Biomedical Engineering Reference
In-Depth Information
an outcome measure in amyotrophic lateral sclerosis,
spinal muscular atrophy and exercise interventions in
aging. It is uncertain whether the technique has the po-
tential of supplanting standard neuromuscular diagnostic
methods, most notably needle EMG, and this remains
a subject of ongoing research.
Electroacupuncture
The secretion of endorphines is obtained with low fre-
quency (2-4 Hz) stimulation, corresponding to the
rhythmic movement of an
acupuncture
needle. Instead
of, or in addition to the mechanical movement, the
needle is used as monopolar electrode, pulsed by a low
frequency in the same frequency range (1-4 Hz). This is
not a TENS method strictly speaking, as the electrode is
invasive and the current not transcutaneous.
4.1.9 Electrotherapy
Transcutaneous stimulation
Transcutaneous electrical nerve stimulation (TENS): is
electrical stimulation through surface electrodes. The
advantage of not using syringe injections is obvious, the
electrical pulses stimulate the body's own mechanisms
for obtaining pain relief. There are three theories as to
how the pain relief is achieved.
4.1.9.1 Electrotherapy with DC
Applied DC through tissue for
long
duration (e.g.
>
10
seconds) is a method almost 200-years old, and is tradi-
tionally called
galvanization.
Today the DC effect is
often ignored, even if it is quite clear that DC through
tissue has some very special effects. Generally the
physical/chemical effect of a DC through tissue is:
electrolysis (local depletion or accumulation of ions)
electrophoresis (e.g. protein and cell migration)
iontophoresis (ion migration)
electro-osmosis (volume transport)
temperature rise.
Some of these effects are special for long duration
(
>
10 seconds) DC or very low frequency AC, in partic-
ular electrolysis. Other effects are in common with AC.
Short term effects of DC through the skin is limited
to the sweat ducts. The current density is much smaller
in the stratum corneum, but long term currents may have
an effect. Proximal to the electrode electrolytic effects
influences the skin. Possible effects in deeper layers are
erythema (skin reddening) and hyperemia (increased
blood perfusion) due to the stimulation of vasomotor
nerves. If the DC is applied transcutaneously, there is
always a chance of unpleasant pricking, reddening and
wound formation in the skin under the electrode.
Gating theory
Pain perception is controlled by a gate mechanism in the
synapses, particularly in central nervous system (CNS) of
the spine. This gate is controlled by separate nerve fibers,
and by stimulating with pulses of high frequency (50-
200 Hz), these fibers are stimulated and pain relief is
obtained.
Endorphines
The body uses natural forms of morphine called endor-
phines for pain relief. The secretion of endorphines is
obtained with low frequency (2-4 Hz) stimulation.
These low frequencies correspond to the rhythmic
movement of an
acupuncture
needle (in classical acu-
puncture it is also necessary to stimulate motor nerve
fibers). The effect of endorphines is probably in the
higher centers of the CNS.
Vasodilation
This effect is usually linked to pain in cold extremities.
Increased blood flow may increase the temperature from
the range 22-24
C to 31-34
C, including in the ex-
tremities not stimulated. The effect must therefore be
elicited in higher centers of the CNS.
The afferent pain nerves have a higher threshold and
rheobase than sensory and motor nerves. Thus it is pos-
sible to stimulate sensory and motor nerves without
eliciting pain. Very short pulses of duration 10-400
m
s are
used, with a constant amplitude current up to about 50
mA and a treatment duration of 15 minutes or more. The
skin electrodes may be bipolar or monopolar. The posi-
tion is in the pain region, an electrode pair may, for ex-
ample, be positioned on the skin on the back of the
patient, or implanted with thin leads out through the
skin. The electrode pair may also be positioned outside
the pain area (e.g. at regions of high afferent nerve fiber
densities in the hand).
Iontophoresis
Iontophoresis in the skin (cf. also Section 4.1.15): if
a drug is in ionic form, the migration velocity and di-
rection are determined by the polarity of the DC. It is
a very famous experiment whereby strychnine was ap-
plied to one of the electrodes attached to a rabbit. With
one polarity the rabbit died, with the reverse polarity
nothing happened. The ions may be transported from the
electrode, and thus have an effect both in the tissue
during passage, and when assembled under the other
electrode. Anestethic agents may be introduced in the
skin by iontophoresis, for minor surgery or the treatment
of chronic pain. Also antibiotics and metallic silver has
been introduced iontophoretically, as well as zinc for is-
chemic ulcers. The mechanism of iontophoresis is of
course accompanied by a possible electrophoretic action
