Biomedical Engineering Reference
In-Depth Information
4.1.6 Electrogastrography
signal is much smaller and must therefore be averaged
with multiple stimuli. However, the method is more
of interest because there is more information in the
response waveform.
The typical electrogastrographical (EGG) signal due to
stomach activity is recorded with a bipolar lead using
a pair of standard ECG electrodes on the skin (e.g. 4 cm
apart). The signal is typically of about 100
m
Vamplitude,
and periodic with a period of about 20 seconds (0.05Hz
fundamental). Best position for the EGG electrodes is
along the projection of the stomach axis on the abdomen.
Internal electrodes are also used, but are in general not
considered to provide more information than external
EGG signals. Of course the internal electrodes are nearer
to the source, implying higher amplitude signals with
more high frequency content. Because of the very low
frequency spectrum, external noise from, for example,
slowly varying skin potentials tends to be greater problem
than from internal recordings.
4.1.8 Electrical impedance
myography
Electrical impedance myography (EIM) refers to a group
of impedance-based methods for the clinical assessment
of muscles. This includes primary disorders of muscle
such as myopathic conditions (Rutkove et al., 2002;
Tarulli et al., 2005) and the sarcopenia of aging (Aaron
et al., 2006) as well as diseases that affect the nerve, such
as localized neuropathies or nerve root injuries (Rutkove
et al., 2005) and generalized problems, such as amyo-
trophic lateral sclerosis (Esper et al., 2005). This neu-
romuscular disease-focused application of bioimpedance
is built on the earlier experimental and theoretical work
of Shiffman and Aaron (see Aaron et al., 1997; Shiffman
et al., 1999; Aaron and Shiffman, 2000). Importantly, the
goal of EIM is not to image the muscle, but rather to
assess quantitatively changes in its microscopic structure
induced by neuromuscular disease states.
All methods utilize a tetrapolar technique and rely on
the placement of voltage sensing electrodes along
a muscle or muscle group of interest. Depending on the
application, current-injecting electrodes can be placed in
close proximity to or at a distance from the voltage elec-
trode array. Both single frequency (50 kHz) and multi-
frequency (up to 2 MHz) methods have been studied with
the former showing very high reproducibility of the major
outcome variable, the spatially averaged phase q
avg
(Rutkove et al., 2006). The application of EIM in the
setting of voluntary or stimulated muscle contraction
represents another provocative and potentially important
area of investigation that may allow assessment of the
contractile apparatus (Shiffman et al., 2003).
Some limited animal work has also been performed.
Nie et al. (2006) showed that consistent measurements
could be obtained on the hamstring muscles of the rat
and that substantial changes occur after experimental
sciatic crush, including reductions in the measured phase
and loss of the normal frequency dependence. Future
animal work will be geared at disease differentiation and
determining the relationship between muscle states and
their impedance patterns.
The most straightforward application of the technique
of EIM for clinical care is for its use as a quantifiable
measure of muscle health, such that treatment or re-
habilitation programs can be effectively monitored.
Indeed, EIM has the potential to serve as a useful new
outcome measure in clinical trials work (Tarulli et al.,
2005) and studies are ongoing to verify the role of EIM as
4.1.7 EMG and neurography
EMG
To record signals from muscles (EMG), both skin sur-
face electrodes and invasive needles are used. The dis-
tance to the muscle is often short, and the muscle group
large, so signals have high amplitude and high frequency
content. But the signal is usually coming from many
muscle groups, and the signal looks rather chaotic. The
muscle activity is related to the rms value of the signal.
If the muscle activity is low and controlled, single motor
units become discernible with needle electrodes. EMG
is often recorded in connection with active neuro-
stimulation, involving both the muscle and the nervous
system.
The frequency EMG spectrum covers 50-5000 Hz,
and the amplitude may be several millivolt with skin
surface electrodes.
ENeG
The sum of activities forming a nerve bundle may be
picked up by skin surface electrodes (e.g. on the arm
where the distance from the electrodes is only a few
millimeters). Action potentials from single nerve fibers
must be
measured
with invasive needle electrodes. They
may be bipolar or unipolar. To find the right position, the
signal is monitored during insertion, often by sounds in
a loudspeaker.
Stimulation
may be done with the same electrode
designs: either transcutaneously right above the bundle
of interest, or by needles. Muscles (e.g. in the hand) are
stimulated by stimulating efferent nerves in the arm.
EMG electrodes can pickup the result of this stimula-
tion, for instance for nerve velocity determination. It can
also be picked up by neurographic electrodes, but the
