STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 341

1

CH1

Voltage

GND

OSCILLOSCOPE

GND

Current

1

CH 2

TENS UNIT

TENS CURRENT WAVEFORM

5

.

1

200, 500, 1k

ELECTRODE 1

6

0.1uF

2.7k

300us 60us

4

ELECTRODE 2

8

.

1

1/10 A

STEPUP TRANSFORMER

TENS TEST LOAD
per ANSI/AAMI NS4 - 1985

1/2 A

A = 0 to 150mA

20us

Figure 7.20 Stimulation pulse delivered to the skin electrodes by the TENS generator of Figure 7.19. The American National Standard for
Transcutaneous Electrical Nerve Stimulators (ANSI/AAMI NS4-1985) specifies the load used to simulate the body impedance.

di erence in frequency between the two carrier signals. When the signals are in phase, the

low-frequency components sum to an amplitude that is su

ff

cient to stimulate, but no stim-
ulation occurs when they are out of phase. The beat frequency of IFC is equal to the
di erence in the frequencies of the two carrier signals. For example, the beat frequency,

and hence the stimulation rate of an interferential unit with signals set at 4 and 4.1 kHz, is
100 Hz.
The interferential method is used most often for the therapeutic stimulation of nerves
and muscles in the treatment of acute pain, edema reduction, and muscle rehabilitation. It
is not a common modality for functional neuromotor stimulation because it requires
greater energy consumption and a larger number of electrodes. Scientists in the former
Soviet Union also use interferential currents delivered through scalp electrodes to produce
narcosis (electronarcosis) and anesthesia (electroanesthesia). This last application, which
does not involve causing convulsions as with ECT, is very controversial and seldom used
in Western psychiatry.
IFC stimulators commonly use carrier frequencies around 4 to 5 kHz, with sinusoidal
waveforms that can reach peak-to-peak voltages of 150 V and force currents of up to 100 mA

ff