|
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.
|
