Biomedical Engineering Reference
In-Depth Information
perception of pain. When used as directed, TENS is a safe, noninvasive, drug-free method
of pain management. TENS has no curative value; it is a symptomatic treatment that sup-
presses pain sensation that would otherwise serve as a protective mechanism on the out-
come of the clinical process. As such, it is used only to of
er a better quality of life for
people with pain when the source of the pain cannot be treated. It must be noted, however,
that none of the studies have proven conclusive, and there is still quite a bit of debate about
the degree to which TENS is more e
ff
ective than placebo in reducing pain. Generally,
TENS provides initial relief of pain in about 70% of patients, but the success rate decreases
after a few months to around 25%.
The most accepted theories of how TENS may work are:
ff
•
Gate control theory:
suggests that by electrically stimulating sensory nerve receptors,
a gate mechanism is closed in a segment of the spinal cord, preventing pain-carrying
messages from reaching the brain and blocking the perception of pain.
•
Endorphin release theory:
suggests that electrical impulses stimulate the production
of endorphins and enkaphalins in the body. These natural, morphinelike substances
block pain messages from reaching the brain, in a manner similar to conventional
drug therapy but without the danger of dependence or other side e
ects.
•
Descending inhibitory pathway theory:
suggests that noxious stimuli excite the smaller
pain
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fibers, leading to activation of the brainstem reticular formation. This releases sero-
tonin (a neurotransmitter related to the feeling of well-being), which in turn inhibits
pain at the spinal cord level. Putting it shortly, “pain inhibits pain.”
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The practitioners of quack treatments credit TENS with restoring “energy lines” and
stimulating “acupuncture points.” Don't waste your time considering such claims.
The circuit for a battery-powered two-channel TENS unit is shown in Figure 7.19. In
this circuit, timer IC1 produces a pulse every time the stimulation channels need to be trig-
gered. The rate at which the trigger pulses are generated is set via R1. With the component
values shown in the circuit, trigger frequency can be varied between 3 and 100 Hz. Con-
stant pulsing at a set frequency is known in TENS jargon as the
conventional mode
.
What is commonly known as
conventional TENS
is to stimulate at a relatively high fre-
quency (40 to 150 Hz) and low current (10 to 30 mA), barely above the threshold of sen-
sation. The reported onset of analgesia with these parameters is virtually immediate. Pain
relief lasts while the stimulus is turned on, but it usually returns when the stimulation is
turned o
. Patients often leave the electrodes in place all day and turn stimulation on for
30 to 60 minutes whenever they need it. In the low-frequency conventional mode, the
TENS unit is set to deliver pulses at 1 to 10 Hz, but the stimulus intensity is increased close
to the patient's tolerance limit. The belief is that this will increase the time of analgesia
after therapy because of the release of natural morphinelike substances. This stimulation
mode is uncomfortable, and not many patients can tolerate it.
Burst TENS
uses low-intensity stimuli
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firing in high-frequency bursts. Each burst deliv-
ers some 5 to 10 pulses at a rate of 100 Hz, with rests between bursts. Patients usually
report that each burst is felt as a single pulse, making it more comfortable than single-pulse
sensation. No particular advantage has otherwise been established for the burst method
over the conventional TENS method. The TENS circuit of Figure 7.19 implements the
low-frequency burst mode when SW2 connects the reset line of timer IC3. This IC3 peri-
odically inhibits IC1 from oscillating. The fundamental burst frequency is approximately
2 Hz, which allows approximately seven trigger pulses to be issued by IC1 per burst cycle.
Ramped modulation is a feature that is often encountered in TENS units to cause a grad-
ual rise and decline of amplitude and frequency, resulting in a comfortable stimulation sen-
sation. It is very similar to the conventional mode, but a modulation in pulse rate and
amplitude is thought to avoid nerve adaptation and accommodation, which diminishes the
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