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The monitor also on-line evaluates the amount of instantaneous burst suppression (BS) in each
thirty-second period of the EEG. This measurement quantifies the amount of "silent" or "flat" EEG period
characteristic of the deepest levels of hypnosis.
These four parameters are used as input to a fuzzy logic classifier system that calculated the Cerebral State
Index.
CSI Scale
The CSI is a unit-less scale from 0 to 100, where 0 indicates a flat EEG and 100 indicates EEG activity
corresponding to the awake state. The range of adequate anaesthesia is designed to be between 40 and 60.
All values in the table are approximate values based on the mean values of the patient behaviour.
EMG
Facial EMG may increase the CSI under certain conditions. The monitor incorporates an EMG filter that
removes most of the potential interfering EMG activity. The EMG% bar shows the energy of the EMG level
in the 75-85 Hz frequency band (0-100 logarithmic). The bar is located on the right side of the display.
EMG activity is expected to be present when the patient is awake. When the patient is asleep, EMG activity
can increase due to:
Reflex reactions to painful stimuli during surgery.
⚫
Lack of muscular relaxation.
⚫
Muscular rigidity caused by some opioids (analgesics).
⚫
Presence of large external electrical fields, e.g., diathermy.
⚫
The EMG bar should be checked frequently, especially in case of a sudden increase in the CSI. If the increase
in CSI is accompanied by an increase in muscular activity, there is a risk that EMG is causing interference.
When this happens, attention must be paid to the stimuli receive by the patient during surgery. In the
presence of hypnotically unrelated EMG, administration of a neuromuscular blocking agent will cause the
CSI to decrease. Since patients receiving neuromuscular blocking agents cannot exhibit movement as a sign
β
=ln(E
30-42.5Hz
ratio
/E
)
11-21Hz
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