Bispectral Index(BIS Monitors)

By Reda abu al majd ismail Mohamed

Lecturer of Anesthesia & ICUFaculty of Medicine

Suez Canal University

Topics

- awareness: definition- incidence- causes .

- Short and long term effect of excessive anesthesia.

- what BIS monitor and how it work.

- calculation of BIS index.

- Artifact and factors affecting.

- uses outside OR.

AWARENESSexplicit memory from the anesthesia period during which the patient should have been unconscious. (nightmares, anxiety and flashbacks)In the United States, the incidence of intraoperative awareness is 0.1% to 0.2% of patients undergoing general anesthesia.In Europe the incidence of recall of intraoperative events and dreams during operation in non-obstetric surgeries 0.2% and 0.9%, respectively.Causes of Intraoperative Awareness:

Light anesthesia:• Difficult intubation• Premature discontinuation of anesthetic• Myocardial depression• Cesarean section

Machine malfunction or misuse of the technique:

• Failure to check equipment• Vaporizers and circuit leaks• Errors in intravenous infusion• Accidental administration of muscle relaxants to patients who are awake

Increased anesthetic requirement :

• Individual variability in anesthetic requirements• Chronic alcohol, opioid, or cocaine abuse

Short term effects from excessive anesthesia

• General anesthetics affect the circulation and cause hypotension by concentration- dependent myocardial depression and effects on blood vessel tone.; postoperative nausea and vomiting (PONV) ; delay recovery, and result in longer stay in the postoperative care unit (PACU).

Long term morbidity and excessive anesthesia

• There is controversy whether anesthetics may have adverse long term effects on cognitive function, in the very young as well as in the elderly.

•

BIS index• Developed by Aspect Medical Systems (Newton, MA), the BIS system includes a sensor,

a patient interface cable, a digital signal converter, a BIS engine (microprocessor), and a monitor.

• Wipe the forehead with alcohol and dry with gauze.• Either side of the head can be used except in the case of stroke were you would use the

unaffected side.• Electrode # 1 is placed in the centre of the forehead 1.5 inches above the bridge of the

nose• Next electrode # 4 is placed directly above the eyebrow• Place electrode #3 on the temple between the outer canthus of the eye and the hairline• Now electrode #2 will be in the proper position to be secured.

Values for BIS Monitoring

General anaesthesia associated with no awareness under anaesthesia occurs from BIS values of 40-60.

Deep Sedation occurs with BIS Values of 60-80

In special needs patients work around 80-90 for sedation.

• The BIS monitor uses frontal EEG passively recorded via a strip containing 4 electrodes applied to the patients' forehead.

• The EEG is composed of a number of sine waves of different frequencies put together.

• The EEG, which is an analogue signal, is digitized, artefact filtered and split into epochs. The epochs, which are small periods of time 2 seconds. then transformed to a power spectrum by the use of a fast Fourier analysis . The fast Fourier analysis splits the EEG epochs into the different frequencies, and estimates the power in each frequency.

• This information is then used to analyze two of the four parameters which sums up to give the BIS index. (the beta ratio and the sync fast slow)

• The EEG is also analyzed to detect periods of burst suppression shown as suppression ratio (SR; the time of isoelectric EEG). A fourth analysis, QUASI, is used to detect background noise to be able to correctly define the burst suppression periods.

• These four parameters are combined, by a proprietary algorithm, to give a dimensionless number between 0-100 called the BIS index where 0 represents the isoelectric EEG and 95-100 the fully awake patient.

• Bis analysis quantifies the phase correlation between different frequencies in the EEG.

While studies have shown BIS monitoring is reliable in a wide variety of patients and situations, in certain circumstances the BIS index may not accurately reflect a patient's level of sedation.

The most frequent source of unreliable BIS readings is signal artifact from muscle activity, typically from the patient's face or forehead muscles. This muscle activity, measured as EMG in decibels, can be caused by pain, twitches, seizures, eye movement, or anything that results in increased muscle tone or movement.

This activity can generate high-frequency signals that can contaminate the EEG and make the BIS index unreliable.

For this reason, BIS monitors include an EMG bar graph. The higher the EMG, the greater the muscle activity, and therefore the less reliable the BIS index.

Other sources of high-frequency activity that can lead to inaccurately high BIS values include medical devices, such as warming blankets, circulatory assist systems, high-frequency ventilators, suction devices, surgical instruments, and pacemakers and defibrillators. The closer the device is to the BIS sensor or equipment, the more likely it is to generate artifact.

Ketamine causes an increase in high-frequency EEG activity; therefore, BIS values may remain high when ketamine is administered, despite the onset of sedation.

Etomidate frequently produces skeletal muscle excitation, which can cause high-frequency EMG activity and, hence, a high BIS reading.

Some clinicians have questioned the accuracy and reliability of the BIS index in patients with abnormal brain structure or function—from stroke or encephalopathy.

BIS outside OR• used to monitor patients during bedside procedures that require procedural sedation or

monitored anesthesia care.

• It has also been used to monitor the level of sedation in patients in neuromuscular blockade or barbiturate coma.

• In some critical care units, BIS monitoring is also used as an adjunct to the neurological exam. And in the perioperative setting, clinicians have used it to confirm brain activity during critical events like cardiac arrest and resuscitation.

summeryThe bispectral index (BIS) is a fairly recent technology used to measure the effects of anesthetics and sedatives on the brain and consciousness

Uses a complex mathematical algorithm based upon descriptive EEG parameters from the frontal cortex to suggest various levels of sedation A sensor, placed on the patient’s forehead, sends raw EEG waveforms to the monitor, where they are analyzed and a BIS index is calculated

This value ranges from 100 (completely awake) to 0 (isoelectric EEG)

summeryOriginally used to monitor the effects of general anesthesia in the OR, BIS monitoring is now used in a variety of settings where patients require sedation, including critical care units, endoscopy suites, and radiology departments.

While BIS monitoring is a valuable tool, you should not make clinical decisions based solely on a BIS index. To make sure your patient's level of sedation is safe and effective, use BIS monitoring in conjunction with your clinical judgment and other methods of assessment, such as vital signs and sedation scales.