Electrode 2

Ui Ut VO

Figure 7.19 Circuit of a battery-powered, two-channel TENS unit. IC1 produces a pulse every time the stimulation channels need to be triggered. The trigger frequency can be varied between 3 and 100 Hz. Burst TENS is activated when SW2 connects the reset line of timer IC3 periodically to inhibit IC1 from oscillating. Modulated TENS is enabled by closing SW3, which allows a triangle wave generated by IC2 to modulate the frequency of IC1. The electrodes are driven by step-up transformer Tl. Stimulation current is controlled via R9, which sets 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 Q can be varied between 0 and 150mA.

effectiveness 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 -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 Q 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 specified 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) effects 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 batteries 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 defibrillators) 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.

Interferential Stimulation

Stimulating deep tissues using surface stimulation electrodes requires that very strong currents be delivered to the skin to yield sufficiently high currents to depolarize target tissue. Strong pulses are often painful, limiting their clinical applicability, especially when electrical 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 current therapy (IFC) is based on the summation of two ac signals of slightly different frequency that are delivered using two pairs of electrodes. Each of the few-kilohertz "carriers" 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

Water Sloset Area Required
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.

difference in frequency between the two carrier signals. When the signals are in phase, the low-frequency components sum to an amplitude that is sufficient to stimulate, but no stimulation occurs when they are out of phase. The beat frequency of IFC is equal to the difference 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

Electrodes

Electrodes

Muscle Stim Ifc

Figure 7.21 Interferential current therapy (IFC) is based on the summation of two ac signals of slightly different frequency that are delivered using two pairs of electrodes. Each of the few-kilohertz "carriers" 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 beat current capable of stimulating the tissue.

Figure 7.21 Interferential current therapy (IFC) is based on the summation of two ac signals of slightly different frequency that are delivered using two pairs of electrodes. Each of the few-kilohertz "carriers" 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 beat current capable of stimulating the tissue.

RMS. The most common beat-frequency therapy ranges are 1 to 10 Hz for edema, 1 to 150Hz for rehabilitation of muscle, and 80 to 150 Hz for the control of pain. Figure 7.22 shows a simple circuit that can generate interferential audio-frequency currents. The power amplifier sections are based around the ST Microelectronics TDA2005 10W + 10 W audio amplifier IC. This IC is intended specifically for use in bridge amplifier designs in car audio systems. It is a class B dual audio power amplifier with a high output current capability of up to 3.5 A, ac and dc output short-circuit protection (one wire to ground only), and thermal shutdown protection, and is capable of driving very inductive loads. Although it can be used as a dual amplifier by operating each half of the device separately, in this application it is configured for operation as a bridge amplifier. With this mode, the current and voltage swings in and around the IC are twice that of a single amplifier, which results in a power output four times greater than that of a single amplifier with the same load connected to the output pins.

Two audio-output transformers are used in reverse to step up the amplifier's output voltage all the way up to the 150 Vpp necessary to push up to 100mA RMS through the body. We used Hammond 1615 audio-output transformers in reverse; that is, the 8-Q speaker outputs were connected to the outputs of the power amplifiers, and the 5-kQ inputs were connected to the electrodes. As with all bridge amplifier designs powered from a single supply, both output terminals of the TDA2005 are held at half the supply voltage, which eliminates the need for the usually large-value dc blocking capacitor in series with the output transformers. The power amplifiers are driven through audio isolation transformers. The complete circuit should be powered from an IEC601-compliant power supply rated for 12 V at 5 A or more, for example using a Condor model MD12-6.8-A medical-grade linear supply.

Ems Stimulation Amplifier

Figure 7.22 An experimental interferential-mode stimulator uses two audio-output transformers as step-up transformers. The primaries are driven by bridged audio amplifiers at earner frequencies around 4kHz with peak-to-peak voltages of 150V and force currents of up to 100mA RMS. Audio signals are generated by the PC sound card running a beat-tone generator program. The most common beat-frequency therapy ranges are 1 to 10 Hz for edema, 1 to 150 Hz for rehabilitation of muscle, and 80 to 150 Hz for the control of pain.

Figure 7.22 An experimental interferential-mode stimulator uses two audio-output transformers as step-up transformers. The primaries are driven by bridged audio amplifiers at earner frequencies around 4kHz with peak-to-peak voltages of 150V and force currents of up to 100mA RMS. Audio signals are generated by the PC sound card running a beat-tone generator program. The most common beat-frequency therapy ranges are 1 to 10 Hz for edema, 1 to 150 Hz for rehabilitation of muscle, and 80 to 150 Hz for the control of pain.

