STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 340

e ectiveness of conventional TENS. When the circuit is operating in the conventional

mode, modulation can be employed by closing SW3. This allows a triangle wave generated
by timer IC2 to be fed to the control voltage input of IC1. The modulating signal frequency
is approximately 1 Hz, which induces a

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40% change in the selected conventional-mode
frequency. Note that when modulation is enabled, the timing characteristics of the trigger
pulses generated by IC1 change such that pulse amplitude is modulated to 25% on each

modulation cycle.
The electrodes are driven by step-up transformer T1. A stimulation pulse is generated
every time Q5 is driven into conduction by the Darlington pair Q2 connected to the output
of IC1 via ac-coupling capacitor C2. LED D6

flashes each time that a stimulation pulse is
delivered. The constant-current amplitude is set via R9, which controls the current that Q1
allows across the primary of the step-up transformer. The peak current of a pulse into a
purely resistive load of 500

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can be varied between 0 and 150 mA. The shape of the pulse
delivered to the skin electrodes is shown in Figure 7.20. The load used to simulate the body
impedance is the one speci

Ω

ed in the American National Standard for Transcutaneous
Electrical Nerve Stimulators (ANSI/AAMI NS4-1985). The preferred waveform is bipha-
sic, to avoid the electrolytic and iontophoretic (whereby ions and charged molecules can
be driven through the skin by an electrical current) e

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ects of a unidirectional current.
Power for the stimulator is controlled independently via the potentiometer switches
SW1 of each channel. When either channel is on, current supplied by the four alkaline bat-
teries in series is delivered to the timer ICs via diode D1 of the active stimulation channel.
Each output channel is isolated from the other, which allows two distinct areas of pain to
be stimulated independently.
TENS electrodes are usually placed initially on the skin over the painful area, but other
locations, such as over cutaneous nerves, may give comparable or even better pain relief.
TENS should not be applied over the carotid sinuses, due to the risk of acute hypotension
(because of stimulation of the vagus nerve), over the anterior neck because of possible
spasm of the larynx, or over an area of sensory impairment where the current could burn
the skin without the patient becoming aware of it. Of course, TENS should not be used in
patients with any active implantable medical device (e.g., pacemakers and implantable
de

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brillators) because of the risk of interfering with or damaging the implantable device.
In addition, TENS should not be used during pregnancy because it may induce premature
labor.
A relatively new TENS-like modality is percutaneous electrical nerve stimulation
(PENS), which is often incorrectly called electroacupuncture . Rather than using surface
electrodes, PENS uses needle probes as electrodes, placed just under the outer layers of
the skin in the region where the patient feels pain. The only advantage of PENS over TENS
is that it bypasses local skin resistance and delivers electrical stimuli at the precise level
desired, in close proximity to the nerve endings.

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Interferential Stimulation

Stimulating deep tissues using surface stimulation electrodes requires that very strong cur-
rents be delivered to the skin to yield su ciently high currents to depolarize target tissue.

Strong pulses are often painful, limiting their clinical applicability, especially when elec-
trical stimulation is used for therapeutic purposes (e.g., TENS, or for the stimulation of
deep muscles such as those of the pelvic floor). As shown in Figure 7.21, interferential cur-

rent therapy (IFC) is based on the summation of two ac signals of slightly di

fl

erent
frequency that are delivered using two pairs of electrodes. Each of the few-kilohertz “car-
riers” on their own do not cause skin sensations or stimulation of the underlying tissues.
However, the tissue causes the signals to mix or interfere with each other, resulting in a
low-frequency current that consists of cyclical modulation of amplitude, based on the

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