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BY COSIMO ARNESANO

 

 

 

 

BME 240-Spring 2009

 

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other techniques


SNAP was introduced in 2002, using the high- and low-frequency platform to calculate the SNAP index. So far, there has been little experience with the SNAP index in order to assess its validity and usefulness for clinical practice. Recent studies have indicated that the SNAP is able to differentiate between different states of anaesthesia. Even if the SNAP index can provide useful additional information for the anaesthesiologist, more detail and information on artefact control and the influence of the EMG is needed.

The SNAP index is a combination of a high-frequency (80–420 Hz) and a low-frequency (0.1–40 Hz) variable created by processing the spectral power from a single-channel EEG.  The high-frequency variable is most pronounced during the awake or lightly anaesthetised state, whereas the low-frequency variable increases with deepening anaesthesia. The high-frequency variable is scaled to result in values from 0.0 to 1.0, and the low-frequency variable to result in values between 0.0 and 100.

The SNAP index ranges in value from 100 to 0. The fully awake patient is represented by a SNAP index of 100, whereas a SNAP index of 50–65 is recommended for general anaesthesia (www.snaphandheld.com). Dominant high-frequency ranges result in higher values, whereas decreasing high-frequency ranges indicate low index values.

Entropy describes the randomness or complexity of a signal. The algorithm used in the GE/Datex-Ohmeda monitor is in the public domain. The monitor provides 2 indices State and Response Entropy. The State Entropy encompasses EEG frequencies of 0.8 – 32Hz i.e. the cortical EEG while the Response Entropy uses the range 0.8 – 47 Hz. The latter includes the EMG which is hoped by some to be an index of the need for analgesia. There has not yet been much clinical literature about the device.