Biomedical Engineering Reference
In-Depth Information
The exact frequency di
ff
erence and phase relationship between the carrier signals has a
tremendous e
erential stimulation will take
place. For this reason, the majority of audio signal generators used in modern IFC stimu-
lators are based on direct-digital synthesizers (see Chapter 6). The simplest way to drive
the experimental IFC stimulator circuit of Figure 7.22 is to use the PC sound card and soft-
ware that is freely available for download from the Web, such as the beat-tone generator
(BEATINBRAIN.EXE, freeware for Windows 9x, 2000, NT) by the Physics Lab of Rutgers
University, a copy of which is supplied for your convenience in the topic's ftp site.
Although this program is not designed speci
ff
ect on the site inside the body at which the di
ff
fi
cally for IFC stimulation,
6
it generates two
sine waves of di
erent frequencies through the left and right sound card channels that
can be used to drive the isolated high-voltage output stages. If you are a Matlab user, you
can devise complex frequency and phase shifts between the carriers. The data streams can
then be played using the
sound(y, Fs)
command, where y is an
N
ff
2 matrix that
contains the sine-wave data to be played through each one of the sound card channels and
Fs is the sampling frequency.
The risks associated with interferential stimulators is much larger than those for
devices that deliver narrow stimulation pulses. The reason is that the carrier signals are
constantly on and convey quite a bit of power through the tissue. IFC stimulators, espe-
cially those that do not have electrode impedance monitoring with automatic turn-off,
,
require special attention to electrode selection, placement, and maintenance. If you ever
use or build an IFC stimulator,
ff
4
-W resistor between one of the elec-
trode pairs. Crank the power up until the resistor bursts into
fi
rst connect a 2-k
Ω
1
flames. This is exactly what
will happen to skin if electrode contact is poor. As such, when placing electrodes for IFC,
it is imperative that they are not touching and will not touch each other, since burns on
the edges of the electrodes as well as on the skin will occur if electrodes are touching dur-
ing stimulation. Even though the electrodes may appear to be far enough apart, remem-
ber that when a muscle comes into contraction, it can bring the electrodes together. In
addition, if you use carbon-loaded silicone electrodes, always remember to apply and
maintain a su
fl
cient amount of an appropriate conductive medium, such as water-soaked
sponges or especially formulated electrotherapy gel to prevent burns during stimulation.
Finally, never even consider passing IFC currents through the brain. Remember the “this
is your brain on drugs” advertising? Well, misapplication of IFC takes the fried-egg anal-
ogy one notch up on the realism scale.
General Safety Precautions and Contraindications for
Transcutaneous Electrical Stimulation Therapies
All transcutaneous electrical stimulation devices (e.g., TENS, EMS, IFC, as well as ion-
tophoresis units) are classi
ed by the FDA as either class II or class III medical devices
7
and require physician prescription to be dispensed to or used on patients. The fact that
some of these devices are sold without a prescription by unscrupulous online vendors may
make a lot of people believe that they cannot do any damage. However, most electrical
stimulation therapies carry signi
fi
cant risks with them. The following should be considered
before using any form of transcutaneous electrical stimulator:
fi
6
The brain is able to combine two pure tones, each sent to a separate ear, to produce a beat tone at the di
ff
erence
frequency. Some researchers believe that this binaural beat e
ff
ect can be exploited to a
ff
ect brain states positively
using di
erence frequencies related to those of the alpha, beta, and theta brainwaves. Even if it would be demon-
strated that the brain can be trained to generate speci
ff
fi
c frequencies, whether or not altering the brainwaves has
any e
ect on mind, body, or mood is subject to considerable debate.
7
For the FDA's de
ff
fi
nition of class II and class III medical device, see the Epilogue.
Search WWH ::
Custom Search