BISPECTRAL =20 INDEX (BIS) MONITORING

Dr.=20 Aravind Narayanan

Consultant=20 Anaesthesiologist,

HOSMAT ( Hospital for Orthopaedics, = Sports=20 Medicine, Arthritis, Accident-Trauma & Neuro Center),=20 Bangalore.

1. = Principle of BIS Monitoring = :

a. =20 What does it = Analyze=20 ?

A BIS Monitor analyzes the EEG of = the patient=20 to arrive at a Numerical Value (BIS Index), the interpretation of which=20 indicates the level of Consciousness and thereby the depth or plane of=20 Anaesthesia / hypnosis.

The EEG signal = originates from the post-synaptic excitatory and inhibitory potentials = produced=20 by the pyramidal cells located in the outer cerebral cortex. Small = voltages thus=20 produced (10 =96 200 mV) are picked up by scalp/skin electrodes and = amplified to=20 give a waveform of variable frequency (0 =96 50 Hz). Normal EEG has Four = frequency=20 bands:

Beta > 12 Hz ; Alpha 8-12 Hz; Theta 4-8 Hz; Delta < 4 = Hz

Alpha & = Beta waves=20 are present in awake persons, and with gradual deepening of Anaesthetic = plane=20 slower frequencies Theta & Delta predominate. Sleep or Surgical = Anaesthesia=20 increases amplitude or Power of the waves (Synchronisation) while = arousal=20 to awake state decreases amplitude (Desynchronisation). Higher = concentrations of=20 anaesthetics cause periods of electrical silence interspersed with brief = episodes of activity =96 this pattern is called =91Burst Suppression=92. At = very deep=20 planes an Isoelectric EEG (flat line) is seen =96 mimicking Hypothermia = or Brain=20 Hypoxia.

The Complex = EEG waveform=20 is digitized and processed using Fourier Transformation algorithms to = segregate=20 amplitudes and frequencies of interest into Epochs or periods ; the = voltage-time=20 data of which is transformed into Power Spectral information. This Power = vs.=20 Frequency information is then graphically displayed over epochs to give = the=20 Compressed Spectral Array or =91Power Spectrum=92. Power Spectral = Analysis=20 gives numeric descriptors known as Processed EEG parameters like=20 :

=B7 =20 Median = Frequency &=20 95% Spectral Edge Frequency: Frequency below which 50 =96 95 % of the = Power /=20 amplitudes are contained

=B7 =20 Relative Delta = Power:=20 percentage of waves with delta (0.5 =96 3.5 Hz) relative to power over = entire=20 spectrum

The BIS Index = is a=20 numerically processed, clinically validated EEG parameter that is = arrived at by=20 utilizing a composite of advanced EEG signal processing techniques like=20 Bispectral Analysis, Power Spectral Analysis and Time Domain=20 Analysis.

b. = How does the = BIS analyze=20 the EEG and how do we get a numerical Value ?

Bispectral = analysis is a=20 sophisticated EEG signal processing methodology that assesses = relationships=20 among signal components and captures Synchronisations within the signal. = Bicoherence Bispectrum and Real Triple product are some statistical = variables=20 generated during this analysis which are integrated with Power Spectral = and=20 Cortical EEG analysis to generate a Numerical value =96 an additional = EEG=20 descriptor of brain activity which was then clinically assessed in more = than=20 5000 volunteers and matched to different levels of anaesthetic depths = &=20 consciousness.

Four=20 Processed EEG descriptors are utilized in computing the BIS Index. Each=20 parameter being accurate in identifying a given stage of=20 anaesthesia:

a. =20 Beta ratio=20 Parameter (Degree of = beta or high=20 frequency (14-30 Hz) activation) reflects Light Sedation

b. = Synch Fast=20 Slow (Amount of=20 low frequency synchronization) reflects Surgical=20 Anaesthesia

c. =20 Burst=20 Suppression Ratio = ( BSR )=20 (Presence of nearly suppressed periods within the EEG) &

d. = QUAZI = (Presence=20 of fully suppressed (i.e. isoelectric, =93flat line=94) periods within = the EEG)=20 reflect Deep=20 Anaesthesia.

