Electrical Stimulation Therapy

The history and origin of electrostimulation, also commonly referred to as electrotherapy, is unique. The therapeutic benefits of electricity were not discovered in a laboratory or clinic and were not a byproduct of someone's accidentally coming into contact with a bolt of lightening. In fact, electrotherapy originates as early as 400 BC from contact with the torpedo fish, which could produce electric shocks between 100 and 150 volts. Taken live from lakes and streams and placed on a painful area of the body, the torpedo fish produced a series of electric shocks that reduced and controlled pain.

In the mid 1700s the development of the Leyden Jar, which is a predecessor to the battery, provided the capacity to store electricity. As a result, physicians had more control over where, when, and how much current could be applied for therapeutic use. The advancement of the battery in the 1800s further developed electrotherapy, and in the latter half of the nineteenth century most physicians in America possessed at least one electrical stimulator. However, as with any new medical technology, electrotherapy was not readily accepted. This skepticism resulted in a decline of interest in electrotherapy toward the end of the [nineteenth] century (eMedicine Clinical Knowledge Base, 1996).

In 1965 electrotherapy regained its popularity when the gate control theory of pain was introduced. This theory proposed that pain perception depends on the balance of large- and small-diameter nerve fiber activity and that an increase in large nerve fiber activity can potentially "close the gate" to information going to the brain from small pain fibers. When the gate is closed, the transmission of pain signals to the brain is blocked.

Clinical evidence came in 1967 by Wall and Sweet, who reported that electrical nerve stimulation provided successful relief of chronic pain. Initially, electrodes had to be surgically implanted but it was discovered that current could be sent directly through the skin, eliminating the need for surgery. This therapeutic effectiveness in pain relief has led to other applications of electrotherapy by rehabilitative clinicians, including treating injured or diseased muscle and other soft-tissue conditions (Gersh, 1992).

This course reviews cell physiology and the response of muscle fibers to electrical stimulation, and presents the principles of electrical stimulation to aid the healthcare professional in decisions regarding indications and clinical applications.

THE PHYSIOLOGY UNDERLYING ELECTROSTIMULATION

Excitable Cell Membranes

The major therapeutic uses of electricity derive from muscle contractions or sensory stimulation or a combination of both, so it is important to review the general physiological effects of electricity on nerve and muscle tissues. Nerves and muscles are both excitable tissues, and this excitability is dependent on permeability of the cell membrane. The nerve or muscle-cell membrane regulates the interchange of substances from inside and outside the cells.

This cell permeability is voltage-sensitive, producing an unequal distribution of charged ions on either side of the cell membrane, which in turn creates a difference in electrical charge between the interior and exterior of the cell. When this charge occurs, the membrane is considered to be polarized. The potential difference between the inside and outside charge is known as the resting potential because the cell tries to maintain this difference in electrical charge as its normal homeostatic environment.

There is a greater concentration of diffusible positive ions outside the membrane than within it. The cell continuously moves positively charged sodium from inside to outside, and balances this by moving negatively charged potassium to the inside through a mechanism called active transport. A higher concentration of potassium occurs inside the cell, but the overall charge difference produces an electrical gradient with positive charge outside and negative charges inside (Gersh,1992).

Nerve and Muscle Fibers

The nervous system is subdivided into the central nervous system (CNS) and peripheral nervous system. The CNS comprises the brain and spinal cord.

The peripheral nervous system consists of nerves emerging from the brain (cranial nerves) and from the spinal cord (spinal nerves). Spinal-nerve fibers of the peripheral nervous system that convey conscious and unconscious information such as pain are termed afferent fibers.

Spinal-nerve fibers that control voluntary muscles are efferent fibers. Both the peripheral afferent input and central efferent activity are of interest to the physical therapist employing electrical stimulation (Noback, 1991).

When an electrical stimulation system is applied to the soft tissues of an extremity, the system provides the force to induce ion movement into the tissues and create an action potential. At the positive electrode (anode), positive ions are repelled and negative ions are attracted. At the negative electrode (cathode), the negative ions are repelled and positive ions are attracted. As the charged ions move across the nerve-fiber membranes beneath the anode and cathode, membrane depolarization occurs.

