Monitoring the depth of anesthesia is very important during any procedure, as anesthesia that's too deep can cause hemodynamic changes. Anesthesia that's too light carries the risk of recall or awareness during anesthesia. Awareness during anesthesia is a very serious complication with potential long-term psychological sequelae such as anxiety and post-traumatic disorder. When the bispectral index (BIS) value is maintained below 60, it decreases the incidence of anesthesia awareness. Several neuro monitors based on the processed electroencephalogram or evoked potentials have been developed to evaluate the anesthetic depth. The BIS monitor is the first method which is FDA approved to assess the hypnotic effects of drugs.
BIS Monitoring and Principle:
The bispectral index monitor processes electroencephalographic signals to obtain a value, which reflects the level of consciousness of the patient. BIS monitor collects raw EEG data through its sensors and uses an algorithm to analyze and interpret the data. The data displays as a number on the BIS view monitor. BIS values range from 0 to 100. A value of 0 represents the absence of brain activity, and 100 represents the awake state. BIS values between 40 to 60 represent adequate general anesthesia for a surgery, values less than 40 represent a deep hypnotic state. BIS value is usually maintained between 40 to 60 to prevent awareness under anesthesia.
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. 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:
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.
Limitations/ Factors Affecting BIS Monitoring:
Alternative technologies to monitor the depth of anesthesia are as follows:
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. 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.
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. Use of this technology requires a multidisciplinary team approach that includes the surgeon, anesthesiology, nurse anesthetist, surgical nurses, and other support staff to ensure the best possible patient outcomes. [Level V]
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