The=20 parameters are ranked and statistically combined to yield a weighted sum = =96 a=20 single numerical value which is called the =93 BIS INDEX=94. This = Value was=20 found to measure the Hypnotic component of anaesthetics and was less = sensitive=20 to the analgesic opiate component. This BIS = algorithm is=20 Proprietary Commercial Property of Aspect Medical Systems Inc. and is = not made=20 public.

BIS = INDEX=20 :

The=20 BIS Index is a number between 0 and 100 scaled to correlate with = important=20 clinical endpoints during administration of anesthetic agent. BIS values = near=20 100 represent an =93awake=94 clinical state while 0 denotes the maximal = EEG effect=20 possible (i.e., an isoelectric EEG). As the BIS Index value decreases = below 70,=20 the probability of explicit recall decreases dramatically. At a BIS = Index value=20 of less than 60, a patient has an extremely low probability of = consciousness.=20 BIS Index values lower than 40 signify a greater effect of the = anesthetic on the=20 EEG. At low BIS values, the degree of EEG suppression is the primary = determinant=20 of the BIS value.=20 Prospective=20 clinical trials have demonstrated that maintaining BIS Index values in = the range=20 of 40-60 ensures adequate hypnotic effect during general anesthesia = while=20 improving the recovery process.=20 During=20 sedation care, BIS Index values > 70 may be observed during adequate = levels=20 of sedation but may have a greater probability of consciousness and = potential=20 for recall.

The=20 BIS Index provides a direct measurement of brain status, not the = concentration=20 of a particular drug. BIS Index values decrease during natural sleep as = well as=20 during administration of an anesthetic agent.=20 The=20 decrease produced during the natural process of sleep, however, is not = to the=20 degree caused by high doses of propofol, thiopental or volatile=20 anesthetics.

The=20 BIS Index values reflect the reduced cerebral metabolic rate produced by = most=20 hypnotics. Using positron emission tomography (PET), a significant = correlation=20 between BIS Index values and reduction in whole brain metabolic activity = was=20 measured.

The=20 BIS Index value is derived from the preceding 15-30 sec of EEG data. As = such, it=20 is a measure of the state immediately prior to the calculation. A = similar=20 analogy would be the data provided by pulse oximetry.

Under = steady-state=20 conditions a BIS Index value predicts subsequent responses to voice = command or=20 memory for words. However, the clinical situation during surgery is = notably=20 different because of the lack of steady-state conditions. Intraoperative = BIS=20 Index values will depend upon a number of variables=20 including:

=95=20 Brain concentration of anesthetic

=95=20 Level of analgesia = & Surgical=20 stimulation

It=20 must be recognized that brain state, as measured by the BIS Index, = changes as a=20 result of these dynamic variables. Nevertheless, the BIS Index is a = highly=20 accurate measure of the net effect and responses of the brain to new = conditions.=20 It is unable, however, to predict future changes.

2. What are the Components of BIS = Monitoring Equipment ?

( The A2000 = BIS Monitor=96=20 Aspect Medical Inc.)

Sensors placed = on the=20 patient's head transmit EEG signals to the Digital Signal = Converter(DSC). The=20 DSC amplifies and digitizes these signals, then sends them to the = monitor. The=20 monitor software filters the data, analyzes it for artifact, and = processes it=20 using digital signal processing techniques. The purpose of processing = the EEG=20 waveform data is to extract characteristic features from the complex = signal in=20 order to provide easier pattern recognition of changes over time during = the=20 recording. These data=20 are=20 displayed on the screen.

