Biomedical Engineering Reference
In-Depth Information
(see below), but is not necessarily so pH-dependent as
the latter. The advantage of iontophoretic instead of local
syringe injection is not always obvious.
we will keep to the traditional meaning, however, which
is more limited to methods stimulating
muscles,
either
directly or via the nerves. Usually such stimulation also
generates data of diagnostic value.
Figure 4.1-10
shows the minimum stimulus current to
an efferent nerve fiber as a function of pulse duration for
obtaining a certain muscle response. The coupling (syn-
apses) between the nerve axon end plates and the muscle
cells is an important part of this signal transmission line.
It is not possible to lower the current under a certain
minimum level, the
rheobase
value. The pulse length
with current amplitude 2
rheobase value is called the
chronaxie.
Pfl¨ger's law relates the muscle effect to the leading or
trailing edge of the pulse, and to anodal or cathodal
polarity:
Electrotonus
Tonus is the natural and continuous slight contraction of
a muscle. Electrotonus is the altered electrical state of
nerve or muscle cells due to the passage of a DC. Sub-
threshold DC currents through nerves and muscles may
make the tissuemore (excitatory effect), or less (inhibitory
effect) excitable. Making the outer nerve cell membrane
less positive lowers the threshold and has an excitatory
effect (at the cathode,
catelectrotonus
), the anode will have
a certain inhibitory effect (
anelectrotonus
). This is used in
muscle therapy with diadynamic currents, see below.
Wound healing
Many controlled studies have shown that a small mi-
croampere current as a long term treatment leads to ac-
celerated healing. There are two classes of cases:
accelerated healing of bone fractures, and of skin surface
wounds. Ischemic dermal ulcers are treated with DC,
and the healing rate is approximately doubled. It has
been found that a monopolar cathodic application during
the first days followed by an anodic application, gives the
best results. In a skin wound it is believed that positive
charge carriers (ions and proteins) are transported to the
liquid wound zone by endogenic migration.
An increased rate of bone formation has also been
found when small currents are applied to each side of
a bone fracture (bipolar electrode system). This is of
particular interest in cases of bone fractures that will not
grow the natural way: so-called
non-union
). Nordenstram
(1983) described the use of DC treatment by applied
needles into tumors for the treatment of cancer.
Leading edge cathodal
Strongest effect
Y
Leading edge anodal
Trailing edge anodal
Trailing edge cathodal
Weakest effect
If a linear triangular current pulse is used instead of
a square wave pulse, an
accommodation
effect will
appear, particularly at pulse duration
>
1 ms. A slowly
increasing DC does not excite a nerve to the same extent
as a DC step change, and the accommodation implies
that the threshold current amplitude will be larger with
triangular than with squared waveform. The current-
time curves can be recorded for diagnostic purposes, the
curves are quite different for degenerated muscles.
The pulses may be of unidirectional current (inter-
rupted DC,
monophasic
), which implies that the current
has a DC component. High voltage pulsed galvanic
stimulation is also used, with pulse currents up to some
amperes, but pulse duration only a few microseconds. If
DC ablation
At higher current densities the acid under the anode
leads to coagulation, and the alkali under the cathode to
liquidification, of the tissue. Warts can thus be treated,
and with a needle cathode in the hair follicle, the local
epidermis is destroyed and the hair removed.
mA
100
DC shock pulses
DC shock pulses have also been used for destroying
calculi in the urinary tract,
electrolithotrity
.
10
2 3
Hydrogen production
The application of a DC current to the inside of the
eyeball by a needle electrode is used to produce bubbles
of hydrogen in the aqueous humor,
electroparacentesis
.
3
rheobase
1
t
(ms)
0.01
0.1
1
10
100
4.1.9.2 Electrotherapy of muscles
chronaxie
Figure 4.1-10 Minimum stimulus current to an efferent nerve fiber
bundle for obtaining a certain muscle response, as a function of
pulse duration.
Electrotherapy is a broad term, and should for instance
include pacing, defibrillation and electroconvulsion. Here
