The basic principle of Interferential Therapy (I/F) is to utilise the strong physiological effects of low frequency (F<250 Hz) electrical stimulation of muscle and nerve tissues without the associated painful and somewhat unpleasant side effects of such stimulation.
To produce low frequency effects at sufficient intensity at depth, most patients experience considerable discomfort in the superficial tissues (i.e. the skin). This is due to the resistance (impedance) of the skin being inversely proportional to the frequency of the stimulation. In other words, the lower the stimulation frequency, the greater the resistance to the passage of the current & so, more discomfort is experienced. The skin impedance at 50Hz is approximately 3200W whilst at 4000Hz it is reduced to approximately 40W . The result of applying this latter frequency is that it will pass more easily through the skin, requiring less electrical energy input to reach the deeper tissues & giving rise to less discomfort.
The effects of tissue stimulation with these 'medium frequency' currents (Medium frequency in electromedical terms is usually considered to be 1KHz-100KHz) is unknown. It is unlikely to be innocuous, but in terms of current practice, little is known of its physiological effects. It is not capable of direct stimulation of nerve in the common context of such stimulation.
Interferential therapy utilises two of these medium frequency currents, passed through the tissues simultaneously, where they are set up so that their paths cross & in simple terms they interfere with each other. This interference gives rise to an interference or beat frequency which has the characteristics of a low frequency stimulation.
The exact frequency of the resultant beat frequency can be controlled by the input frequencies. If for example, one current was at 4000Hz and its companion current at 3900Hz, the resultant beat frequency would be at 100Hz, carried on a medium frequency 3950Hz amplitude modulated current.
By careful manipulation of the input currents it is possible to achieve any beat frequency that you might wish to use clinically. Modern machines usually offer frequencies of 1-150Hz, though some offer a choice of up to 250Hz or more. To a greater extent, the therapist does not have to concern themselves with the input frequencies, but simply with the appropriate beat frequency which is selected directly from the machine.
The use of 2 pole I/F stimulation, where there is clearly no interference within the body, is made possible by electronic manipulation of the currents - the interference occurs within the machine.
The magnitude of the low frequency interference current is approximately equivalent to the sum of the input amplitudes. In other words, the result of the interaction between the two input currents is a low frequency current, which has an amplitude greater than either of the individual input currents.
Excitable tissues can be stimulated by low frequency alternating currents. Although to some extent, all tissues in this category will be affected by a broad range of stimulations, it is thought (Savage 1984) that different tissues will have an optimal stimulation band, which can be estimated by the conduction velocity of the tissue, its latency and refractory period. These are detailed below:
Sympathetic Nerve 1-5Hz
Parasympathetic Nerve 10-150Hz
Motor Nerve 10-50Hz
Sensory Nerve 90-100Hz
Nociceptive fibres 90-150Hz (?130Hz specific)
Smooth Muscle 0-10Hz
The clinical application of I/F therapy can be based logically on this data together with a knowledge of physiological behaviour of stimulated tissue. Selection of a wide treatment band can be considered less efficient than a smaller selective band in that by treating with a frequency range of say 1-100Hz, the appropriate treatment frequencies can be covered, but only for a relatively small percentage of the total treatment time. Additionally, some parts of the range might be counterproductive for the primary aims of the treatment.
The are 4 main clinical applications for which I/F appears to be used:
Increased blood flow
Reduction of oedema
In addition, claims are made for its role in stimulating healing and repair.
As I/F acts primarily on the excitable tissues, the strongest effects are likely to be those which are a direct result of such stimulation (i.e. pain relief and muscle stimulation). The other effects are more likely to be secondary consequences of these.
