Electroencephalography

Bispectral Index (BIS)

Intraop monitoring of anesthetic depth remains challenging yet critically important to a patients overall intra-operative course. Volatile anesthetics provide a MAC value that can guide depth of anesthesia and help prevent awareness. 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.

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 genrally fit the pattern of, a lower BIS value coorolates with less brain activity. However, certain drugs like ketamine and Nitrous Oxide actually produce anesthesia despite high BIS values.

Patient Pathology

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 if 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 unconscious patient.

Superficial pathology, such as infection or pathology on the skin, may limited 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 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.