Interferential Therapy Definition
Interferential therapy (IFT) is a part of electrotherapy and form of electrical treatment in which two medium frequency currents are used to produce a low frequency effect. Medium frequency currents alternate the direction of the flow of electrons at a frequency between 2000-4000 Hz.
Physiological Effects of Interferential Therapy
- Relief of pain
- Increase of local circulation
- Stimulation of autonomic nerves result in vasodilatation
- Stimulation of thick and thin nerve fibers
- Helping in absorption of exudates and decrease swelling
Therapeutic Effects of Interferential Therapy
- Pain relief
- Muscle stimulation
- Increased local blood flow
- Reduction of edema
Interferential Therapy Method of Application
- Metal plate electrodes and pads
- Quadripolar plate electrodes
- Suction cup electrodes
- Quadripolar suction cup probe electrodes
Wave length Frequency and Beat Frequency of IFT
The basic principle of Interferential Therapy (IFT) is to utilize the strong physiologic portable IFT effects of low frequency (<250 PPS) electrical stimulation of nerves without the associated painful and somewhat unpleasant side effects sometimes associated with low frequency stim.
To produce low frequency effects at sufficient intensity at depth, patients can experience considerable discomfort in the superficial tissues (i.e. the skin). This is due to the impedance of the skin being inversely proportional to the frequency of the stimulation. In other words, the lower the stimulation frequency, the greater the impedance to the passage of the current & so, more discomfort is experienced as the current is ‘pushed’ into the tissues against this barrier. The skin impedance at 50 Hz is approximately 3200* whilst at 4000 Hz it is reduced to approximately 40*. The result of applying a higher 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 effect of tissue stimulation with these ‘medium frequency’ currents (medium frequency in electromedical terms is usually considered to be 1K Hz-100K Hz) has yet to be established. It is unlikely to do nothing at all, 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 utilizes two of these medium frequency currents, passed through the tissues simultaneously, where they are set up so that their paths cross & they literally interfere with each other. This interference gives rise to an interference (beat frequency) which has the characteristics of low frequency stimulation – in effect the interference mimics 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 4000 Hz and its companion current at 3900 Hz, the resultant beat frequency would be at 100 Hz, carried on medium frequency 3950 Hz 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 250 Hz 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.
Part of Machine and Beat Frequency
- Part of machine:
An interferential therapy machine will have the following facilities:
- On/off knob
- Time setting knob
- Intensity knob
- Modes (Bipolar, Quadripolar and clover-Leaf method)
- Sweep/Swing sitting knob (constant/variable)
- Beat frequency: It is an interference effect. The where the medium frequency current. The different in frequency between the two alternate currents which are slightly out of phase. Example-
Current 1 = 4000
Current 2 = 3900
Beat frequency = 4000-3900 = 100
Sweep Frequency in Interferential Therapy
Nerves will accommodate to a constant signal & a sweep (or gradually changing frequency) is often used to overcome this problem. The principle of using the sweep is that the machine is set to automatically vary the effective stimulation frequency using either pre-set or user set sweep ranges. The sweep range employed should be appropriate to the desired physiological effects (see below). It has been repeatedly demonstrated that ‘wide’ sweep ranges are ineffective in the clinical environment
Note : Care needs to be taken when setting the sweep on a machine in that with some devices, the user sets the actual base and top frequencies (e.g. 10 and 25 Hz) and with other machines the user sets the base frequency and then how much needs to be added for the sweep (e.g. 10 and 15 Hz). Sweep triangle sweep rectangle sweep trapezoidal (A) Triangular sweep pattern
(B) Rectangular Sweep Pattern
(C) Trapezoidal Sweep Pattern
The pattern of the sweep makes a significant difference to the stimulation received by the patient. Most machines offer several sweep patterns, though there is very limited ‘evidence’ to justify some of these options. In the classic ‘triangular’ sweep pattern, the machine gradually changes from the base to the top frequency, usually over a time period of 6 seconds – though some machines offer 1 or 3 second options. In the example illustrated, the machine is set to sweep from 90 to 130 Hz (figure A) employing a triangular sweep pattern. All frequencies between the base and top frequencies are delivered in equal proportion.