Electrical stimulation for pain relief has widespread clinical use, thought the direct research evidence for the use of I/F in this role is limited. Logically one could use the higher frequencies (90-150Hz) to stimulate the pain gate mechanisms & thereby mask the pain symptoms. Alternatively, stimulation with lower frequencies (1-5Hz) can be used to activate the opoid mechanisms, again providing a degree of relief. These two different modes of action can be explained physiologically & will have different latent periods & varying duration of effect. It remains possible that relief of pain may be achieved by stimulation of the reticular formation at frequencies of 10-25Hz or by blocking C fibre transmission at >50Hz.
Stimulation of the motor nerves can be achieved with a wide range of frequencies. Clearly, stimulation at low frequency (e.g. 1Hz) will result in a series of twitches, whist stimulation at 50Hz will result in a tetanic contraction. The choice of treatment parameters will depend on the desired effect, but to
combine muscle stimulation with an increase in blood flow and a possible reduction in oedema, there is some logic in selecting a range which does not involve strong sustained tetanic contraction & a sweep of 10-25Hz is often used.
There is no primary nervous control of oedema reabsorption & the direct electrical stimulation of blood flow is limited in its effectiveness. It is suggested therefore that in order to achieve these effects, suitable combinations of muscle stimulation can be made.
Electrode positioning should ensure adequate coverage of the area for stimulation. In some circumstances, a bipolar method is preferable if a longitudinal zone requires stimulation rather than an isolated tissue area. Placement of the electrodes should be such that a crossover effect is achieved in the desired area. If the electrodes are not placed so that a crossover is achieved, the physiological effects of I/F can not be achieved.
Nerves will accommodate to a constant signal & a sweep (or gradually changing frequency) is often used to overcome this problem (as well as generating a range of effects). The sweep (range) should be appropriate to the desired physiological effects, though again it is suggested that an excessive range may minimise the clinical effect.
The mode of delivery of the selected sweep varies with machines. The most common application is the 6 second rise and fall between the pre-set frequencies. For example, if a 10 - 25Hz range has been selected, the machine will deliver a changing frequency, starting at 10Hz, rising to 25Hz over a 6 second period. Once this upper limit has been achieved, the frequency will once again fall, over a 6 second period to its starting point at 10Hz. This pattern is repeated throughout the treatment session
Other patterns of sweep can be produced on many machines as illustrated:
Treatment times vary widely according to the usual clinical parameters of acute/chronic conditions & the type of physiological effect desired. In acute conditions, shorter treatment times of 5-10 minutes may be sufficient to achieve the effect. In other circumstances, it may be necessary to stimulate the tissues for 20-30 minutes. It is suggested that short treatment times are initially adopted especially with the acute case in case of symptom exacerbation. These can be progressed if the aim has not been achieved and no untoward side effects have been produced. There is no research evidence to support the continuous progression of a treatment dose in order to increase or maintain its effect.
Interferential stimulation is concentrated at the point of intersection between the electrodes. This concentration occurs deep in the tissues as well as at the surface of the skin. Conventional TENS and Neuromuscular stimulators deliver most of the stimulation directly under the electrodes. Thus, with Interferential Stimulators, current perfuses to greater depths and over a larger volume of tissue than other forms of electrical therapy. When current is applied to the skin, capacitive skin resistance decreases as pulse frequency increases.' For example, at a frequency of 4,000 Hz (Interferential unit) capacitive skin resistance is eighty (80) times lower than with a frequency of 50 Hz (in the TENS range). Thus, Interferential current crosses the skin with greater ease and with less stimulation of cutaneous nociceptors allowing greater patient comfort during electrical stimulation.
In addition, because medium-frequency (Interferential) current is tolerated better by the skin, the dosage can be increased, thus improving the ability of the Interferential current to permeate tissues and allowing easier access to deep structures. This explains why Interferential current may be most suitable for treating patients with deep pain, for promoting osteogenesis in delayed and nonunion fractures and in pseudothrosis, for stimulating deep skeletal muscle to augment the muscle pump mechanism in venous insufficiency, and for depressing the activity of certain cervical and lumbosacral sympathetic ganglia in patients with increased arterial constrictor tone.
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