The cathode is usually the area of the depolarization and, as a concentration of negatively charged ions increases, the membrane's voltage potential becomes low and is brought toward the threshold of depolarization. The anode makes the nerve cell membrane more positive, increasing the threshold necessary for depolarization. In terms of muscle excitation, a twitch muscle contraction results. This contraction, initiated by electrical stimulus, is the same as a twitch contraction coming from voluntary efforts or activity (Prentice, 2001).

THE PHYSICS OF ELECTROSTIMULATION

Alternating and Direct Current

Electrotherapeutic devices used in rehabilitation generate two different types of current that, when introduced into biological tissues, are capable of producing specific physiologic changes. These two current types are referred to as alternating and direct current. In alternating current, the electrons constantly change directions, reversing its polarity. Electrons flowing in alternating current always move from the negative to positive pole, reversing direction when the polarities are reversed.

Conversely, direct current is a unidirectional flow of electrons toward the positive pole. However, on most modern direct-current devices, the polarity and thus the direction of current flow can be reversed. Electrotherapeutic devices are usually further classified as being either high-voltage generators or low-voltage generators. The high-voltage devices produce waveforms (the visual representation of the current or voltage) within an amplitude of 115 volts and are greater and of relatively short duration (less than 10 msec) (Gersh, 1992).

Pulsed Current

Pulsed current is the unidirectional or bidirectional flow of charged particles that periodically stop for a limited period of time before the next event. More specifically, a pulse is an isolated electrical event separated by a finite period of time from the next event. A constant current source is preferable to a constant voltage source for most physiologic applications (Gersh, 1992; Prentice, 2001).

Three types of current.

Galvanic Current

The terms galvanic current and direct current are often used interchangeably. Historically, the term galvanic has been used to describe an uninterrupted direct-current form. High-volt pulsed galvanic electrical stimulators are considered to be useful in acute injuries associated with major tissue trauma accompanied by bleeding or swelling. Their direct current creates an electrical field over the treated area that, theoretically, changes blood flow.

Connected to two pads, galvanic stimulation uses a positive pad that behaves like ice, causing reduced circulation to the area under the pad and an associated reduction in swelling, and a negative pad that behaves like heat, causing increased circulation and reportedly speeding healing (Gersh,1992).

Interference Current

Interference current is based on the summation of two alternating-current signals of slightly different frequency. This results in current having a recurring modulation of amplitude, based on the difference in frequency between the two signals.

TYPES OF ELECTRICAL STIMULATION

Iontophoresis

Iontophoresis, the process of increasing the penetration of drugs into the skin by application of an electric current, is commonly used by physical therapists for the purpose of delivering anti-inflammatory medications such as corticosteroids. The groundwork for iontophoresis dates back to the early 1900s, with initial scientific experiments performed by a researcher named LeDuc.

The majority of units consist of a compact phoresor that operates with a 9-volt battery and two wire leads, each connected to an electrode. One electrode is the drug-delivery electrode intended for the anti-inflammatory, and the other is used as a dispersive electrode charged opposite to the anti-inflammatory ion. When the electrodes contain solutions of ions, negatively charged anions are repelled from the cathode into the body and positively charged cations are repelled into the targeted body area from the anode.

This effect is specific for ions of the same polarity as the electrode and, conversely, ions of the opposite polarity are not transferred into the body. Physical therapists use iontophoresis based on this penetration and distribution of ions primarily for controlling and reducing inflammation. This is applied while minimizing the systemic concentration caused by circulatory removal of the desired medication from the targeted area.

Two typical prerequisites for treatment with iontophoresis are that the medication must be charged (or modified to carry a charge) and that the inflammatory process be near the body surface (i.e. a superficial muscle or tendon rather than a deeper muscle tendon bursa) (Costello, 1995).

The effectiveness of the ion transport system remains controversial. For example, some researchers have proposed that all the material delivered through the skin with iontophoresis is removed by the subcutaneous circulation and circulated around the body, providing little if any local concentration to the intended region. Conversely, other researchers have shown with animal studies that ions and other substances do penetrate and do provide local concentration.

