DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

344 STIMULATION OF EXCITABLE TISSUESThe exact frequency difference and phase relationship between the carrier signals has atremendous effect on the site inside the body at which the differential stimulation will takeplace. For this reason, the majority of audio signal generators used in modern IFC stimulatorsare based on direct-digital synthesizers (see Chapter 6). The simplest way to drivethe experimental IFC stimulator circuit of Figure 7.22 is to use the PC sound card and softwarethat 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 RutgersUniversity, a copy of which is supplied for your convenience in the book’s ftp site.Although this program is not designed specifically for IFC stimulation, 6 it generates twosine waves of different frequencies through the left and right sound card channels thatcan be used to drive the isolated high-voltage output stages. If you are a Matlab user, youcan devise complex frequency and phase shifts between the carriers. The data streams canthen be played using the sound(y, Fs) command, where y is an N 2 matrix thatcontains the sine-wave data to be played through each one of the sound card channels andFs is the sampling frequency.The risks associated with interferential stimulators is much larger than those fordevices that deliver narrow stimulation pulses. The reason is that the carrier signals areconstantly on and convey quite a bit of power through the tissue. IFC stimulators, especiallythose that do not have electrode impedance monitoring with automatic turn-off,require special attention to electrode selection, placement, and maintenance. If you everuse or build an IFC stimulator, first connect a 2-kΩ 1 -W resistor between one of the electrodepairs. Crank the power up until the resistor bursts into flames. This is exactly what4will 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 onthe edges of the electrodes as well as on the skin will occur if electrodes are touching duringstimulation. Even though the electrodes may appear to be far enough apart, rememberthat when a muscle comes into contraction, it can bring the electrodes together. Inaddition, if you use carbon-loaded silicone electrodes, always remember to apply andmaintain a sufficient amount of an appropriate conductive medium, such as water-soakedsponges or especially formulated electrotherapy gel to prevent burns during stimulation.Finally, never even consider passing IFC currents through the brain. Remember the “thisis your brain on drugs” advertising? Well, misapplication of IFC takes the fried-egg analogyone notch up on the realism scale.General Safety Precautions and Contraindications forTranscutaneous Electrical Stimulation TherapiesAll transcutaneous electrical stimulation devices (e.g., TENS, EMS, IFC, as well as iontophoresisunits) are classified by the FDA as either class II or class III medical devices 7and require physician prescription to be dispensed to or used on patients. The fact thatsome of these devices are sold without a prescription by unscrupulous online vendors maymake a lot of people believe that they cannot do any damage. However, most electricalstimulation therapies carry significant risks with them. The following should be consideredbefore using any form of transcutaneous electrical stimulator:6The brain is able to combine two pure tones, each sent to a separate ear, to produce a beat tone at the differencefrequency. Some researchers believe that this binaural beat effect can be exploited to affect brain states positivelyusing difference frequencies related to those of the alpha, beta, and theta brainwaves. Even if it would be demonstratedthat the brain can be trained to generate specific frequencies, whether or not altering the brainwaves hasany effect on mind, body, or mood is subject to considerable debate.7For the FDA’s definition of class II and class III medical device, see the Epilogue.