Biomedical Engineering Reference
In-Depth Information
methods more accurate but perhaps less robust in the case
of for instance critically ill patients or validity before and
after surgery. Bernstein and Osypka (2003), and Bernstein
and Lemmens (2005) introduced an indexof transthoracic
aberrant conduction (e.g. by excess extra-vascular lung
water) in their equation. Suttner et al. (2006) tried it on
critically ill patients with acceptable results.
Since ICG is a very simple and low cost technique very
valuable applications may appear. It must also be re-
membered that a golden standard method for
SV
and
CO
does not exist; the intramethod variability may be larger
than the intermethod variability. Reference methods have
been thermodilution, dye dilution, oxygen Fick tech-
nique, radionuclear radiography and transoesophageal
Doppler echocardiography.
50
m
V, and the frequency content 1-50 Hz, so DC
voltages are filtered out. The low frequency content is
a clear sign that the scull has a detrimental effect on the
signal transmission. Even so the number of electrodes
indicate that the information content is sufficient to
roughly localize a source. It is believed that the brain
centers are less synchronized the higher the activity,
resulting in smaller amplitude and more high frequency
content of the EEG. At sleep the waves are with largest
amplitude and lowest frequency content. The electrodes
used are rather small, often made of tin/lead with
collodium as contact/fixation medium.
The brain can be stimulated (e.g. by a sound or by
looking at changing patterns). The EEG signal can be
time averaged based on synchronization pulses from the
stimulator. Electrical activity and the brain electric re-
sponse can by this method be extracted from noise. In
this way hearing sense of small children and babies can be
examined.
Electrocorticography with electrodes placed directly
on the cortex during surgery permits direct recording of
high amplitude, high frequency EEG.
EOG (electro-oculography) is an electrophysiological
method where DC potentials are utilized, and therefore
AgCl-electrodes are used. The DC potential is de-
pendent on the position of the eye, and is of particular
interest, for example, when the eye lids are closed (REM
sleep). As a DC recording method EOG tends to be
prone to drift which makes the spatial localization of the
point of gaze problematic. It is also sensitive to facial
muscle activity and electrical interference. The signals
are due to a potential between the cornea and the fundus
of an eye with a functioning retina, and are not from the
ocular muscles (Geddes and Baker, 1989).
ENG (electro-nystagmography) is also the recording
of corneo-retinal potentials, usually used to confirm the
presence of nystagmus (special eye movements). The
electrodes are placed to the side (lateral), above and
below each eye. A reference electrode is attached to the
forehead. A special caloric stimulation test is performed,
with cold and/or hot water brought into the canal of one
ear. The electrodes record the duration and velocity of
eye movements that occur when the ear is temperature
stimulated.
ERG (electro-retinography) records the AC potentials
from the retina. The electrode system is unipolar, with
a gold foil or AgCl recording electrode embedded in
a special saline filled contact lens in contact with the
cornea. The eye may be considered as a fluid-filled sphere
in contact with the retina as a thin, sheet-like bioelectric
source. The ERG signal caused by a light flash is a very
rapid wave with an initial rise time less than 0.1ms (early
receptor potential) and an amplitude around 1 mV,
followed by a late receptor potential
4.1.4 Tissue characterization
in urology
Bioimpedance techniques are interesting candidates for
diagnosing disorders in the urinary system. There are
many possible applications, ranging from measurements
of uteric tone and contractility to assessment of prelabor
ripening and detection of cervical cancer. Some examples
are as follows.
Mudraya et al. (2007) used different instruments
such as a multichannel impedance spectrometer for
tetrapolar measurements on an array of nine electrodes,
enabling simultaneous recording of six locations along
the urinary tract. They found such measurements to be
valuable for quantitative assessment of ureteric peri-
stalsis, and they could also locate stricture regions by
monitoring the ratio between low and high frequency
impedance,
Z
low
/Z
high
.
Abdul et al. (2005) developed a tetrapolar probe
for impedance measurements in the frequency range
2-1200 kHz. They found their system to give similar
sensitivity and specificity to currently used screening
tests for cervical intraepithelial neoplasia, but with the
evident advantage of providing instant results.
Gandhi et al. (2006) measured cervical stromal im-
pedance (CSI) in non-pregnant women and women in
different stages of pregnancy. They found the impedance
to increase during pregnancy and their results suggest
potential utility of CSI measurement for quantifying
gestation-dependent changes in cervical stromal tissue.
4.1.5 EEG, ENG/ERG/EOG
The electrical activity of 10
11
brain cells are recorded on
the skin of the scull with a standardized electrode net-
work of 21 electrodes. The leads may be bipolar or
unipolar. The signal amplitude is only of the order of
lasting many
milliseconds.