In the physical therapy setting, constant direct current has been commonly used in iontophoresis applications. However because of concern over pH changes, some researchers contend that a method of producing a more "consistent" constant current should be used to provide current while the skin resistance is changing. Because of potential skin charge accumulation and skin irritation due to pH changes, modulated currents have been used with success on laboratory animals. Pulsed currents have proved to be as effective or more effective in the delivery of small ions. Such studies indicate the need for physical therapists to consider and investigate the use of currents other than the traditional continuous monophasic current for iontophoresis.

Corticosteroids are the principal drugs used with iontophoresis in physical therapy because they have an anti-inflammatory effect and are relatively inexpensive. Dexamethasone is available in a somewhat more stable dissolved form and is therefore often used with iontophoresis. Some clinicians recommend treatments using a current of 4 mA for 10 minutes. This current is thought necessary to penetrate into the deeper tissues; however, treatment times greater than 10 minutes are less likely to achieve any greater tissue concentration due to circulatory removal of the medication.

Still other clinicians propose a current of 2.0 mA for 20 minutes for more superficial areas with a chronic inflammatory condition. More recent advances in this technology have introduced a disposable single-use iontophoresis system with an internal battery and current limiting circuitry. This method provides a constant drug delivery for an 80 mA-minute treatment and can deliver both negatively and positively charged drug ions. It operates at a low current and is worn for 24 hours to deliver the desired dose. The unit is designed to begin a treatment as soon as it is hydrated and applied to the skin, and stop the treatment at approximately 80 mA-minutes (Morris, 2003; Reena Rai, 2005).

Transcutaneous Electrical Nerve Stimulation (TENS)

Transcutaneous electrical nerve stimulation (TENS) is one of the most commonly used forms of electrostimulation for pain relief. Numerous clinical reports exist regarding the use of TENS for conditions such as low back pain, myofascial and arthritic pain, neurogenic pain, and postsurgical pain. The method of pain reduction produced by TENS is explained by the gate control theory proposed by Melzack and Wall in 1965. The "gate" between the level of the spinal cord and the pain centers of the brain usually is closed, thereby inhibiting constant nociceptive transmission by way of C fibers from the periphery to the T cell.

When painful peripheral stimulation does occur, the information carried by C fibers reaches the T cells and opens the gate, allowing pain transmission centrally to the thalamus and cortex, where it is interpreted as pain. Recall that the gate control theory postulated a mechanism by which the gate is closed again, preventing further central transmission of the nociceptive information to the cortex. The proposed mechanism for closing the gate is inhibition of the C-fiber nociception by impulses in activated myelinated fibers (eMedicine Clinical Knowledge Base, 1996; Gersh, 1992; Noback,1991).

A TENS unit consists of one or more electric signal generators, a battery, and a set of electrodes. The units are small and programmable, and the generators can deliver uninterrupted forms of stimuli with variable current strengths, pulse rates, and pulse widths. The preferred waveform is biphasic, which helps avoid the electrolytic and iontophoretic effects of a unidirectional current. A variety of newer transcutaneous or percutaneous electrical stimulation modalities are emerging as technology advances (Jarzem, 2005).

Interferential Current Therapy (IFC)

Interference current, utilized as interferential current therapy (IFC), is based on the summation of two alternating-current signals of slightly different frequency. This results in current having a recurring modulation of amplitude, based on the difference in frequency between the two signals.

When these signals are in phase, they sum to an amplitude sufficient to stimulate; when the signals are out of phase, no stimulation occurs. To determine the stimulation rate of an IFC unit, you must understand that the beat frequency of IFC is equal to the difference in the frequencies of the two signals. For example, the beat frequency and, hence, the stimulation rate of a dual-channel IFC unit with signals set at 3400 and 3300 Hz is the difference of 100 Hz.

Interferential current therapy can deliver high currents compared to other stimulators, and can use 2, 4, or 6 applicators, arranged in either the same plane for use on regions such as the back or in different planes in complex regions such as the shoulder (eMedicine Clinical Knowledge Base, 1996; Gersh,1992).

Neuromuscular Electrical Stimulation (NEMS)

Neuromuscular electrical stimulation (NMES) is the application of current to elicit a muscle contraction. The use of NMES in orthopedic and neuromuscular rehabilitation has grown significantly in recent years. A nerve action potential may be elicited either by a command originating in the motor cortex of the brain or by an electrically induced stimulus at the periphery. NEMS is addressed below under Stimulating Muscle Contraction.