Other patterns of sweep can be produced on many machines, for example a rectangular (or step) sweep. This produces a very different stimulation pattern in that the base and top frequencies are set, but the machine then ‘switches’ between these two specific frequencies rather than gradually changing from one to the other. The diagram (figure B) illustrates the effect of setting a 90 – 130 Hz rectangular sweep.
There is clear difference between these examples – even though the same ‘numbers’ are set. One will deliver full range of stimulation frequencies between the set frequency levels and the other will switch from one frequency to the other. There are numerous other variations on this theme, and the ‘trapezoidal’ sweep is effectively a combination of these two. The only sweep pattern for which ‘evidence’ appears to exist is the triangular sweep.
Principle of Interferential Therapy
The main problem in applying low frequency currents (the faradic or sinusoidal currents) is the very high skin impedance encounter by such low frequency currents (50-100 Hz). Medium frequency currents of around 4000 Hz, while able to stimulate motor and sensory nerves, encounter much lower skin impedance. Impedance is inversely proportional to frequency: the applicable formula is-
Where, Z + impedance in ohms
F = frequency in hertz
C = capacitance of skin in micro farads.
The resistance of the skin at 50 Hz is in the region of 3200 Ω where as with a frequency of 4000 Hz skin resistance is 40 Ω.
The second principle upon which IFT is based is that when two currents cross each other, the resulting current will be in the range of their difference. So, two medium frequency currents, will easily pass the skin impedance, and will be allow to cross each other deep in the patient’s tissue (e.g. in deep muscles as pelvic floor). Hence, this muscle will be stimulated by a low frequency current which is the result of their difference. For example, we design two circuits (A and B) of medium frequency current as follows.
- Circuit A : 4000 Hz
- Circuit B : 3900 Hz
When the currents of these two circuits are applied is such a way so as the directions across each other, then their resulting current (i.e. interferential current) will be in the range of their difference i.e. 100 Hz, which is a low frequency current.
The frequency of both circuits can be made fixed with a resulting “fixed” low frequency current, e.g. current A : 4000 Hz and circuit B : 3900 Hz, with resulting 50 Hz interferential current, or A : 4000 Hz and B : 3980 Hz, with a resulting 50 Hz interferential current, or A : 4000 Hz and B : 3980 Hz with a resulting 20 Hz interferential current.
Production of a low frequency interference current in tissue. Electrodes A and A’ are part of a circuit with an a.c. (current at 4000 Hz), electrodes B and B’ are part of a separate circuit, passing an a.c. (current at 3900 Hz).
Indications of Interferential Therapy
- Pain relief (nerve pain)
- Reflex sympathetic dystrophy
- Phantom limb pain
- Recent injuries
- Stress incontinence
- Edema or swelling
- Back pain or disc lesion
- Knee joint pain
- Sudeck’s atrophy
- Rheumatoid arthritis
- Shoulder-hand syndrome
- Neck pain
Contraindications for Interferential Therapy
- Crdiac patient with pacemaker
- Open wounds
- Severe diabetic patient (DM)
- Unreliable patient
- Sensory loss
- Limb ischemia
Danger of Interferential Therapy
- Electrical burn
- Chemical burn
- Poor result for faulty positioning of the electrodes.
Precautions of Interferential Therapy (IFT)
Care should be taken in the following applications-
- Check main supply and equipment of machine.
- Electrodes should not be placed on the eye.
- Electrodes should not be placed on broken skin sites.
- Check if any contraindication.
- Check switch and machine
- Skin lesion must be insulated with petroleum jelly.
Technique of Application of Interferential Therapy (IFT)
Preparation of The Patient and Treated Part
The patient position will suitable and comfortable. A suitable support will be arranged for the treated area. Treatment part is washing carefully, dried and clean. Any contraindication should be avoided and any skin lesions insulated with petroleum jelly.
Preparation of The Therapist
The therapist position will be suitable and comfortable. Check the machine which input and output impulse correctly and counseling with the patient necessary.
