Waveforms Definition .
The term waveform means the graphical representation of the direction, shape, amplitude, duration and pulse frequency of the electrical current produced by the electrotherapeutic device. The instrument which is used to display the electric current is called an oscilloscope.
Pulses, Phases and Direction of Current Flow .
The waveform as shown by an oscilloscope is referred to as a pulse. A pulse may contain either one or two phases . It rises above or goes below the baseline for some specific period of time. Direct current, also referred to as monophasic current, produces waveforms that have only a single phase in each pulse. Current flow is unidirectional, always flowing in the same direction toward either the positive or negative pole.
Conversely, alternating current, also referred to as biphasic current, produces waveforms that have two separate phases during each individual pulse. Current flow is bidirectional, reversing direction or polarity once during each pulse. Biphasic waveforms may be symmetrical or asymmetrical. If both phases of the waveform may be symmetrical, the shape and size of each phase is identical.
pulsed current waveforms are called polyphasic currents and are representative of electrical current that is conducted as a series of pulses of short duration followed by a short period of time, when current is not flowing called the interpulse interval. Single current may flow in one direction as in direct current or may reverse direction of flow as in alternating current. With pulsed currents, there is always some interruption of current flow .
Waveform Shape .
Waveform shape could be of any type like sine, rectangular, or triangular waveform depending on the capabilities of the generator producing the current. Alternating, direct and pulsed currents may be of the following waveform shapes .
Pulse Amplitude .
The maximum amplitude of a pulse can be shown by the tip of highest point of each phase. The amplitude of each pulse reflects the intensity of the current. The term amplitude is synonymous with the terms voltage and current intensity. The higher the amplitude, the greater is the voltage or intensity. The total current cannot be confused with the tip of highest point of a phase. The total current delivered to the tissues can only be calculated by averaging the current flowing per unit time including the interpulse intervals.
The electrical generators that produce short duration pulses, the total current produces (coulomb/sec) is low compared to peak current amplitudes due to long interpulse intervals. Thus, the average current or the amount of current flowing per unit of time is relatively low. Average current can be increased by either increasing pulse duration, increasing pulse frequency, or by some combination of the two.
Pulse Charge .
The term pulse charge indicates the total amount of electricity that is delivered to the patient during each pulse. In monophasic currents, the phase charge and the pulse charge are the same and are greater than zero. With biphasic currents the pulse charge is equal to the algebric sum of the phase charges. If the pulse is symmetrical the net pulse charge is zero. In asymmetrical pulses, the net pulse charge cannot be zero.
Rise and Decay Time .
The rate of rise in amplitude or the rise time indicates the time taken by a pulse to reach its maximum amplitude in each phase. Conversely, decay time refers to the time taken by a pulse to return to neutral. The rate of rise is important therapeutically so as to avoid accommodation of the nerves to the constant amplitude current, which results in constant level of depolarization and nerves become unexcitable at that same intensity or amplitude.
Rate of rise and decay times are generally short, ranging from nanosecond to millisecond. By observing the three different waveforms, it is apparent that the sine wave has a gradual increase and decrease in amplitude for both alternating and direct currents. The rectangular wave has an almost instantaneous increase in amplitude, which plateaus for a period of time and then abruptly falls off. The shape of these waveforms as they reach their maximum amplitude or intensity is directly related to the excitability of the nervous tissue.
The more rapid the increase in amplitude or the rate of rise, the greater the current’s ability is to excite nervous tissue. Most modern DC generators make use of a twin-peak triangular pulse of very short duration and peak amplitudes as high as 500 V. Combining high-peak intensity with a short-phase duration produces a very comfortable type of current as well as an effective means of stimulating sensory, motor and pain fibers.
Asymmetric Waveforms .
The use of asymmetrical waveforms for therapeutic purposes is now of the past. The true faradic waveform is also no longer being used. The so-called true faradic current is like a biphasic pulsed current with asymmetric waveform. The original faradic current is like an alternating current because there was always a reversal of direction of current flow.
The amplitude of the portion of the wave in the negative direction was not great enough to produce any physiologic response. In the monophasic saw—tooth or exponential waveform the amplitude rises very gradually and then falls abruptly. Current that uses this waveform stimulates denervated muscle without affecting normally innervated muscle, since the gradual rise in amplitude allows for accommodation of the normal muscle.
Exponential Current .
The basic phenomenon is to rise the current impulses gradually. When represented graphically, these impulses display a similarity to a triangle, which is why this form of current is called triangular current. As the current does not increase in a straight line, but rather in accordance with a mathematical exponential equation, the current is also called exponential current.
Pulse Duration .
The length of time that current is flowing in one cycle indicates duration of each pulse. With monophasic current the phase duration is the same as the pulse duration. It is the time from initiation of the phase to its end. With biphasic current the pulse duration is determined by the combined phase durations. In some devices, it is prefixed and in some the uses can alter it.
The phase duration as well as pulse duration may be as short as few microseconds or may be a long-duration direct current that flows for several minutes. In pulsed currents and also in some cases with alternating and direct currents, the current flow can be off for some period of time. The combined time of the pulse duration and the rest duration or interpulse interval is known to as the pulse period.
Pulse Frequency .
Pulse frequency is the number of pulses per second. Each individual pulse either rises or falls from its base value. As the frequency of any waveform is increased, the amplitude tends to increase and decrease more rapidly. The muscular and nervous system responses depend on the length of time between pulses and on how the pulses or waveforms are modulated.
Muscle will respond with individual twitch contraction to pulse rates of less than 50 pulses per second. At 50 pulses per second or greater, a tetany will result, regardless of whether the current is biphasic, monophasic, or polyphasic.
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