Functional Electrical Stimulation (FES)

Functional electrical stimulation (FES) is another form of electrotherapy that utilizes electrical currents to activate the nerves that serve extremities affected by paralysis. This paralysis can be the result of a spinal cord injury, head injury, stroke or other neurologic disorders. The goal of FES is to help restore function in people with disabilities. The many possibilities for patient management using FES are beyond the scope of this course. However, a description of the use of FES for normalizing gait pattern in included under Stimulating Muscle Contraction, below.

TERMS USED IN ELECTRICAL STIMULATION

Voltage

The electrical force needed to produce a flow of electrons (current) is called a volt. The volt is a unit that describes the difference in the concentration of electrons between two points; the electrons cannot move unless such a difference exists. Voltage measures the potential energy of an electric field that results from the accumulation of electrons at one point in an electrical circuit and a corresponding shortage of electrons at another point in the circuit. If the two points are connected by a suitable conductor, the potential difference will cause electrons to move from an area of higher population to an area of lower population. Commercial current flowing from wall outlets produces either 115 volts or 220 volts. Electrotherapeutic devices used in injury rehabilitation are capable of modifying voltages.

Resistance and Impedance

The opposition to electron flow in a conducting material is referred to as resistance. Resistance is measured in ohms. The mathematical relationship among current, voltage, and resistance is current = voltage/resistance (Gersh, 1992 ; Hayes, 2000).

Impedance is the resistance the body's tissue to the passage of electrical current. Bone and fat are high-impedance tissues; nerves and muscles are low-impedance tissues. If a low-impedance tissue is located under a large amount of high-impedance tissue, the current may never become high enough to cause depolarization.

Waveforms

The term waveform refers to a graphic representation of the shape, direction, amplitude, and duration of the current being produced by the electrotherapeutic device. Both alternating and direct currents may take on a design of sine, square, or triangular waveform arrangement, depending on the capabilities of the electrostimulation device producing the current. The basic difference between alternating and direct current is that for each shape the alternating current reverses direction one time in each cycle; the direct current does not reverse direction. If the modality has capabilities of automatically reversing polarity, a direct current will elicit the same physiological response as an alternating current (Hayes, 2000).

Essential Parameters

Electrostimulation parameters for therapeutic applications are defined in terms of duration, strength, frequency, on-off time, rise-fall time, and polarity. Some parameters are time-dependent; pulsed current is described by special time-dependent properties of the pulse. The term phase describes the current moving in one direction for a predetermined period of time. The pulse waveform may be monophasic or biphasic. Monophasic refers to the current located on one side of the baseline, whereas biphasic current is present on both sides of the baseline.

Phase duration is the time elapsed from the beginning to the end of one phase. Pulse duration, also known as "pulse width," is the time elapsed from the beginning to the end of all phases in one pulse (Figure 2). The rise time measures the time for the leading edge of the phase to increase from the baseline to the peak amplitude of phase (Figure 2). The fall time is the time for the terminal edge of the phase to return to the zero baseline from the peak amplitude of the phase. Frequency is the repetition rate of the waveform expressed in pulses per second or cycles per second. Both alternating and pulse currents are described by frequency-dependent properties.

Graphs showing pulse duration and rise time.

For clinical purposes, pulse and alternating currents can be varied within a specific time frame. Pulse duration, phase duration, and frequency may also be modulated. Ramp, or surge, modulations are increases or decreases in the phase charges over time. A train is a continuous repetitive sequence of pulses or cycles of pulsed current. A burst is an interruption in a train separated by an inter-burst interval. The duty cycle is the ratio of one-time to total-time of trains. The duty cycles are generally expressed as a percentage. For example, if the pulse duration of the waveform is 25 msec and the period is 100 msec, the duty cycles are 25/100 or 25%.

Also of clinical importance is the intensity or strength of the electrical stimulus. Increasing the intensity or strength of the stimulus allows the current to reach more deeply into the tissue. The depolarization of more fibers is then accomplished by depolarizing higher threshold fibers within the range of the first stimulus. Additional fibers with the same threshold but located deeper to the structure are depolarized by deeper spread of the current (Gersh, 1992; Hayes, 2000).