The BIS sensor = is a=20 sophisticated electrode system specifically designed to work with BIS = systems. A=20 family of sensors tailored to different clinical applications or = different=20 patient sizes is available. After minimal skin preparation, the = single-use=20 sensor is placed on the forehead of the patient with a specific = orientation over=20 either the left or right hemisphere. Advanced electrode technology = results in=20 low impedance values, allowing reliable capture of raw EEG data and = increasing=20 the fidelity of the EEG signal. Filtering and signal processing steps = occur in the=20 DSC to identify and reject certain types of electrical artifact. The = digitized=20 EEG data travels through the DSC cable to the BIS engine, the heart of = the BIS=20 system, containing the microprocessor responsible for rapid signal = processing=20 and computation of the BIS Index. The Monitor displays the BIS, SQI, = EMG,EEG=20 & other parameters.

3. =20 How to set up = BIS=20 Monitor and interpret the Displayed Data ?

There are a = variety of=20 sensors. Each sensor terminates in a tab which allows connection to the = patient=20 interface cable.

Quatro Sensor = :=20 incorporates 4 = circular=20 areas which need to be positioned accurately on the patient=92s = forehead. Sensor=20 application should be done in the following = sequence:

=95=20 Orient the sensor so that circle 3 can be secured to that temple area = and the=20 sensor can be applied on forehead at an angle.Position circle 1 = approximately 2=20 inches above the nose. Position circle 4 above and adjacent to the = eyebrow. =95=20 Position circle 3 on either temple area between the corner of the eye = and the=20 hairline in a vertical fashion. Press circles firmly for 5 seconds to = assure=20 proper contact.

=95=20 Insert sensor tab into patient interface cable.

Pediatric = Sensor : is=20 designed for better fit on smaller patients. The sensor incorporates 3 = circular=20 areas which need to be positioned accurately on the patient=92s head.=20

Standard = Sensor :=20 incorporates=20 3 circular areas which need to be positioned accurately on the = patient=92s head as=20 shown in the figure. =

The Sensor is = connected=20 to the Patient Interface Cable by snap on connectors. The PIC is then = clipped on=20 to the Digital Signal Convertor whose pins are aligned and connected to = the=20 Monitor

The=20 Monitor Screen- displays = numerous=20 data as detailed further.=20

The=20 BIS (Bispectral Index) Numeric Region

The BIS = Numeric Region=20 is in the upper left corner of the screen and displays the current = numeric value=20 of the BIS. The number displayed changes to outlined characters during = periods=20 of poor signal quality when SQI is below 50, and disappears altogether = when=20 signal quality is very low. If limit alarms have been set, this number = will=20 alternately display as a solid or outlined numeric character whenever a = limit is=20 reached.

&n= bsp; =20

&nbs= p; =20

The=20 Signal Quality Region ; Signal Quality Index (SQI) Bar=20 Graph

The SQI=20 is an indication of the quality of the EEG signal that is received and=20 processed

into=20 screen data. The SQI graph represents a scale of 0 - 100, marked off at=20 intervals of

10.=20 Optimal signal quality for the EEG channel source is indicated when the = bar=20 extends

to the=20 right side (+) of the graph.

The=20 Electromyograph (EMG) Bar Graph: The EMG=20 graph shows the power (in decibels) in the frequency range 70 - 110 Hz. = This=20 frequency range contains power from muscle activity (i.e., = electromyography or=20 =93EMG=94) as well as power from other high-frequency artifacts. A low = level of EMG=20 is indicated when the bar is not present or at the left side of the=20 graph.

The=20 Electroencephalogram (EEG) Waveform Display: Filtered=20 EEG waveforms are displayed with a sweep rate of 25 millimeters per = second and a scale of 25 microvolts = per=20 division.

The=20 Suppression Ratio (SR) Number

Suppression=20 ratio is a calculated parameter to give the user an indication when an = isoelectric (flatline) = condition may=20 exist. Suppression ratio is the percentage of time over the last = 63-second=20 period that the signal is considered to be in the suppressed state. For = example:=20 SR=3D11 (isoelectric over 11% of the last 63 second review). The SR is = displayed=20 in the upper right corner of the screen.

