Electroencephalography

Bispectral Index (BIS)

Intraop monitoring of anesthetic depth remains challenging yet critically important to a patients overall intra-operative course. While volatile anesthetics provide a MAC value that can guide depth of anesthesia, TIVA techniques do not have a similar method to determining adequacy of anesthesia. Minimal anesthetic agent runs the risk of intraoperative movement or awareness and recall. Anesthetic overdose exposes the patient to excessive hemodynamic changes. Therefore, a balanced and titrated anesthetic is critical. One tool to assist the anesthesiologist is the Bispectral Index (BIS)

This BIS consists of 4 electrodes across the forehead to measure and processes electroencephalographic (EEG) signals. This raw EEG signal is processed though an algorithm resulting in a BIS value between 0-100, with 100 representing an awake state and 0 representing no brain activity. Generally, values between 40-60 represent an appropriate depth of general anesthesia.

The BIS monitor will display the BIS value derived from a proprietary algorithm, a EEG waveform, a signal quality index (SQI), and an electromyography (EMG) signal. SQI is proportional to signal reliability (limitations discussed below). EMG signal represents movement tone or muscle stimulation. Increased EMG signal tends to produce a higher BIS value.

Limitations

Like all technological tools utilized in anesthesia, an understanding of the devices function helps to guide the user to a devices specific limitations, inaccuracies, and failure points.

BIS failure modes can be generally categorized into drug induced, patient pathology, or electromagnetic interference.

Drug induced

Each anesthetic drug will cause specific and reproducible changes in EEG pattern. Propofol and volatile anesthetics generally fit the pattern of a lower BIS value correlating with less brain activity. However, certain drugs like ketamine and Nitrous Oxide actually produce general anesthesia despite higher BIS values.

Patient Pathology

Paralysis, or globally decreased muscle tone, results in no EMG signal. This is often interpreted by the BIS algorithm as lower BIS value. Paralysis without amnesia is an important source of anesthesia awareness and may not reliably be detected by the BIS.

Neurologic pathology can represent abnormal cortical brain function. This is especially important if impairment is located in the frontal lobes, where the BIS records EEG data. This can result in variable BIS values, generally tending to result in a falsely low number. Nevertheless, certain pathology could result in increased values such as status epilepticus. One case report describes a BIS value of 90 in a patient with status epilepticus despite being unresponsive.

Superficial pathology, such as infection or pathology on the skin, may limit the quality of data received by the BIS monitor and therefore will result in unreliable BIS values.

Patient body temperature can also change the BIS value, approximately decreased by 1 point per 1C

Electrical signal interference

Electrocautery or other electromagnetic waves such as a pacemaker could influence BIS values. One such case report describes an external pacemaker following cardiac bypass giving a falsely high BIS number. Another report describes an aortic balloon pump producing artifact resulting in a BIS value of 80 despite an otherwise deeply anesthetized patient.