USING ELECTROSTIMULATION DEVICES

The Electrodes

Electrodes are fabricated of electrically conductive material that is used to transfer electric charge to biological tissue. Similar to the electrodes of recording systems such as electromyography (EMG), therapeutic stimulating electrodes may be used on the skin surface or percutaneously; however, percutaneous electrodes are less commonly used in therapeutic electrostimulation for rehabilitation purposes.

The percutaneous type of electrode system comes into direct contact with body fluids and has lower impedance than surface electrodes. Surface electrodes normally require some type of skin preparation to reduce skin impedance, as well as the use of an electrolyte couple medium. An electroconductive gel applied between the electrode and skin serves to reduce skin impedance. More recent electrode designs are pre-packaged with both a conductive gel and an antibacterial component. The couple medium needs to be kept moist because skin irritation can occur in as many as 33% of patients, due partly to drying out of the electrode gel.

Electrode shapes can be round, oval, or rectangular and can also be custom fitted; sizes vary from 3 to 5 inches in diameter. With consistent electrode conductivity, current density is inversely proportional to the electrode size. While the contact area of the stimulating electrode decreases for specific current intensity, the current density increases. This phenomenon explains why the smaller electrode in a monopolar configuration is selected to be the active electrode, while the larger electrode serves as the dispersive or reference electrode. Because electrode size is directly proportional to the current level, larger electrodes have lower impedance—that is, for a given voltage a larger electrode provides a greater current level (Gersh, 1992; Hayes, 2000).

Placement Techniques

Guidelines for placing electrodes are the same no matter what protocol is used for stimulation of sensory nerves. These guiding principles are designed to help clinicians select the appropriate sites for electrode placement. The TENS applications use electrodes sized similarly to other electrostimulating devices; they are placed according to a pattern and moved about in a trial-and-error manner until pain is decreased.

Electrodes may also be placed directly on or around the painful area, and over specific dermatomes, myotomes, and sclerotomes corresponding to the painful region. Electrodes may also be placed near a spinal cord segment that innervates the area that is painful. Another option to the clinician is the peripheral nerves that innervate the painful area. The peripheral nerves can be stimulated by placing electrodes over sites in which the nerve becomes more superficial and is stimulated more easily.

The orientation of electrodes used in therapeutic electrical stimulating systems can be monopolar, bipolar, or quadripolar. When applying a monopolar orientation, the electrode of the stimulating circuit is placed over the target tissue and is referred to as the active electrode. This location is where the greatest effect is desired; a second, larger dispersive electrode is placed some distance from the active electrode.

In the bipolar configuration, a dispersive electrode is not required because two electrodes from one circuit are placed over the target tissue; one electrode is positive and the other is negative. In the quadripolar design, electrodes from two or more circuits are positioned so that currents intersect; this type of electrode placement may be used for interferential stimulation technique (Gersh, 1992; Hayes, 2000).

APPLICATIONS

Managing Pain

Electrical stimulating systems are commonly used in the treatment of acute and chronic pain. The most successful application of TENS continues to be the control of postoperative incision pain. It is most effective when the postoperative pain is confined to a small area, is well-defined, and is generally self-limiting as to time, course, and severity. Clinicians often expose patients to TENS during instruction about stimulus parameters to be used postoperatively. During this pre-operative instruction, attention should be given to the patient's own sensory requirements (personal feedback) regarding selection of stimulation parameters.

The concurrent use of analgesic medication to address postoperative pain may skew the need for TENS. In addition, medication history may be a compounding factor in evaluating the effect of TENS on postoperative pain. Researchers evaluating the effect of TENS on patients following a laminectomy noted that TENS was most effective for managing pain in the drug-inexperienced patients—described in the study as patients who had had no more than 2 weeks' narcotic medication in the 6 months prior to surgery. Patient cost benefits from the use of TENS include reduced narcotic intake and a potential decrease in incidence of depression. Early mobility, fewer instances of postoperative pulmonary complications, and, in some cases, a reduced length of stay in the ICU are also possible benefits of TENS.

