Learn a few tips and clear up any confusion on how to get the best results when using interferential therapy

Interferential therapy, in its basic form, has been around for more than 40 years. Recently, advancements in electronics and manufacturing capabilities have made improvements possible in these devices. Even those individuals who are knowledgeable in therapy and electronic terminology can get confused. Terms such as constant current versus constant voltage, true versus premodulated interferential, and premodulated bipolar or premodulated interferential are confusing to many clinicians.

The Third Degree
Hans Nemec, MD, an Austrian physician, proposed that by crossing two slightly different medium frequency currents within the tissue, a third frequency current of greater intensity is created in the deeper tissue.

As an example, a frequency of 4,000 Hz interfering with another frequency of 4,080 Hz creates a third current of 80 Hz. This is caused by the in-phase and out-of-phase relationship of the two original currents as they alternate from positive to negative polarity.

The third current, referred to as the beat frequency becomes the actual therapeutic frequency. One output of the unit is a constant 4,000 Hz, while the second output is adjustable from perhaps 4,001 Hz to as much as 4,250 Hz. This form of interferential therapy has become known as true interferential or frequency difference interferential.

Frequency versus premodulated. A second method of creating the interference effect has been developed in recent years and is known as premodulated interferential. With this method, both outputs of the unit provide a carrier frequency of 4,000 Hz. However, each output has the ability to premodulate or burst the frequency within the unit. It is important that the unit has the capability of perfectly synchronizing these bursts in the same polarity, at the same time in order to create premodulated interferential.

Units capable of premodulation are not necessarily premodulated interferential. They may only provide premodulation for the purpose of bipolar (two electrodes) stimulation. When considering the relative merits of these two methods, many clinicians have noted that while both create the interferential effect, there may be some advantages to the premodulated technique.

Since the true interferential provides an uninterrupted, constant 4,000 Hz frequency to the tissue, a condition known as Widensky Inhibition (depolarization of the nerve fibers) can occur beneath the electrodes. This will create a numbness and what will be perceived by the patient as a reduction in the intensity of the current. With premodulated interferential, however, since the current is being burst inside the unit itself, Widensky Inhibition does not occur and a larger treatment area is established with the actual therapeutic frequency.

Constant current versus constant voltage. Electrotherapy devices may be of either a constant current or constant voltage design. Simply put, constant current is better. With constant voltage units, the output (milliamps) will tend to creep upward as tissue resistance to the current breaks down. With constant current units this does not occur as the current always remains the same regardless of the resistance.

Let us compare the flow of electrical current through tissue to the speed of an automobile on an icy street. The actual movement of the automobile is the output in milliamps, the reading on the speedometer is the voltage, and the icy street represents the resistance to current flow through the tissue.

As the accelerator is pressed, the speedometer reflects the rate at which the wheels are turning. However, since the street is icy, the actual speed of the automobile is less than what is shown on the speedometer. A speedometer reading of 60 mph may only generate 15 mph of actual movement.

If we wish to travel at a rate of 25 mph, we would have to reduce the pressure on the accelerator as our tires gradually increased traction and we increased our actual speed. Otherwise, we would eventually reach a speed of 60 mph.

With electrical current, poor contact of electrodes, faulty electrodes, or faulty lead wires may result in greater resistance to the flow of current (icy street). By increasing the output of the device, the voltage (speedometer reading), not shown on the face of the device rises rapidly. The milliamps of output that is shown on the device may indicate 15 mA (actual miles per hour). As tissue resistance is reduced, either by the pressing down on an electrode or merely prolonged flow of current through the tissue, the milliamps may escalate similar to the miles per hour the automobile is actually traveling as the tires get a better grip on the icy street.

With a constant voltage unit, the delivered milliamps will increase because the voltage must remain the same (keeping the accelerator pressed down to maintain the speedometer at 60 mph). With a constant current device, the delivered milliamps will remain the same and the voltage will be reduced automatically by the device (the reduced pressure on the accelerator to keep the actual speed of the automobile at 25 mph).

What this all means is that the milliamps of output will always remain at the level you set with a constant current device, even if you press on the electrodes to increase contact or if tactile tissue resistance breaks down over the treatment time.

Treatment frequencies. While frequency ranges vary with manufacturers, basic therapy ranges are fairly consistent. Frequencies that vary from approximately 80–120 Hz are considered most effective for acute pain, while lower frequencies of 3–5 Hz or 2–10 Hz are preferred for chronic pain treatment. Some units feature a nerve-block setting where both channels produce an output of 4,000 Hz to create an interferential nerve block to quickly block out acute pain. Most clinicians prefer settings of 1–15 Hz for treating acute edema. Some newer devices even allow for setting up to two frequency ranges to be administered back to back during the same treatment session.

Treatment time. When treating acute pain with the 80–120 Hz setting, interferential therapy will provide a release of enkephalin with a treatment time of 10–12 minutes. Chronic pain requires 15–20 minutes of the 3- to 5-Hz setting to provide release of beta-endorphins. Nerve-block techniques at 4,000 Hz normally requires 10 minutes or more, depending on the size or the area to be treated.

Intensity of current. Interferential therapy provides a comfortable, soothing stimulation and should never be strong enough to cause any discomfort to patients. Higher intensities should not be considered better as far as obtaining results. Once the comfort level is established at the onset of therapy, the intensity should not be increased during the treatment. This could cause overstimulation of the tissue and even minor burns, particularly with a unit that produces true interferential due to the Windensky Inhibition effect.

Contraindications and precautions. Interferential therapy is considered a safe modality when used properly for appropriate conditions. Most manufacturers list similar contraindications and precautions, most of which are the same as other electrotherapy devices. It is always recommended that clinicians review each manufacturer’s contraindications and precautions statements prior to treatment with any device.

The Legal Limit
Doctors are often left in the dark as to the legality of using therapy devices. Interferential and other electrotherapy devices must be properly registered with the Food and Drug Administration to be legally sold and/or used in the United States. Potential purchasers should request proof of this registration prior to buying the equipment. Request a copy of a manufacturer’s FDA Section 510(k) notification complete with date and 510(k) reference number. If unsure about the legality, consult with your malpractice insurance carrier. CP

Suzi Plank is a contributing writer for Chiropractic Products. During the past 15 years, she has provided technical and management services to health care providers, specializing in radiology and neurology.