in BIS value.21,26,28,38


image Neurologic injury: Decrease in BIS value3,8,16 depending on location of injury and degree to which overall cerebral metabolism is affected.1,3,18


image Encephalopathic states: Severe anoxic or ischemic encephalopathy (decrease in BIS value).1,3


image Electromyographic (EMG) activity (high-frequency activity from muscle activity across forehead)3,6 may cause increase in BIS value independent of hypnotic state.8,17,19,40


image High-frequency electrical artifact from patient care equipment, such as pacemaker, or muscle activity, such as rapid head or eye movement (increase in BIS value).


• Knowledge is needed of BIS display screen, monitor controls, and information array available on BIS monitor (Figs. 86-2, 86-3

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Jun 4, 2016 | Posted by in CRITICAL CARE | Comments Off on 86: Bispectral Index Monitoring

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