Other electrical stimulating alternatives to conventional TENS for pain control include the application of interference current. Recall that IFC consists of waves of constant amplitude and slightly different frequencies from two independent low-voltage AC currents. The waves are superimposed, resulting in higher-amplitude wave produced secondary to the summation of current values at that specific point in time.

Some clinicians who advocate interference current for treatment of muscle pain contend that IFC penetrates deeper than other forms of TENS based on the rationale that skin impedance decreases in response to higher frequencies of alternating current.

Interferential current therapy may be applied in such manner that combined "beat" characteristics produce a stimulus that corresponds to those produced by brief, intense TENS. Setting the beat frequency between 100 and 120 bps produces beat durations of between 10 and 8 msec, respectively. By setting amplitude to the maximum tolerable output, a strong continuous tingling paresthesia combined with mild to moderate muscle facilitations is produced. Using this method of exciting motor and sensory nerve fibers at the same time, pain reduction can be induced within 15 minutes (ProMax, 2007).

Determining electrode sites for optimal stimulation when treating acute and chronic pain is not specifically documented. Electrode placement varies but it is commonly at the site of pain, adjacent to the spinal column at the spinal nerve root, or over the course of the peripheral nerve serving the painful region. The selection of preferred stimulation sites is made once the nature, location, and structural source of pain have been determined. The spinal-cord segments and peripheral nerves innervating that structure are identified on initial examination.

Preferred stimulation sites along the innervating structures may be located using motor points, trigger points, or acupuncture-point charts or tables, or by palpation and knowledge of the anatomy. Clinicians can also search for regions of high conductivity by holding one electrode of a TENS circuit and giving the patient the second electrode. After turning on the electrical current, clinicians places their index finger on the patient's skin overlying the course of the peripheral nerve that innervates the painful region. As the finger is moved along the skin, the patient reports sites at which the electrical stimulation is perceived most acutely. These are points of low skin resistance and they are thus more susceptible to electrical stimulation (Gersh, 1992; Hayes, 2000; ProMax, 2007).

Reducing Edema

Electrical stimulation can be used for various types of edema reduction. Traumatic edema resulting from the disruption of blood vessels often accompanies musculoskeletal injuries such as acute strains and sprains. Voluntary muscle pump activity or muscle pump facilitation through electrical stimulation may be effective in lymphatic and venous drainage and thus aid in resolution of posttraumatic edema.

The second approach for reducing traumatic edema through electrostimulation employs sensory-level stimulation that does not result in muscle contraction. Clinicians theorize that perhaps an effective current in managing edema would be a low-intensity, continuous, unidirectional current that would be expected to have the appropriate polar effects dependent upon the specific polarity used for treatment (Gersh, 1992; Hayes, 2000).

Stimulating Muscle Contraction

USING NMES

Neuromuscular electrical stimulation (NMES) is the application of current to elicit a muscle contraction. As mentioned earlier, the use of NMES in orthopedic and neuromuscular rehabilitation has grown significantly in recent years. A nerve action potential may be elicited either by a command originating in the motor cortex of the brain or by an electrically induced stimulus at the periphery.

The optimal size of the electrode is based on the desired muscular response, the size of the target muscle or muscles, and the chosen electrode placement. Larger electrodes are effective in generating torque from large muscles or groups of muscles that contract together; for example, a 5 cm x 10 cm electrode may be used to stimulate the quadriceps and hamstring muscles.

When small individual muscle stimulation is desired, a small electrode that increases the current density is placed over the motor point of that muscle; a second, larger reference electrode is often placed distally over the tendinous portion of the muscle. The second electrode provides a return path for the current but has inadequate current to cause significant polarization of excitable tissue under the electrode.

Recall that monopolar stimulation occurs when one electrode is placed over a motor point with the other placed at a site away from that area, while bipolar stimulation describes placement of both electrodes on the muscle or muscle group to be activated. Bipolar placement tends to be used most often in neuromuscular electrostimulation, because, for a given intensity of stimulation, more current reaches the muscle to be stimulated.

For optimal bipolar positioning, the clinician should place the muscle at resting length or in a slightly lengthened range, avoiding any close-packed positions of the limb or joint. Isometric contractions are used to achieve greater tension and should be applied at several points in the range of motion. Greater tension is built with higher frequencies but it can cause fatigue, and a frequency of 50 pulses per second (pps) provides as much as the higher frequencies. The amplitude is increased until strong maximal contraction is obtained; however, patient tolerance is the guide and stimulation should not be painful.