Pad Placement or Electrode Placement
The site for treatment is accurately located and the two currents are over or within the lesion. Two pairs of electrodes with different colors are placed in such a way that the currents with different colors are placed in such a way that the currents passing between both pairs of electrodes cross each other in exactly the desired place of treatment. Avoid too close approximation of the electrodes. The patient is warned the he will feel a tingling sensation.
Application Of Interferential therapy
An appropriate treatment frequency is selected (e.g. 80-100 Hz, rhythmic for relive pain and the current intensity in turned up until the patient experiences a mild tingling sensation; after a few minutes accommodation occurs, and the intensity may be turned up. The current should be sufficient to produce motor stimulation, if this is required. After 15 minutes of treatment it is common for the machine to be turned down and another frequency applied for a different effects. (e.g. 1010 Hz. Rhythmic for reduce swelling).
Duration and Frequency of Treatment of Interferential Therapy
Stimulation is usually given for 10-20 minutes daily according to the patient condition.
Dosage of Interferential Therapy (IFT)
For acute pain= 90-100 Hz, rhythmically for 10 minutes.
For chronic pain= 100 Hz, constant for 10 minutes.
During edema with pain= 100 Hz, constant for 10 minutes.
Only edema= 1- 100 Hz, rhythmic, 15 minutes.
For weak muscle with innervation= 1-10 Hz rhythmic, 5-10 minutes.
For parasthesia= 90-100 Hz rhythmic, 10-15 minutes.
For pelvic floor muscle= 100 Hz constant, 10 minutes.
For phantom pain= 100 Hz constant, 10 minutes.
Termination of interferential therapy: The Interferential therapy (IFT) application is removed on the skin and inspected the treatment area once more.
Difference Between TENS and IFT
|SI No||TENS ( Trans-cutaneous Electrical Nerve Stimulation)||SI No||IFT ( Interferential Therapy )|
|1.||Low Frequency, high intensity, high frequency, low intensity||1.||Two medium frequency current. Current are used to produce low frequency current.|
|2.||One circuit of electricity is used.||2.||Two circuit of electricity is used.|
|3.||Usually two electrodes are used but rarely four electrodes are used.||3.||Four electrodes are must be used.|
|4.||Used for only sensory stimulation.||4.||Used for both sensory and motor stimulation|
|5.||No muscle contraction||5.||Muscle contractions.|
|6.||No beat frequency no influence.||6.||Beat frequency can be area to influence the tissue healing.|
|7.||Portable and no expensive.||7.||Not portable and expensive.|
Operational Skills of Interference Therapy (IFT)
It includes proper operation and maintenance of the instruments along with knowledge about contraindication and treatment record:
- Patients who do not comprehend the physiotherapist’s instructions or are unable to co-operate should not be treated.
- Patients with pacemakers- Some pacemakers are relatively immune to interference from electrical stimulation whilst others can demonstrate serious adverse behavior. It is suggested that as a general rule, if the patient has a pacemaker, it is best to avoid all electrical stimulation, but like TENS, if it is a treatment that is needed. The stimulation should be tried in a carefully controlled environment where appropriate equipment is available to correct any pacing problems should they arise.
- Patients who are taking anti coagulation therapy or have a history of pulmonary embolism or deep vein thrombosis should not be treated with the vacuum electrode applications.
- Similarly patients whose skin may be easily damaged or bruised.
- The trunk or pelvis during pregnancy (through this may be modified in time in line with the TENS advice. At the present time, it is suggested that it is best avoided in these regions).
- Active or suspected malignancy except in hospice/palliative/terminal care.
- The eyes.
- The anterior aspect of the neck.
- The carotid sinuses.
- Dermatological conditions, e.g. dermatitis, broken skin.
- Danger of hemorrhage or current tissue bleeding (e.g. recent soft tissue injury).
- Avoid active epiphysis region in children.
- Trans-thoracic electrode application is considered to be risky by many authorities.
Interferential Therapy Treatment Record
- Electrode number (2 pole, 4 pole) and positions.
- Frequency applied.
- Sweep setting employed (if applicable).
- Current intensity applied (or patient reported sensation).
- Treatment duration.
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