Interferential therapy (IFT) is a transcutaneous electrical nerve stimulation technique developed in the 1950s. It involves applying two medium frequency alternating currents (usually around 4000 Hz) through the skin that interfere with each other to generate a low frequency amplitude modulated current for therapeutic purposes. This provides deeper penetration than a single current while being more comfortable than low frequency currents. The interference produces a rotating vector that varies the stimulation frequency, preventing nerve accommodation. IFT is used to relieve pain, stimulate muscles, increase local blood flow, and reduce edema.
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DEFINITION
Developed by Dr. Hans Nemec of Vienna, Austria in the
early 1950’s
It may be described as the transcutaneous application of
alternating medium frequency electrical currents,
amplitude modulated at low frequency for therapeutic
purposes.
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Why IFT?..
MF currents associated with a lower skin resistance (impedance),
more comfortable than low frequency currents
more tolerable penetration of current through the skin is possible
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SKIN IMPEDANCE
The word impedance comes from the Latin impedire meaning to
prevent, to stop from going on.
Impedance signifies the resistance of a conductor when an electric
current passes.
Conventionally speaking, the term resistance refers to the obstacle to
the direct current, and it is represented by the letter R.
The terms impedance refers to the obstacle to the alternating current
and it is represented by the letter Z.
Impedance Z, as resistance R, is expressed in ohms.
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SKIN IMPEDANCE
SKIN IMPEDANCE is calculated as Z = ½ π f C Where,
Z = skin resistance
f = frequency in Hertz (Hz)
C = capacitance of skin in microfarads
From above formula, skin impedance is inversely proportional to
frequency and capacitance of the skin.
Capacitance of the skin cannot be changed; hence by increasing the
frequency the skin impedance can be lowered.
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SKIN IMPEDANCE
It is assumed that a capacitance of 1 microfarad (1 μF) i.e. equivalent
to 0.000001 (10 to the -6th power).
For a 50-Hz alternating current we obtain:
Z = 1/2π x 50x 10-6 = 3200 Ω
For a 4,000-Hz alternating current this means:
Z = 1/2π x 4000x 10-6 = 39.8 Ω
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PRINCIPLE
The principle of IFT is to pass two Medium Frequency
Alternating Currents which are slightly out of phase,
through the tissues, where the currents intersect to
produce a low frequency effect.
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Principles of wave interference: Combined Effects
Constructive, Destructive, & Continuous
Constructive interference: when two sinusoidal waves that are exactly
in phase or one, two or three wavelengths out of phase, the waves
supplement each other in constructive interference
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+ =
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Principles of wave interference: Combined
Effects
Destructive interference: when the two waves are different by 1/2 a
wavelength (of any multiple) the result is cancellation of both waves.
+ =
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Principles of wave interference: Combined
Effects
Continuous Interference: Two waves slightly out of phase collide and
form a single wave with progressively increasing and decreasing
amplitude
=
+
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WORKING
In the interferential therapy method, two medium-frequency alternating
currents that interact with each other are used.
One of the alternating currents has a fixed frequency of 4,000 Hz while
the frequency of the other alternating current can be set between 4,000
to 4,250 Hz.
The superimposition of one alternating current on the other is called
interference
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Carrying Frequency
The frequency of the new medium-frequency alternating current can be
calculated as follows:
f1 + f2 /2 where
Example:
f1 = 4,000 Hz
f2 = 4,150Hz
Resultant Carrying Frequency = 4,000 + 4,150/2 = 4,075 Hz
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Beat Frequency
The frequency with which the amplitude varies is referred to as the
Beat Frequency.
In interferential therapy the AMF (treatment frequency) corresponds to
frequencies which are used in low-frequency electrotherapy.
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Beat Frequency
The AMF has a value of f2 - f1
Example:
f1 = 4,000 Hz
f2 = 4,150 Hz
f 2 - f 1 = 4,150 - 4,000 = 150 Hz (AMF or treatment frequency)
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SWEEP
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.
Such sweep prevents accommodation of nerves.
Example:
An AMF of 20 Hz is set and a 50 Hz spectrum is added.
The current begins with an AMF of 20 Hz and passes successively
through all frequencies up to 70 Hz, after which it decreases gradually
to 20 Hz.
This process is repeated automatically.
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Triangular sweep pattern Rectangular Sweep Pattern
Trapezoidal Sweep Pattern 21
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APPLICATION
Methods of applications are:
1. Static interference
2. Dynamic interference
3. Bipolar interference
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STATIC INTERFERENCE
Here the area in which IFT
is set up remains
stationary.
This area of static
interference gives an
appearance of ‘clover leaf
’.
It lies to 45°angles to the
perpendicular lines from
each electrode. 24
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DYNAMIC INTERFERENCE
This is moving the area of
interference current in a to and
fro manner through 45°.
It is obtained by varying the
current intensity from 50 to
100%.