A rest cycle that is 5 to 6 times as long as the hold cycle allows the muscle adequate time to recover between contractions and produces same amount tension on each subsequent contraction. A hold time of 6 to 10 seconds builds optimal tension and 8 to 10 contractions in a single treatment session are sufficient for strengthening. Strength gains are likely to begin peaking about 20 to 25 sessions. Treatment should be provided daily, or at least every other day (Gersh, 1992; Hayes, 2000).

Electricity has been used to stimulate denervated muscle for over a century, yet the success rate is unclear; while this form of treatment has been shown to be effective in laboratory animals, studies are not consistent regarding its effectiveness in humans. The rationale for electrically stimulating denervated muscle is to exercise the muscle in an attempt to maintain it in a healthy condition while the injured axons regenerate and re-innervate the muscle.

Following the denervation of a muscle, the tissue undergoes a number of changes that are physiologic, biochemical, and anatomic. Progressive muscle fiber atrophy is the most obvious change, and the neuromuscular junctions also begin to degenerate. To achieve successful treatment of denervated muscle, electrical stimulation should be initiated as soon after the injuries as possible. In some cases, atrophy that has already occurred cannot be reversed. A stretched muscle produces more tension, and greater tension is produced with isometric contractions when they are feasible.

Denervated muscle requires a stimulus of long duration, and greater than 100 msec is recommended. Alternating current of 5 Hz has been known to provide a phase duration of 100 msec and to be preferred for the patient's skin. The use of continuous direct current that is manually interrupted can also provide a desired response. If direct current is used, the cathode should be the stimulating electrode. A 100 msec phase duration can be delivered only 10 times per second, assuming no rest between stimuli.

The amplitude is increased until a strong contraction is obtained, within the patient's tolerance, and sustained for at least 2 seconds. A period of rest follows the contraction that is 5 times as long as the length of the contraction. Permit 10 to 25 contractions per session which is to be completed 3 times a day with at least 10 minutes of rest between sessions. Treatment can be uncomfortable for the patient and consistent encouragement may be needed to ensure continued compliance at these parameters (Gersh, 1992; Hayes, 2000).

USING FES

Functional electrical stimulation (FES) is another form of electrotherapy that utilizes electrical currents to activate nerves that serve extremities affected by paralysis. The goal of FES is to restore function in people with disabilities. There are gait deviations associated with orthotic management, and electrostimulation that is timed to coincide with the individual gait pattern can be a useful addition to many gait training programs. This can take form of a conventional ankle orthosis made of plastic/metal and leather with the addition of electrical stimulation using a remote control switch.

When the clinician chooses electrostimulation as an orthotic substitute, the timing of the stimulus during ambulation is usually controlled by a pressure-activated heel switch. As a result, corrective positioning provided during the swing-through phase will be terminated during stance. The corrected positioning for stance will be terminated during swing-through, producing a more normal gait pattern.

If dorsiflexion during swing-through is the goal of stimulation, there should be no interference with push-off during stance, because weight-bearing on the heel switch during stance would inactivate stimulation of the dorsiflexors. Similarly, where knee extension is stimulated for stance stability, there would be no interference with knee flexion swing-through because non–weight-bearing on the heel switch at end of stance would inactivate stimulation of the quadriceps muscle.

When electrostimulation is used to provide dorsiflexion during swing, the electrical stimulus stops upon weight-bearing, thereby allowing the ankle to move freely into plantar flexion from heel-strike to foot-flat. As a result, no increase in flexion is produced at the knee. This can be important for the hemiparetic patient, where instability is often a problem.

Relatively small changes in electrode position can noticeably influence the effect in joint positioning. When electrostimulation as an orthosis is used outside the clinic, it is essential to have the patient or attendant reliably duplicate effective placement and be aware of changes in joint positioning during the wearing period.

Because the electrostimulation protocol is more complex than a functional orthosis, there is a greater chance of mechanical problems and/or human error. Safety is a primary concern and any human error can result in a change in joint positioning, placing the patient at risk of falling. These factors are much less problematic during supervised use of electrostimulation in the clinical setting (Gersh, 1992 ; Hayes, 2000).

