Biomedical Engineering Reference
In-Depth Information
9.2
Nature of the OR Environment
The term “electrically hostile” scarcely does just justice to the operating room envi-
ronment. The signal of interest, the EEG, is one or two orders of magnitude lower
amplitude than the electrocardiogram (ECG) and shares its frequency spectral range
with another biological signal cooriginating under the scalp, the electromyogram
(EMG). While the patient is awake or only lightly sedated, facial grimacing is asso-
ciated with an EMG signal amplitude many times the EEG signal. In fact, the
desynchronization of EEGs seen in anxious patients leads to an EEG with a broader
frequency spectrum and diminished amplitude, even worsening the EEG SNR. For-
tunately, EMG activation usually mirrors activation in the EEG and accentuates the
performance of EEG-derived variables predicting inadequate anesthesia. Other cra-
nial sources of biological artifact include the electro-oculogram (from movement of
the retina-corneal dipole) and swallowing and the ECG as its projected vector
sweeps across the scalp.
The electrochemistry of silver/silver chloride electrodes creates a relatively sta-
ble electrode potential of several hundred millivolts (depending on temperature and
molar concentration of electrolyte) at each skin contact [18, 19]. Any change in con-
tact pressure or movement of the skin/electrode interface will provoke changes in
the electrode potential that will be orders of magnitude larger than the EEG signal
because it is unlikely that a change at one site will be exactly balanced and thus can-
celed out by the electrode potential at the other end of an electrode pair. Silver/silver
chloride electrode potentials are also sensitive to ambient light.
The next source of artifact to contend with in the OR is pickup of the existing
electromagnetic field in the environment. The two dominant sources are the
power-line frequency field that permeates all buildings that are wired for electricity
and the transmitted output of electrosurgical generators. Power-line frequencies are
fairly easily dealt with using effective common-mode rejection in the input stage
amplifiers and narrow bandpass filtering. Electrosurgical generators (ESUs) are a
far greater problem for EEG recordings. ESU devices generate large spark dis-
charges with which the surgeon cuts and cauterizes tissue. Several different types of
ESUs are in use and the output characteristics vary, but in general, the output volt-
age will be in the range of hundreds of volts, the frequency spectrum very broad and
centered on about 0.5 MHz, with additional amplitude modulation in the subaudio
range.
Some ESU devices feature a single probe whose current flow proceeds through
volume conduction of the body to a remote “ground” pad electrode. This “unipo-
lar” ESU is associated with the worst artifact at the scalp that will exceed the linear
dynamic range of the input stages producing rail-to-rail swings on the waveform
display. The EEG data is usually lost during the surgeon's activation of a unipolar
ESU. Other ESUs are bipolar in that the surgeon uses a tweezer-like pair of elec-
trodes that still radiates enormous interference, but most of the current is contained
in the tissue between the tweezer's tips. Some monitoring companies have gone to
great lengths to reduce the time during which ESU artifacts render their product
offline.
Search WWH ::
Custom Search