The dynamic area of
interference is also called as
vector sweep, vector scan,
rotating vector, etc.
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BIPOLAR INTERFERENCE
Here pre-modulated IFT are
applied through 2 surface
electrodes.
IFT is generated inside the
equipment and is delivered
to the patient through 2
electrodes.
Useful for smaller areas.
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INSTRUMENTATION
AMF PARAMETER To choose the basic value of the LF modulation that is
desired.
SPECTRUM/ SWEEP PARAMETER to set the range of variation in the AMF
value that is desired; for ex. AMF at 100Hz and spectrum at 50Hz will give an
AMF variation from 100hz up to 150Hz and back to 100Hz.
ROTATION PARAMETER is applicable in case of vector currents only and
sets the rate of rotation and the direction change of the AMF field within the
tissues.
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PHYSIOLOGICAL EFFECTS
This depends upon:
1. Magnitude of current
2. Type of mode used i.e. Rhythmic or constant
3. The frequency range used
4. Accuracy of electrode positioning
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PHYSIO.EFFECTS (CONT…)
Main clinical applications are :
1. Pain relief
2. Muscle stimulation
3. Increased local blood flow
4. Reduction of edema
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PAIN RELIEF
Higher frequencies of 90 - 130 Hz to stimulate the pain gate
mechanisms & thereby mask the pain symptoms.
Lower frequencies of 2 – 5 Hz can be used to activate the opioid
mechanisms
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MUSCLE STIMULATION
For patients who can not generate useful voluntary
contraction, IFT may be beneficial.
Stimulation at low frequency, for e.g. 1 Hz will result in a
series of twitches
Stimulation at 50 Hz will result in a tetanic contraction.
Most effective motor nerve stimulation range is between
10 - 25 Hz.
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BLOOD FLOW
Experimentation demonstrated vascular changes at 10 -
25Hz.
Effects of blood flow is believed to be due to:
1. Effects of suction electrodes on tissues
2. Effects of muscle stimulation on circulation.
3. Possibility that the IFT is acting as an inhibitor for
sympathetic activity. (Sympathetic nervous system stimulation
causes vasoconstriction of most blood vessels.)
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OEDEMA
A rhythmical pumping action at a frequency of 1-10 Hz
The effects are associated with the effects on Muscle
Stimulation & Blood Flow.
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CONTRAINDICATIONS
1. Cardiac pacemaker
2. Advanced cardiac disease
3. Hypertension
4. Thrombosis
5. Hemorrhage
6. Pregnancy
7. Neoplasm
8. T B
9. Fever
10. Infections
11. The eyes
12. Skin disorders
13. Epiphyseal region in children
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PRECAUTIONS
IFT apparatus must be at least 6 meters away from SWD
machine
Patients with febrile conditions
Epileptic patients
Over anterior chest wall
Position suction electrode below the level of damage or
discomfort.
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Plate Electrodes
Made of conducting rubber
which are comfortable and
long lasting
larger plates electrodes give
more comfortable treatment
and deeper effect
Smaller electrodes are used
for a localized effect, but this
effect is more superficial
Attached to the patient by
means of straps to assure a
good contact and greater
tolerance of treatment
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Vacuum Electrodes
Excellent for treating flat
smooth areas for example
back or a plump knee
Not indicated for treatment of
the hairy areas because can’t
get an airtight seal
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Overview
These are medium frequency alternating current in the low kHz
range, which is delivered in a pulsed (or burst or interrupted)
output.
The pulsing or bursting is at a 'low' frequency, and as a result,
nerves will respond.
It is primarily employed to generating a motor response.
Burst Mode Alternating Current (BMAC).
The credit for the early work in this field is ascribed to Dr Yakov Kots,
based in Russia, in the late 1970s.
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The term 'Russian currents' refers to sinusoidal AC of frequency
2.5 kHz which is burst-modulated at 50 Hz.
It consists of 10 ms bursts of AC separated by 10 ms 'off' periods.
The waveform repeats every 20 ms (1/50th sec) so the burst or
modulation frequency is 50 Hz.
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Characteristics
Carrier Frequency: 2500Hz.
Waveform: Polyphasic sinusoidal
waveform.
Bursts Frequency: 50 Hz.
Burst Duration: 10 ms
Interburst interval: 10 ms
Phase Duration: 0.2 ms
Current Amplitude: The
maximum current amplitude is
100mA. Clinical used at 70mA.
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2.5 kHz sinusoidal AC, burst modulated
at 50 Hz i.e. 10 ms 'on' and 10 ms 'off'.
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Dosing Parameters
Base Frequency : 2500Hz (2.5kHz)
Burst @ 50Hz
10 ms ON : 10 ms OFF (50% duty cycle)
Stimulation delivered thus for 10 seconds
Rest period of 50 seconds
Repeated 10 minutes per session
Maximum tolerable intensity
Applied once daily over a period of weeks
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