Decreased EEG in anaesthesia seen in (AIIMS Nov 2011)
Ref: Basics of anesthesia by Miller, 5th Ed, page 507
a. Bispectral index (BIS) is one of several technologies which purport to monitor depth of anesthesia. BIS monitors are intended to replace or supplement Guedel's classification system for determining depth of anesthesia.
b. Titrating anesthetic agents to a specific bispectral index during general anesthesia in adults (and children over 1 year old) allows the anesthetist to adjust the amount of anesthetic agent to the needs of the patient, possibly resulting in a more rapid emergence from anesthesia.
c. Use of the BIS monitor is thought to reduce the incidence of intraoperative awareness in surgeries. The bispectral index is a statistically based, empirically derived complex parameter.
d. It is a weighted sum of electroencephalographic subparameters, including a time domain, frequency domain, and high order spectral subparameters.
e. The BIS monitor provides a single dimensionless number, the , which ranges from 0 (equivalent to EEG silence)to 100 (equivalent to fully awake and alert). A BIS value between 40 and 60 indicates an appropriate level for general anesthesia, as recommended by the manufacturer.
f. The BIS monitor thus gives the anesthetist an indication of how "deep" under anesthesia the patient is. The essence of BIS is to take a complex signal (the EEG), analyse it, and process the result into a single number. Most anesthetics (except Ketamine) cause dose dependant decrease of EEG frequency and increase in amplitude.
g. This includes volatile agents, propofol, barbiturates, benzodiazepines and narcotics.
h. Progressive hypothermia slows the EEG frequency spectrum with gradual reduction in amplitude of EEG waves.