Improving Circulation

Circulation can be improved with NMES by increasing metabolic demand through sporadically activating the muscle pump around the circulatory network. In this application, a low frequency of 20 to 30 pps has been shown to be most effective. A contraction of only 10% to 30% of maximal effort is sufficient. Duty cycle should be one that is not fatiguing; treatment may be given for 10 to 30 minutes and as frequently as is comfortable for the patient (Prentice, 2001; Gersh, 1992; Hayes, 2000).

Healing Wounds

There is now an ample body of evidence supporting the use of electric stimulation for wound healing. Guidelines for the use of stimulation have not been determined for all conditions, yet enough information exists to guide the clinician to a reasonable plan of care. Although not conclusive, a body of evidence suggests that polarity is an important treatment issue for both acute and chronic wounds.

RATIONALE

Literature on wound healing describes the body as having its own bioelectric system, which influences wound healing by attracting the cells of repair and changing cell membrane permeability. When there is a rupture in the skin, a current is generated between the skin and inner tissues that continues until the skin defect is repaired. Healing of the injured tissue is impeded or will be incomplete if these currents no longer flow while the wound is open. A rationale for applying electrical stimulation is that it mimics the natural current of injury and will initiate and/or accelerate the wound healing process.

Also of clinical importance is the theory that a moist wound environment is required for the bioelectric system to function. By keeping the wound moist with normal saline, the ideal electrical charge is maintained and dressings such as hydrogels and occlusive dressings can help promote the body's own bioelectric system by keeping the wound moist (Gogia, 1995).

Clinicians understand that wound debridement is enhanced if the tissue is solubilized with enzymatic debriding agents. Electric stimulation using negative current has been shown to solubilize clotted blood, whereas the positive electrode has been found to induce clumping of leukocytes and formation of thrombosis in the small vessels. This helps explain why clinical observations demonstrate that hemorrhaging at the wound margin is dissolved and reabsorbed following application of high-volt pulsed current (HVPC) with the negative pole.

Treatment parameters differ considerably in recent studies of successful healing. Typically the active electrode, which is one-fourth the size of the dispersal electrode, is positioned directly over the wound. To use this form of stimulation, electrodes may need to be cut to the appropriate sizes.

Placement of the dispersal electrode in relation to the active electrode is another area of controversy. Some researchers recommend the placement of dispersive electrodes closely proximal to the origin of the spinal nerve; in other words, while treating wounds with negative polarity, the dispersive or positive electrode should be placed close to the spinal cord relative to the negative electrode.

Electrical stimulation affects each phase of wound healing differently, beginning with the inflammatory phase that initiates the wound repair process. In this phase, increasing blood flow can help in the removal of debris by way of phagocytosis; in addition, by increasing blood flow, electrical stimulation enhances tissue oxygenation.

Next, electrical stimulation has been determined to promote the proliferation phase by stimulating the fibroblasts and epithelial cells needed for tissue repair. Membrane transport is improved, which supports the body's natural current.

Eventually, in the proliferation stage, better collagen is produced, which helps in the stimulation of wound contraction. Clinical studies suggest settings for the proliferation phase with the polarity as negative, pulse rate at 100 to 128 pps, intensity of 100 to 150 volts, and a duration up to 60 minutes, once daily, 5 to 7 times per week.

In the later phase of epithelialization, electrotherapy can stimulate epidermal cell reproduction and migration, helping to produce smoother and thinner scar tissue. Settings during the epithelialization phase include alternating the polarity every 3 days—for example, 3 days negative followed by 3 days positive. Pulse rate is recommended at 64 pps, an intensity of 100 to 150 volts, and a duration of 60 minutes, 5 to 7 times per week.

Based on scientific rationale from early studies, the application of electrical stimulation using direct current reported long treatment times of 20 to 40 hours per week. Four controlled clinical studies and three uncontrolled studies with HVPC reported a mean healing time of 9.5 weeks with 45- to 60-minute treatments, 5 to 7 times per week (Sussman, 2000).

PREPARING THE PATIENT FOR WOUND TREATMENT

Having the supplies ready before removing the wound dressing saves time and helps avoid unnec