User manual

User Manual for Multi-parameter Patient Monitor
45
Chapter 7 Parameters Monitoring
7.1 CSM Monitoring
7.1.1 The Principle for Measurement
The monitor is intended for use in monitoring the hypnotic state of the brain by data acquisition of EEG signals of
the anaesthetized or sedated patient in all areas of the hospital. The monitor is a non-invasive measurement tool to
be used by a trained professional to measure the level of consciousness during general anaesthesia and sedation by
use of variations in the frequency content of the spontaneous EEG. It analyses the frequency shifts that take place
in the EEG signal as the level of consciousness changes. Based on this principle, the monitor calculates the
Cerebral State Index (CSI), which is used to estimate the level of consciousness of the patient.
7.1.2 CSM Parameters
 CSI
The performance of the CSI is based on the analysis of the frequency content of the EEG signal.
The energy of the EEG is evaluated in specific frequency bands. These are used to define two energy ratios
called alpha (α) and beta (β). Both of these show a shift in energy content from a higher to the lower
frequencies during anaesthesia. The relationship between these quantities is also analysed as a separate
parameter (β-α).
α
ratio
=ln(E
30-42.5Hz
/E
6-12Hz
)
β
ratio
=ln(E
30-42.5Hz
/E
11-21Hz
)
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.