Biomedical Engineering Reference
In-Depth Information
DC effects are to be avoided (e.g. to reduce electrolytic
effects or electrode metal corrosion), the current is bi-
phasic. Faradic currents are biphasic currents of the type
generated by an induction coil. If the pulses are slowly
increased in amplitude, then reduced, and after a pause
again increased, we have a ramp or surged current. As
many effects are current controlled, it is often better to
use a constant amplitude current mode than a constant
amplitude voltage mode of the stimulator output.
not contain any new frequencies. The linear summation
of two currents at two different frequencies remains
a current with a frequency spectrum with just the two
frequencies, no current at any new frequency is
created.
The current density in the treated volume must be
high enough to create non-linear effects. The process can
then be described mathematically by a multiplication. If
u 1 zu 2 zu, f ( t ) is a signal of double frequency and half
the amplitude, together with a signal of the low beat
frequency u 1 - u 2 also of half the amplitude. The double
frequency signal is not of interest, but the low beat fre-
quency u 1 - u 2 is now present in the tissue.
A third electrode pair can be added, with additional
flexibility of frequency and amplitude selection. Anyhow
it must be taken into account that muscle impedance
may be strongly anisotropic, with a possible 1:10 ratio
between two directions.
Pulse waveform treatment of innervated
muscles (faradization)
Short (0.5-5ms) triangular pulses for tetanic muscular
contractions, interrupted or with varying amplitude. In-
terval between pulses 10-25 ms. For muscle pain relief
rectangular pulses of length 2 ms and interval 7 ms are
used. TENS for pain relief is different. It is based on
stimulating afferent nerve fibers with much shorter
pulses (e.g. 0.2 ms).
Pulse waveform treatment of denervated muscles
As there are no innervation, the stimulation is directly
to the muscle. The paralyzed part of the muscle mass
can be stimulated selectively, because such muscles has
a smaller accommodation at long pulse duration. Very
long (1 second) triangular waveforms are used, with even
longer intervals between the pulses.
4.1.10 Body composition analysis
The parameters of interest in body composition analysis
(bioelectric impedance analysis BIA) are total body water
(TBW), extracellular/intracellular fluid balance, muscle
mass and fat mass. Application areas are as diversified as
sports medicine, nutritional assessment and fluid balance
in renal dialysis and transplantation.
One of the first to introduce BIA was Thomasset
(1965), using a two-electrode method and 1 kHz signal
frequency. With just two electrodes it is important to
use large area band electrodes in order to reduce the
contribution from the current constrictional zones near
the electrodes. With a tetrapolar electrode system it is
easier to select the preferred volume to be measured.
The small circumference of the lower arm, wrist and
fingers causes those body segments to dominate mea-
sured impedance in a so-called whole body measure-
ment. With measuring electrodes, for example, on one
hand and one foot the chest contribution is very small.
The impedance of the chest segment is therefore the
most difficult one to determine accurately, both be-
cause it is much lower than the impedance of the limbs
and because it varies with respiration and heartbeats, as
exploited in ICG (Section 4.1.3). Thoracic measure-
ments were compared with whole body measurements
by Nescolarde et al. (2006).
By using more than four electrodes it possible to
measure more than one body segment. One method uses
eight electrodes, two electrodes at each hand and foot.
The body impedance is then modelled in five segments:
arms, legs and chest ( Fig. 4.1-11 ). One segment imped-
ance is determined by letting two limbs be current car-
rying and use a third limb for zero current potential
reading (five electrode lead, pentapolar). The leads are
Diadynamic currents for the treatment
of pain and increase of blood perfusion
This is the summation of two currents: a pulse current
superimposed on a DC. Each current is separately ad-
justed. Often the DC level is first increased slowly so
that no perception occurs ( electrotonus ), and then with
a constant DC flowing the pulse amplitude is increased
until a weak vibration is felt. The pulse waveform may be
a power line 50 Hz half-rectified (50) or fully rectified
(100 Hz) current.
Interferential currents
Two-electrode pairs are used to set up two different
current paths crossing each other in the target tissue
volume. Each pair is supplied by a separate oscillator,
adjusted to, for example, 5000 and 5100 Hz. The idea is
that the target volume is treated with the frequency
difference, 100 Hz. The advantage is the possible selec-
tive choice of a limited treated volume deep in the tissue,
together with lower electrode polarization and skin im-
pedance, plus less sensation in the skin and the tissue
outside the treated volume.
If the current level is low enough for linear condi-
tions, the resultant current density in the tissue is the
linear summation of the two current densities.
According to the superposition theorem in network
theory and Fourier analysis, the new waveform f ( t ) does
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