BIS monitoring involves the application of four electrodes on the forehead. The skin on the forehead is cleaned with an alcohol swab and 2 to 5 seconds of digital pressure applied over the sensor leads.[3] The sensor comprises of disposable wet gel electrodes. Electromyographic activity of the frontalis muscle is measured by lead 4, which is the ground electrode as well. The 2-Channel monitor includes a user-configurable display. The 4-Channel monitor has enhanced bihemispheric capabilities. BIS extended sensors are available for use in ICU where patients require long-term monitoring. To measure brain activity in the pediatric population BIS pediatric sensor is available which utilizes 'zipper' technology and can capture low voltage EEG. 

The BIS view monitor displays:

• BIS number
• Trend graph of BIS values over time
• Raw EEG waveforms in real time
• Various signal quality indicators as SQI, EMG
• Alarm indicators and messages

The signal-quality-index (SQI) bar predicts the reliability of the signal; the higher the SQI, the more reliable is the BIS number. The electromyographic (EMG) bar indicates EMG activity, which reflects muscle stimulation caused by an increase in muscle tone or muscle movement.[4]

Limitations/ Factors Affecting BIS Monitoring:

1. Anesthetic Agents: The anesthetic agents used affect BIS values. The patient anesthetized with one anesthetic drug may be more sedated than another patient with the same score anesthetized with a different combination of drugs. BIS monitor is unreliable with certain anesthetics, such as ketamine and nitrous oxide (NO). In 2017, Mishra et al. studied the effect of nitrous oxide on the bispectral index and found out that the BIS value increases with the addition of nitrous oxide. This effect could be due to the decrease in the suppressant effect of the inhalational anesthetic agents on EEG with the use of NO and the neuro-stimulant properties of NO. NO increases cerebral blood flow velocities along with an increase in cerebral consumption of oxygen (CMRO).[5]
2. Age: It is difficult to titrate anesthetic agents in the infants younger than 6 months of age with BIS; this could be due to a difference in EEG in this population than older children as brain maturation and synapse formation occurs during that period.  In 2001, Bannister et al. found that there were no significant differences between the standard practice and BIS groups in anesthetic use or recovery measures in the children 6 months to 3 years of age.[6]
3. Hypothermia: Doi et al.,[7] studied 12 patients undergoing hypothermia for a cardiopulmonary bypass pump; they noticed a wide variation in BIS values during the decrease in body temperature. The BIS goes down by 1.12 units for each degree Celsius reduction in body temperature. Reduction in temperature results in decreased cerebral metabolic rate for oxygen which is reflected on the EEG by the progression to an isoelectric/burst suppression pattern dominated by isoelectric periods. Aortic cannulation causes  EEG slowing.  The onset of cardiopulmonary bypass produces a transient EEG depression most likely due to perfusion of the brain with a crystalloid prime solution. EEG slowing that may persist into the post cardiopulmonary bypass period is seen during decreased cardiopulmonary bypass flow rates with the aortic cross-clamp release. Decreased CPB flow rates requested by the surgeon during aortic cross-clamp release often correlated with EEG slowing that may persist into the post-CPB period.[8]
4. Neurological impairment: Neurological impairment alters the ability of the BIS to monitor the depth of consciousness; it is not a reliable tool to assess the level of consciousness in this patient population. Generally, BIS values are lower in patients with neurological impairment. BIS values reflect the activity of the cortical structure of the brain but do not reflect the activity of subcortical structures such as a spinal cord. 
5. Interference with medical devices: Interference from the medical devices causes artifacts and impairs the ability of the BIS monitor to assess the changes in the depth of anesthesia accurately. Chan et al. found that SQI decreases during the use of electrosurgical cautery.[9]

Alternative Technologies:

Alternative technologies to monitor the depth of anesthesia are as follows[10]:

• SNAP EEG monitor system
• Auditory Evoked Potential monitor
• 4-channel processed EEG
• EEG monitor
• Spectral edge frequency 95 
• Automated responsiveness test

Current Evidence of BIS Monitoring:

BIS is in current use in the operating rooms, ICUs and some emergency departments. In 2007, Carlos et al. found that time to extubation and discharge from the operating room, and PACU is reduced with the use of BIS monitoring. The risk of postoperative nausea and vomiting was decreased by 12% in patients monitored with BIS.[11] Tong et al., in 2017 demonstrated that the addition of BIS monitoring could result in economic benefit by reducing the use of propofol and faster recovery compared to standard practice.[12]

So far, there is no gold standard to span the entire spectrum of anesthetic effect on the central nervous system. BIS is one of the most studied monitors derived from EEG used nowadays. BIS has been shown to have a positive cost-benefit ratio and lower morbidity than more invasive monitoring methods.[13] Use of this technology requires an interprofessional team approach that includes the surgeon, anesthesiology, nurse anesthetist, surgical nurses, and other support staff to ensure the best possible patient outcomes. [Level V][13]