Electrical Stimulation Therapy continued from page-1
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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 . 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.
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).
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