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The exact frequency difference and phase relationship between the carrier signals has a tremendous effect on the site inside the body at which the differential stimulation will take place. For this reason, the majority of audio signal generators used in modern IFC stimulators 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 software 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 book's ftp site. Although this program is not designed specifically for IFC stimulation,6 it generates two sine waves of different 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 X 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, especially 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, first connect a 2-kQ j-W resistor between one of the elec-

trode pairs. Crank the power up until the resistor bursts into 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 during stimulation. Even though the electrodes may appear to be far enough apart, remember 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 sufficient 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 analogy 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 iontophoresis units) are classified by the FDA as either class II or class III medical devices7 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 significant risks with them. The following should be considered before 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 difference frequency. Some researchers believe that this binaural beat effect can be exploited to affect brain states positively using difference frequencies related to those of the alpha, beta, and theta brainwaves. Even if it would be demonstrated that the brain can be trained to generate specific frequencies, whether or not altering the brainwaves has any 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.

• Cardiac demand pacemakers that detect a user's heart rate and turn on a pacemaker when the heart rate falls below a predetermined level can in certain circumstances be affected by stimulation. This is because the pulses from the stimulator may be confused with the heart's intrinsic signals and fool the pacemaker into thinking that the heart is beating faster than it is. A pacemaker or implantable defibrillator should be considered to be an absolute contraindication to upper limb and shoulder stimulation. Electrical stimulators can sometimes be used with caution for lower limb applications in these patients as long as the action of the pacemaker/defibrillator and heart is checked by a cardiologist while the stimulator is in use.

• Electrical stimulation should be used with extreme care in patients with any other type of active implantable medical device, such as a spinal cord stimulator or intrathe-cal pump. This is because electrical stimulation currents can interfere with the operation or even damage the circuitry of an implanted device. In addition, electrical stimulators should not be used over metal implants, whether active (pacemakers, spinal cord stimulators, etc.) or passive (orthopedic nails, metal plates, prosthetic joints, etc.) because these will distort the flow of current through the body and may cause internal hot spots with high current density that can destroy adjacent tissues.

• Electrical stimulation should not be applied over the eyes or the carotid sinuses (side of neck), due to the risk of acute hypotension through a vasovagal reflex.

• Electrical stimulation electrodes should not be placed over the anterior neck because of possible laryngospasm due to laryngeal muscle contraction.

• Electronic equipment such as ECG monitors, ECG alarms, sleep apnea monitors, and so on, may not operate properly when electrical stimulators are in use.

• Electrical stimulation should not be used during pregnancy because it may induce premature labor.

• With the exception of electrical stimulation that is specifically intended for the stimulation of the brain, electrodes should never be placed to cause current to flow tran-scerebrally (through the head), as this may induce seizures and have other undesirable neurological consequences.

• With the exception of electrical stimulation that is specifically intended for stimulation of the heart, electrodes should never be placed to cause current to flow across the chest (e.g., both arms simultaneously) because electrical stimulation currents may cause or lead to arrhythmic events. For the same reason, patients should be warned never to handle the electrodes while a stimulator is on since a current path through the heart could be created by accident.

• There are rare anecdotal reports that people who have poorly controlled epilepsy have had symptoms increased after using electrical stimulation. There is no known mechanism for this effect, but our advice is that electrical stimulators should not be used in patients with epilepsy that is not well controlled by drugs.

• Because electrical stimulation (especially EMS and IFC) will increase local blood circulation, it is possible that stimulation in the area of a malignant tumor might increase the rate of metastasizing and therefore the spread of the cancer. Electrodes should never be placed over the area of a known tumor.

• Long-term stimulation at the same electrode site may cause skin irritation through possible allergic reaction to tape or gel. Poor skin condition can be a problem when self-adhesive skin electrodes are used. This is because there is a greater chance of skin irritation. Electrodes should never be placed over broken skin or over rashes, blisters, spots, and so on.

• Electrical currents used in some modes of electrical stimulation (e.g., IFC) are large enough that they may cause skin burns under the electrodes. For this reason, the electrodes should not be placed in an area of sensory impairment (e.g., in cases of nerve lesions, neuropathies) where the possibility exists that the patient would not feel that the skin is being burned.

• People who have a spinal cord injury may be subject to episodes of autonomic dysreflexia. This is characterized by a rise in blood pressure elicited by a noxious stimulus such as electrical stimulation (e.g., FNS) applied below the level of the lesion.

• Electrical stimulators should not be used while driving or operating dangerous machinery.

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