Graphic=20 Display Region

The=20 Graphic Display Region displays trend graphs of the BIS and an optional=20 secondary variable. The BIS Trend display shows a plot of the Bispectral = Index=20 over a 1-hour time period. The BIS trend is indicated with a thick line = and its=20 unit markings appear on the left axis The name BIS is displayed above = the left=20 corner of the graph. =20

4. = Clinical=20 Applications of BIS

The=20 clinical applications for BIS Index monitoring can be broadly = categorized as=20 patient-related or case-related. Specifically, the potential indications = for use=20 of this new monitoring technology can be listed = by:

Patient = profile b. Procedure type & c. Anesthesia=20 technique

a. PATIENT=20 PROFILE

BIS=20 monitoring benefits a broad range of patients. It enables maintenance of = adequate depth of anaesthesia in face of decreased anaesthetic = requirements and=20 necessity to maintain stable hemodynamics. These profiles=20 include:

=95=20 Elderly patients :

=95=20 Medically-compromised patients : eg. Severe cardiopulmonary=20 disease

=95=20 Hemodynamically Labile patients : allows the provider to confidently = treat=20 alterations in blood pressure or heart rate without compromising level = of=20 hypnosis.It also permits the provider to determine if alterations in = anesthetic=20 dosage correlate with changes in blood pressure or heart rate.=20

=95=20 =93At risk=94 patients: to monitor the hypnotic effect independently of = hemodynamic=20 parameters.

=95=20 Pediatric patients : enable faster recovery without overdosing on=20 hypnosis

=95=20 Trauma patients

=95=20 Obese patients: dosing of intravenous anesthetics is more challenging = due to=20 altered uptake, distribution and adipose tissue sequestration & = resultant=20 respiratory depression

=95=20 Patients with MULTIPLE organ dysfunction : BIS monitoring allows an = independent,=20 patient-specific assessment of the anesthetic effect. =

= b.=20 TYPES OF PROCEDURES

Since=20 its introduction, BIS monitoring has been utilized successfully to = monitor the=20 effects of anesthesia in a wide variety of procedures=20 including:

=B7 =20 Outpatient=20 procedures: Improves recovery profiles, Increases Phase 1 recovery room = bypass=20 & Reduces postoperative nausea and vomiting

=95=20 Prolonged surgery: faster recovery due to reduced agent=20 dosage

=95=20 Cardiac surgery : BIS value of 50 has been reported to provide an = effective=20 anesthetic effect yet provide control of cardiovascular responses and = prevent=20 awareness due to hemodilution on Bypass (Incidence upto 1.5% as against=20 0.1-0.2%. in routine cases)

=95=20 Neurosurgery : This technology has been used to=20 facilitate:

=B7 =20 Precise=20 agent titration during =93awake=94 craniotomy for seizure, motor, speech = mapping=20 & Intra-operative = =93Wake Up=20 Test=94

=B7 =20 Rapid=20 emergence for neurosurgical assessment

BIS-guided=20 titration of short-acting sedatives and analgesics during =93awake=94 = craniotomy=20 allows the provider to appropriately reduce sedative effects during = cortical=20 mapping of motor or speech locations around tumors or seizure foci. = BIS-guided=20 titration also allows adjustment of agent administration to ensure rapid = emergence and return to consciousness for immediate neurosurgical = assessment in=20 wake of cerebral edema & potential vascular = affect.

=95=20 Procedures complicated by increased risk of awareness : Cardiac surgery, = Obstetric surgery, Trauma surgery

=95=20 Remote/Office procedures

C. = ANESTHESIA=20 TECHNIQUES

There=20 is a wide variety of anesthetic approaches utilized by anesthesia = providers=20 including: